Hughes iieport on photo-electricity THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA LOS ANGELES Vol. 2. Part 2 APRIL, 1921 Number 10 BULLETIN OF THE NATIONAL RESEARCH COUNCIL REPORT ON PHOTO-ELECTRICITY Including Ionizing and Radiating Potentials and Related Effects BY ARTHUR LLEWELYN HUGHES Research Professor of Physics, Queen's University Kingston, Canada PUBLISHED BY THE NATIONAL RESEARCH COUNCIL OP THE NATIONAL ACADEMY OF SCIENCES WASHINGTON, D. C. 1921 Announcement Concerning Publications of the National Research Council The Proceedings of the National Academy of Sciences has been designated as the official organ of the National Research Council for the publication of accounts of research, committee and other reports, and minutes. Subscription rate for the "Proceedings" is $5 per year. Business address: Home Secretary, National Academy of Sciences, Smith- sonian Institution, Washington, D. C. The Bulletin of the National Research Council presents contributions from the National Research Council, other than proceedings, for which hitherto no appropriate agencies of publication have existed. The "Bulletin" is published at irregular intervals. The sub- scription price, postpaid, is $5 per volume of approximately 500 pages. Numbers of the "Bulletin" are sold separately at prices based upon the cost of manufacture (for list of bulletins see third cover page). The Reprint and Circular Series of the National Research Council renders available for purchase, at prices dependent upon the cost of manufacture, papers published or printed by or for the National Research Council (for list of reprints and circulars see third cover page). Orders for the "Bulletin" or the "Reprints and Circulars" of the National Research Council, accompanied by remittance, should be addressed: Publication Office, National Research Council, 1701 Massachusetts Avenue, Washington, D. C. BULLETIN OF THE NATIONAL RESEARCH COUNCIL Vol. 2, Part 2 APRIL, 1921 Number 10 REPORT ON PHOTO-ELECTRICITY Including Ionizing and Radiating Potentials and Related Effects BY ARTHUR LLEWELYN HUGHES Research Professor of Physics, Queen's University, Kingston, Canada PREFACE The writing of this report on the recent progress in photo- electricity was suggested by the Photo-Electric Committee of the Division of Physical Sciences of the National Research Council. The author wishes to express his appreciation of the valuable suggestions given by the other members of the Committee, viz., Professors R. A. Millikan, K. T. Compton, J. Kunz, and C. E.- Mendenhall, to whom the report was sent for comment. Thanks are also due to Professors Compton and Mendenhall for supplying periodicals and summaries of articles, not accessible to the author, and to the authorities of Queen's University for giving ample opportunity for the preparation of the report. A. LL. HUGHES QUEEN'S UNIVERSITY, KINGSTON, CANADA, January, 1921 CONTENTS I. Introduction 84 II. lonization of gases and vapors by light 86 III. The energy of photo-electrons 90 Method of measurement of velocities 92 Experimental results 93 The photo-electric threshold 96 Fluctuations in the photo-electric threshold. Dependence on the contact difference of potential 99 IV. Total photo-electric effect 104 Photo-electric current and light intensity 104 Photo-electric photometry 107 84 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES V. The photo-electric effect as a function of the frequency and state of polarization of the light no Richardson's statistical theory HO Experimental results 112 Normal and selective effects 115 VI. Photo-electric properties of thin films 119 VII. Photo-electric effects of non-metallic elements and in- organic compounds 122 VIII. Photo-electric effects of dyes, fluorescent and phos- phorescent substances 124 IX. Positive rays produced by light 125 X. Sources of light used in photo-electric experiments. ... 125 XI. Ionizing and radiating potentials Experimental methods 1 27 Collected results 135 Bohr's theory for hydrogen and helium 139 Spectral series notation ] 50 Single and multiple line spectra 153 Cumulative effects low voltage arcs 158 Compound gases 165 ABBREVIATIONS FOR JOURNALS P. R. Physical Review. N. A. S. P. National Academy of Sciences, Proceedings. A. P. J. Astrophysical Journal. J. O. S. A. Journal of the Optical Society of America. J. A. C. S. Journal of the American Chemical Society. P- M. Philosophical Magazine. Proceedings of the Royal Society of London. P. L. P. S. Proceedings of the London Physical Society. A. d. P. Annalen der Physik. P. Z. Physikalische Zeitschrift. V. d. D. P. G. Verhandlungen der Deutsche Physikalische Gesellschaft. Z. f. P. Zeitschrift fiir Physik. C. R. Comptes Rendus. CHAPTER I INTRODUCTION The principal monographs which have hitherto appeared on photo-electricity are the following: R. Ladenburg: In the Jahrbuch fiir Radioaktivitat , 1909. H. S. Allen: "Photo-Electricity" (Longmans), 1913. - LI. Hughes: "Photo-Electricity" (Cambridge University Press), 1914. . Pohl and P. Pringsheim: "Die Hchtelektrische Erscheinungen" (Vieweg) 1914 *. v^ Schweidler: "Photo-Elecktmitat" (in Graetz' "Handbuch der Elek'trizitat mid des Magnetismuss" Earth), 1914. \V. Hallwachs: "Die Uchtelektrizitat" (published privately). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 85 The present report gives an account of the progress which has been made since 1913. It may, therefore, be regarded -as supple- menting, and bringing up to date, Allen's "Photo-Electricity" and Hughes 's "Photo-Electricity." The division into chapters is that followed in the author's "Photo-Electricity." A chapter has been added on the new and most interesting group of investi- gations which may, for want of a shorter title, be designated as "Ionizing and Radiating Potentials and Related Effects. " This subject, which in many aspects is so closely related to photo- electricity that it is natural to consider it a part of photo-elec- tricity in an enlarged sense, has attracted an immense amount of attention from both theoretical and experimental physicists in recent years. Certain investigations, frequently designated as photo-electrical, because they deal with the change of resistance of materials like selenium under illumination, have not been touched upon. These investigations seem to have little in common with those dealt with in the report; it would be well if some such title as "photo-resistance effects" could be used for them to dis- tinguish them from the effects commonly classed as photo-electric effects. Perrin has recently published a remarkable paper 1 in which he suggests the important and possible decisive r61e played by radia- tions in determining chemical reactions, fluorescence and phos- phorescence, radioactivity, cosmical evolution and changes of state. According to this theory, the energetics of every change in the configuration of a structure from state A to state A' may be represented by equations of the type A -f- hv = A' + hv' , where v and v' are the radiation frequencies which change A to A' and A' into A, respectively, and h(v v'} is the heat of reac- tion. There is considerable direct support for this theory and it leads to a satisfactory explanation of certain facts of velocity of chemical reactions which were not explained on the older kinetic theory. Recent calculations by Langmuir 2 prove that Perrin's theory cannot be applied to many cases of molecular dissociation in the simple form proposed, but suggest some additional source of energy for which the light, or the collision, may act as a releasing "trigger." Yet there is, in the theory, a suggestion that photo- electric action, in an enlarged sense, may be one of the most fun- damental and important occurrences in nature. l A.d. P., n,5-108 (1919). 2 J. A. C. S., 42, 2190 (1920). 86 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES Throughout the report, it will be often necessary to make use of the quantum relation Ve = hv = he/*. It is generally con- venient to express V in volts and X in Angstrom units, in which case the numerical relation between them is given by : _ ((1547 X 10-" X 2.9986 X 10") x ^^ x 10 4.774 X II)- 10 X 12331 X CHAPTER II IONIZATION OF GASES AND VAPORS BY LIGHT Comparatively little has been done on this subject since 1913, although it was obvious that the subject was very incompletely covered. The technical difficulties are very formidable. In the first place, most gases, if they can be ionized at all, require light of such short wave-length to effect ionization, that very special arrangements have to be made to secure the right kind of light. Fluorite, the most transparent of known substances, begins to absorb strongly just in the region of wave-lengths where ioniza- tion of air begins. As it is desirable to have a window between the source of light and the ionization chamber, it becomes very difficult to find a window transparent to the active light. Another obstacle to securing reliable results is that in the region where the ionization of gases begins, the photo-electric effect of metals is enormous, and it becomes a problem how to disentangle the small ionization in the gas from the very large photo-electric effect of the electrodes (even when apparently well shielded) which are generally necessary to separate the ions in a gas. Ionization of Air. Hughes 1 had obtained results which indicated that air could only be ionized by wave-lengths shorter than about X 1350. From time to time, various investigators (e. g., Bloch 2 ) had obtained results which seemed to show that a weak ionization of air could be obtained by intense light from a mercury lamp (long wave-length limit probably X 1800). In view of the spurious effects of slight traces of impurities, dust particles, nuclei, etc., so clearly shown in the extensive investigations of Lenard and Ramsauer, 3 it may well be that the slight ionization observed with 1 Proc. Camb. Phil. Soc., 15, 483 (1910). C. R., 1912, 903, 1076. Ber. d. Heid. Akad., 1910-1911. REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 87 ultraviolet light from a mercury lamp is not due to a real ionization of the air molecules themselves, but to some obscure subsidiary effect. Ionization of Alkali Vapors. Theoretical considerations indicate that the vapors of the alkali metals should be ionized by light of much longer wave-length than is necessary to ionize air. Against this advantage is to be set the difficulties of working with the alkali metals, the necessity for working at high temperatures to secure an appreciable vapor pressure, and the fact that, if the electrodes become covered with a film of the metal, their photo- electric effect becomes enormous and may well mask any ionization effect. Gilbreath 1 obtained results which were interpreted as indicating a true ionization of potassium vapor at temperatures not over 65 C. The light used was that from an arc, or from a 500-watt lamp, filtered through glass (shortest wave-length transmitted, probably about X 3300). Kunz and Williams 2 recently found that caesium vapor was ionized by light of wave-length X 3190, and that wave-lengths longer than this were quite ineffective. Special care was taken to ensure absence of surface effects. Discussion. In view of the remarkable utility of the quantum theory in linking up facts in other regions of photo-electricity one naturally looks to it for confirmation of some of the results obtained in ionization of gases by light. Other things being equal, one may perhaps be justified in accepting, tentatively at any rate, those results which link up best with the quantum theory, rather than those which have no obvious connection. We know that gas molecules, struck by electrons, give out radiation when the energy of the electrons exceeds a certain critical value and become ionized when the energy exceeds another critical value. The potentials associated with these critical values are called radiating potentials and ionizing potentials, respectively. As will be shown in a later chapter, the radiating potential VR is related to the frequency VR of the radiation emitted, as follows : when h, e, m and V R are Planck's constant, the charge on and mass of an electron, and the velocity of the electron acquired from the fall of potential VR. The ionizing potential Vj is related to a 1 P. R., 10, 166 (1917). - To be published shortly. 88 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES certain convergence frequency vi in the spectrum of the gas by a similar equation, l Now, in view of the results in the X-ray region, we should rea- sonably expect a certain interchangeability in the effects of electron collision and radiation. We may perhaps expect a gas to be ionized whether it is struck by an electron moving with the critical velocity (corresponding to Vi) or illuminated by radiation of the corre- sponding wave-length, for the same quantum of energy is involved in both cases. 1 Thus, in mercury vapor, we know that direct ionization is produced by electrons of velocity V : = 10.4 volts, which corresponds to X 1188, and we should also expect that radia- tion of this wave-length would be effective also. This is too far in the ultraviolet to allow of easy verification. No direct ionization of a primary character, of mercury vapor, has been definitely proved for electrons of velocity 4.9 volts, the radiat- ing potential corresponding to the strong line X 2536 (though effects associated with "low voltage arcs," to be discussed later, show that a strong ionization of a secondary character can be obtained even here). In line with this is the fact that when mer- cury vapor is illuminated by light of wave-length X 2536, as in R. W. Wood's 2 experiments, no ionization is observed, although the illuminated gas re-emits the radiation strongly. Similarly, with the alkali metals, we find that no ionization is produced by electrons with velocities above the radiating potential but below the ionizing potential, and we should, therefore, expect that no ionization should be produced by light whose frequency is less than that of the convergence frequency, or limit, of the principal series. This result seems to be borne out by Kunz and Williams' recent experiment. The ionizing potential and radiating potential for caesium are 3.9 volts and 1.5 volts, respectively, which corre- spond to the limit (X 3191) and the first line doublet (X 8946 and X 8523), of the principal series of doublets in the spectrum of Cs. Kunz and Williams showed that caesium vapor only began to be ionized by light whose wave-length is not far from X 3191. Should 1 It should, however, be pointed out that ultraviolet light of frequency, say X 2536, incident upon most metals produces a copious flow of electrons. Electrons having the same quantum of energy 4.9 volts falling on the metal instead of the radiation, do not, as far as we know, produce any analogous effect, so that some caution perhaps is to be observed in carrying ideas from the X-ray region to the ultraviolet light region. 1 P. M., 23, 689 (1912). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 89 this view be correct, it is difficult to account for Steubing's 1 result that mercury vapor can be ionized by light from a mercury lamp, i. e., by light whose wave-length is longer than X 1800, and, there- fore, far longer than the limit X 1188 for mercury. It is certainly worth while investigating carefully where ionization of mercury vapor by ultraviolet light sets in, as Steubing's criterion for dis- tinguishing between surface effects and true ionization of the vapor is far from conclusive. Indirect evidence that there is no ionization by light of wave- length of the frequency corresponding to the radiating potential, is obtained from the fact that when mercury vapor, or any mon- atomic gas, is subjected to bombardment by electrons of energy just sufficient to call out the radiation, no ionization can be de- tected, although molecules are being illuminated by radiation from their neighbors. (However, one should bear in mind some recent work of Compton 2 on helium, w^hich indicates that a gas under the influence of its own radiation, when sufficiently intense, is more readily ionized by electron impact than in its normal state.) Hughes's results on air indicated that air is ionized by light of wave-lengths shorter than X 1350, corresponding to about 9 volts. Now 9 volts is not far from the values 7.5 volts for nitrogen and 9.5 volts for oxygen, the values formerly associated with the ion- izing potentials. Later results have shown that 7.5 volts for nitro- gen, however, is a radiating potential rather than the ionizing potential, which is about 18 volts. A similar test has not been made for oxygen, but the analogy of some recent experiments on metallic vapors and on hydrogen and helium would imply that the 9.5 volts for oxygen was the radiating potential and not the ionizing potential. 3 Ionization of air by light of wave-length X 1350 apparently then contradicts the result tentatively arrived at, that the wave-length which ionizes air corresponds to the ion- izing potential and not the radiating potential. It is just possible 1 P. Z., 10, 787 (1909). 2 P. M., 40, 553 (1920). 3 Mohler and Foote (Jour. Opt. Soc. Amer., 4, 49 (1920)) in some recent experiments found that the radiating and ionizing potentials for Nj are 8.18 and 16.9 volts, for O 7.91 and 15.5 volts and for H 2 10.4 and 13.3 volts, thus apparently following the rule observed for monatomic gases that the lower critical potential is always a radiating potential. However, Franck, Knipping and Kruger, and Compton find that the lower critical potential for H 2 is definitely an ionizing potential. In view of this, one may perhaps require further proof that the lower critical potential for diatomic gases is always a radiating potential as appears to be the case for monatomic gases. 90 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES that when a molecule is already in an abnormal state due to the absorption of radiant energy of wave-length correponding to the radiating potential, further radiant energy of the same wave-length may cause it to be ionized. (This speculation, and it must be recognized merely as such, implies, in other words, that intense radiation may produce ionization when feeble radiation would not.) The recent work of Compton, just referred to, is suggestive in this connection. It is recognized, of course, that the effect of radiation impinging on an atom is not exactly analogous to the effect of a collision with an electron, for the absorption of radiation is probably a con- tinuous process, while the absorption of energy through an elec- tron collision is essentially discontinuous. Again, from the experi- ments of McLennan and others, there is only strong absorption at the isolated wave-lengths of the principal series, whereas elec- trons moving with any velocity between the radiating potential and the ionizing potential give rise to radiation. The whole subject is full of obscurities and suffers from lack of experimental data. A systematic investigation, with improved methods, of the ionization of gases and vapors by light, should repay investigation. It is of considerable interest to ascertain definitely whether the wave-length of the light which ionizes a gas is related to the ionizing potential by the quantum relation. CHAPTER III THE ENERGY OF PHOTO-ELECTRONS Exact measurements of the velocities of photo-electrons are 'of the utmost importance in furnishing evidence for testing the theories of the photo-electric effect. The photo-electric effect was among the first effects to which the quantum theory was applied. In 1905, Einstein proposed a law governing the relation between the maximum emission energy of the photo-electron and the frequency of the light causing its emission. Each advance in experimental accuracy has given a more accurate verification of its correctness. According to Einstein, the energy of a photo-electron ejected from a substance by light of frequency v is equal to the energy associated with a quantum of light of that frequency, viz., hv, less the loss of energy in getting out of the substance. The relation may be written Ve = hv V e REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 01 where Ve is the energy of the electron after leaving the surface (V being the voltage necessary to stop it), and V e is the energy lost in getting out of the substance. Some implications of this equation are as follows: (1) The energy of a photo-electron is a linear functon of the frequency of the light. (2) The slope of the line connecting the energy and the frequency is the well-known quantum constant "h." (3) The long wave-length limit of the photo-electric effect, or the "photo-electric threshold" (to introduce a convenient term), is that for which the electron escapes with zero energy. Hence, the frequency v of the photo-electric threshold is given by so that, once we know the threshold for any substance, we know the work necessary to pull a photo-electron out of the substance. Up to the date at which this report begins, these implications had been confirmed through the work of Richardson and Compton Case Lighf Earth FIG. 1. and of Hughes. In view of the importance of testing the implica- tions to as high a degree of accuracy as possible, further work was necessary as the researches referred to yielded a value of "h" from 10 per cent to 20 per cent smaller than the accepted value, and the limited range of wave-lengths naturally restricted the precision with which the other relations could be tested. (When these results were obtained, there was a certain amount of speculation as to whether one could expect complete agreement between the 92 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES photo-electric value and the theoretical value of "h," as it was conceivable that the slope of the line connecting energy of photo- electrons and frequency might in some way be slightly affected by the nature of the substance or by other conditions.) METHOD OF MEASUREMENT OF VELOCITIES The most common way of measuring the velocities of photo- electrons is to measure the photo-electric current with various Currenr r\cTarcf/f} = hv hv . (2) If now a surface, B, be substituted for A and illuminated by the same light, we have a similar equation (V + K> = hv hv' (3) where V, K', and v ' refer to the stopping potential, the contact potential with respect to the case, and the photo-electric threshold of the surface B. Subtracting (2) from (3) we get (K' - K) = h - (, - ;> - (V - V) (4) 1. P. R., 7, 18 (1916). /Wa., 7, 18,355 (1916). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 97 /contact potential \ _ ^(frequency difference) ( difference between) \betweenAandBj e\ of thresholds / \ stopping potentials /' This equation is one which can be checked up by experiment. Before doing so, it is well to look at it from another angle. Let us regard p in equation (1) as being made up of two parts, pi, the energy lost when the electron leaves its parent system and becomes a "free" electron, and pz, the energy lost by the free elec- tron in passing through the surface, pt may be conveniently written as V s e, where V s is the intrinsic potential of the metal. (V s e is probably identical with the energy lost by an electron in passing through the surface in the thermionic effect.) Equations (2) and (3) now become (V + K> = hv V s e p l (V + K> = hv V' s e p[ which on subtracting yield (V V) + (K' K) = (V s Vs) Pl ~ pl . As the difference between the intrinsic potentials (V s V s ') is nothing other than the contact difference of potential (K' K), we get (V - v) = *L=A (5) If this view of the mechanism of the photo-electric effect be correct, then a careful comparison of equation (4) with the experimental data will tell us whether (V - V) for two metals is finite or zero, and then by (5) whether any difference exists in the pi's, i. e., in the work necessary to extract the photo-electron from the parent system and make it a "free" electron. The experimental evidence obtained by all observers is now in agreement that when .any two different metals are brought into rapid succession before a Faraday cylinder into which they dis- charge photo-electrons, the stopping potential for a given value of the wave-length incident upon them is exactly the same. Hence, the difference (V V) in each case is shown by experiment to be zero, and equation (4) degenerates into the following equation:* * The stopping potential for a given wave-length is, however, often observed to change with time as in Millikan's experiments, so that if much time elapses between measure- ments of the stopping potentials for two different metals, the experimental equation will have the form of (4) and the observed values of (V V) will measure a contact 1, M. F. which is due to a transient surface charge rather than to an intrinsic property 98 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES K'-K = -( r.0- (6) e As Millikan points out 1 the interpretation of equation (6) can only be that the energy after escape of the electron from the atom is always equal to hv, the absorbed energy being, contrary to the physical theory which guided Einstein, greater than hv, or else that the same energy is required to detach an electron from all atoms (an impossible conclusion) or else that the photo-electrons are from the beginning the free electrons rather than the elec- tronic constituents of the atoms. The first of these conclusions is exactly the one to which Barkla was led by his work on X-rays. On the other hand, the last conclusion is very difficult to reconcile with the photo-electric properties of insulators, which have no free electrons, and also with the independence of photo-electric currents of the temperature. Whichever of the foregoing interpretations be made as to equa- tion (6), it requires that the whole of the energy hv of the electron be expended in passing through the surface and hence that hv is a measure of V s , the intrinsic potential of the metal. Now, V s is given by thermionic experiments, and V may be obtained from the photo-electric thresholds (V rent is proportioiiaj to the intensity of the light^ Yet some careful investigations indicated a departure from the proportionality FIG. 6a. FIG. Qb. law. In view of important practical applications of the law to the measurement of light intensity, many researches have been carried out to test its validity. Investigations have been made chiefly on the alkali metals inasmuch as these are sensitive to visible light. The photo-electric cell consists essentially of a surface to be illuminated, forming one electrode, and another electrode, whereby an accelerating potential can be applied to drive the photo-electrons across the cell. Two extreme types are shown in the diagrams (figs. 6a, 66). The pure photo-electric effect is that obtained in the absence of any gas. However, to use the magnification (which may conveniently amount to a 100-fold or more) of the photo-electric current by ioniza- tion by collision, it is customary to fill the cell with gas at a pres- REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 105 sure of about 1 mm. To avoid reactions between the sensitive surface and the gas, the gas is usually He, A, or Ne. By passing a glow discharge through the cell when it contains hydrogen, a hydride of the alkali metal is formed which is much more sensitive even than the pure metal. In such cells it is the practice to replace the hydrogen after the new sensitive surface has been formed by an inert gas. Now, in testing the proportionality law, the fundamental thing to be investigated is the ratio of the number of photo-electrons liberated at the surface to the intensity of the light. In the absence of any gas, it is evident that saturation must be obtained, while, in the presence of a gas, the multiplying factor due to ionization by collision must not vary. An extensive series of experiments was made by Ives 1 and also by him in conjunction with Dushman and Karrer. 2 In the earlier of these papers, results were given indicating that the ratio of the current to the intensity of the light was not constant, but was a complicated function of the intensity, applied potential, illuminated surface, and gas. The cells usually contained potassium treated by a glow discharge in hydrogen, and filled with an inert gas at a low pressure to magnify the effect. Finally, the source of the departure from the linear relation was traced down to the presence of insulating surfaces in the cells as usually constructed. These surfaces generally acquire charges from photo-electrons, or ions, hitting them, and these charged surfaces, in turn, distort the elec- tric field which affects the flow of electricity across the cell. In view of the fact that these insulating surfaces (which may not be perfect insulators) may acquire different surface charges under different conditions of illumination, field intensity, etc., it is little wonder that the effect could be otherwise than an incalculable modification of the strict proportionality. The final result of this series of investigations is that a strictly linear relation can be obtained between illumination and photo-electric current, pro- vided close attention is paid to the design of the cell. They point out that the design of the cell due to Hughes, 3 in which the inside of a bulb is almost completely covered with distilled sodium and the other electrode is a brass rod (as shown in fig. 66 above), so that there are no exposed insulating surfaces anywhere near 1 A. P. J., 39, 428 (1914); 40, 182 (1914); 46, 241 (1917). 2 Ibid., 43, 9 (1916). ' P. M., 35, 679 (1913). 106 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES where the current flows, satisfies the necessary conditions. Ives found it impossible to make cells identical in properties. The curve of sensitivity with wave-lengths varied considerably from one cell to another prepared under identical conditions. Kunz 1 found that when the potential, pressure and light intensity were so adjusted that a visible glow was to be seen in the photo-electric cell, the current was not proportional to the light intensity. Evi- dently, the magnification due to ionization by collision is no longer a constant under these conditions. Elster and Geitel 2 discussed the various ways whereby faulty design of a photo-electric cell causes an apparent departure from the proportionality law. When the possibility of charges on the glass wall was avoided, they found strict proportionality from 30,000 lux 0/3 of sunlight) down to 6 X 10 ~ 4 lux. (A lux = 1 candle metre.) The lowest illumination with a potential of 200 volts across the cell was 2.4 X 10 ~ 6 lux and gave a current of 1.8 X 10 ~ 12 amp. In a second paper 3 they found an apparent departure from the proportionality law with strong light, but this was traced down to a discontinuity in the magnification of the photo-electric current by ionization by collision. In this region, they state that if the cell is used completely evacuated, the strict proportionality is observed. In a later paper 4 they found that the proportionality law held down to light as feeble as 3 X 10 ~ 9 erg per sq. cm. per sec. for blue light and 2 X 10 ~ 7 erg. per sq. cm. per sec. for orange light, the former value being below the "thres- hold" value for the human eye. Kunz and Stebbins 5 found that the proportionality law held exactly for a rubidium cell containing neon. Kunz 6 found a slight departure from the proportionality law with high intensities. With the older type of spherical cell with illuminated central electrode, there was a marked departure from the proportionality law, while a cell in which the two electrodes were parallel plates gave the best agreement with the law. Proportionality was found to hold over a range from 1 to 1300. A consideration of these results, taken altogether, seems to point beyond all doubt to the result that there is a strict propor- P. R., 13, 310 (1919). P. Z., 14, 741 (1913). Ibid.. 15, 610 (1914). P. Z., 17, 268 (1916). P. R., 7, 62 (1916). A. P. 7., 45, 69 (1917); P. R., 9, 175(1917). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 107 tionality between intensity of illumination and the number of photo-electrons emitted, and that this holds over a wide range extending from illumination too feeble to be visible, to illumination comparable with sunlight. Departures from the law appear to be due to faulty design in the cell which permits spurious effects, or else to the alteration in the multiplying factor of ionization by collision under extreme conditions. PHOTO-ELECTRIC PHOTOMETRY The numerous articles on the applications of the photo-electric cell to the measurement of light intensities show how valuable such cells have been found for this purpose. They are of particular value under conditions where thermopiles are unsatisfactory and difficult to work with, as, for example, with weak visible light or in the ultraviolet. For satisfactory work as measurers of light intensity, it is, of course, necessary to ensure that the design of the cell is such that there is no departure from strict proportionality between the light intensity and current. For applications of the kind contemplated here, it is well to arrange for as high a degree of sensitivity as one can get. First, arrange to use as much of the incident light as possible. When light falls upon a clean sodium surface, probably over 99 per cent is reflected and so, in a cell of the first type in fig. 6a, is lost so far as the photo-electric effect is concerned. By means of a design of cell suggested by "black body enclosures" as used in the study of heat radiation, such as the cell described by Hughes 1 (fig. 66) it is possible to trap almost all the light and so make it effective photo-electrically. Such a cell is prepared by distilling an alkali metal on to the inside of a suitably shaped bulb, and arranging to leave a small aperture just big enough for the light to enter. Light which enters is re- flected to and fro inside and each time contributes to the photo- electric effect. To increase the sensitivity further, it is customary to pass a glow discharge through hydrogen in the cell to convert the metal into the more sensitive hydride. Finally, one makes use of the magnification of the original photo-electric current from the surface, by filling the cell with an inert gas, such as argon, and choosing the most favorable accelerating potential to give the most convenient magnification. One disadvantage of photo-electric cells is that no two cells have exactly the same sensitivity for different wave-lengths. So, 1 P. M,, 35,679 (1913) 108 REPORT ON PHOTO-ELECTPICITY: A. LL. HUGHES when one uses photo-electric cells to compare energies in different parts of the spectrum, it is necessary to standardize each cell, either by a thermopile (using intense enough light, of course, to affect it) or by using a source of light in which the distribution of energy is known. For studying variations in monochromatic light, no such calibration is necessary. As we have seen, a cell of the type designed by Elster and Geitel is more sensitive than the human eye. Such a cell so arranged that its effect is strongly magnified by the use of three electrode valves, should open up possibilities of a big field of quantitative measurements in regions hitherto unexplored (e. g., weak phos- phorescence, scattering of light, resonance emission, etc.). A method of magnifying the effect of a photo-electric cell by a three electrode valve has been described by Kunz, 1 and by Pike. 2 Pike obtained amplifications as large as 15,000. Meyer, Rosen- berg and Tank 3 obtained amplifications up to about 15,000. In later experiments values up to 125,000 were obtained. They found that the amplification was quite constant for small photo- electric currents. The photo-electric cell is well adapted for measuring stellar magnitudes, fluctuations of star light, and corona intensities during eclipses. It will be remembered that unless spectral resolution be resorted to, the sensitivities of photo-electric cells differ from each other and from that of the human eye. Just as the eye is most sensitive to yellow-green light, so cells containing Cs, Rb, K, Na, Cd and Zn have their own proper maxima running from yellow far into the ultraviolet, in the order indicated. Miss Seiler 4 found that the maxima for photo-electric cells were at X 4050, X 4200, X 4410, X 4730, and X 5390 when the surfaces were Li, Na, K, Rb, and Cs. In every case, sensitizing the surface by passing a glow discharge through hydrogen over the alkali metal caused a shift in the maximum towards the longer wave-lengths. Elster and Geitel 5 describe photometers with Cd or Zn as the sensitive surface, specially suitable for measurements on the ultra- violet radiation of the sun and stars. Guthnick 6 used a photo- 1 P. R., 10, 205 (1917). 2 Ibid., 13, 102 (1919). Archeves des Sciences, 2, 260 (1920). * P. R., 15, 550 (1920). 5 P. Z., 15, 1 (1914); 16, 405 (1915). Nature, 103, 53 (1919); Astr., Nachr., No 4976. REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 109 electric cell to determine the amount of light reflected by the planets from the sun at different parts of their orbit. A. F. and F. A. Lindemann 1 discuss the various applications of photo-electric cells to astronomy. Among other applications may be mentioned the study of the light reflected by the planets for evidence as to fluctuations in the light of the sun. They also point out the use- fulness of photo-electric cells in the study of the light from diffused luminous areas such as nebulae and comets. Kunz and Stebbins 2 gave details of the use of photo-electric cells for studying the light from the corona during the eclipse of June 8, 1918. Stebbins and Dershem 3 used potassium cells for studying the fluctuations of the light from Nova Aquilae, No. 3. Hulburt 4 used a photo-electric cell successfully for investigating the reflecting power of metals in the ultraviolet region. Nathan- son 5 investigated the reflecting power of the alkali metals by a photo-electric method. A recent paper by Elster and Geitel 6 deals with an apparent continuation of the photo-electric effect for potassium for a short time after the light is cut off. This was found to be due to a weak phosphorescence in the photo-electric cell. This effect may intro- duce an error in photo-electric photometry when light intensities are changing rapidly. It is convenient here to give a short account of the discovery of a new and exceedingly interesting kind of photo-electric effect, which was discovered by Mr. Shenstone in the Physics Laboratory of Princeton University. The photo-electric current from a bismuth plate was found to depend very much on the magnitude of a current passing through the plate. A thin square plate of bismuth was mounted in a vacuum so that an electric current could be passed across it, i. e., from one side of the square to the opposite side. The photo-electric current from the bismuth, illuminated by light from a mercury arc, was found to be inde- pendent of the current through the bismuth, as long as it was below 1.1 amperes. Any increase beyond this up to 3 amperes 1 Roy. Astr. Soc. Monthly Notices, 79, 343 (1919). 2 A. P. y.,49, 137 (1919). 3 Ibid., 49, 343 (1919). 4 Ibid., 41, 400 (1915); 46, I (1917). 5 Ibid., 44, 137 (1916). 6 P. Z., 21, 361 (1920). 110 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES or more caused an increase in the photo-electric current, a change amounting to as much as 60 per cent being observed as the curr ent was altered from 1 to 3 amperes. The effects showed considerable time lag, thus the equilibrium value of the photo-electric current was not obtained until more than an hour had elapsed after the current through the bismuth had been maintained at a steady value. Control experiments showed that the heat effect and the magnetic effect of the current were not responsible for the increase in the photo-electric current. (The effects persisted after the cur- rent through the bismuth was stopped, fatigue, however, setting in and reducing it.) A similar, but much smaller, effect was obtained with zinc. CHAPTER V THE PHOTO-ELECTRIC EFFECT AS A FUNCTION OF THE FREQUENCY AND STATE OF POLARIZATION OF THE LIGHT RICHARDSON'S STATISTICAL THEORY A number of investigations on the photo-electric effect as a function of the frequency of the light have been carried out. The only theoretical expressions for this function are those due to O. W. Richardson. 1 They are deduced along the following lines. Consider a cavity in a piece of matter, containing an atmosphere of electrons in equilibrium, so that just as many enter the matter through the surface as leave it. Richardson finds, by applying the usual methods of the kinetic theory of gases, that the number of electrons impinging on a unit area of the surface from the atmos- phere is r (1) where A is a constant, T the temperature, R the gas constant for a single molecule, and w the work done by the electron against the forces tending to retain it in the matter. The number of electrons leaving the surface, on the other hand, is assumed to be determined by the total radiation (acting photo-electrically) which is in equilibrium with the matter at that temperature and, therefore, traversing it in all directions. The energy density of the radiation between the frequencies v and v + dv is > P. M., 23, 594 (1912); 24, 574 (1912); 27, 476 (1914). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 111 E(v, T)o (v)hv 3 e RT dv = AT 2 RT (3) or c . 5 f tf W _ 4 - (4) *-! according to whether Wien's or Planck's expression is adopted. A solution of (3) is F V = , when w v 40 20 0. / 1 / \ / \ 7 v 80 90 100 no lao /jo 140 /so Frequency FIG. 7. tropic radiation. Also, the functions given above are solutions of the equations, but not necessarily complete solutions. The solutions imply that the function has the same shape for all sub- stances, the nature of the substance only enters by fixing v , the frequency of the photo-electric threshold. Compton and Richardson 1 investigated the photo-electric sen- sitivity of Pt, Al, Na and Cs with monochromatic light of different frequencies to test the expression given in (5). The results for Al and Na are shown in the accompanying figures. For both Al and Na, there is a definite maximum sensitivity which, however, is much more sharply defined than the theoretical maximum. Its position is in accordance with the predictions of the theory. 1 P. M., 26, 549 (1913). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 113 It will be noticed that Na has two well defined maxima. The shape of the experimental curves differs considerably from that given by the theory. The available ultraviolet region is so limited that no second maximum could be looked for with Al, nor could the curve be carried as far as the first maximum for Pt. In a later paper, Richardson and Rogers 1 reduce these experimental results to absolute values which are expressed in coulombs per calorie. ^ Richardson 2 went into the question of whether it was possible to explain thermionic emission of electrons by a photo-electric emission due to the radiation from the hot body illuminating 80 V 50 60 70 80 90 100 110 /^0 /30 14-0 150 Frequency FIG. 8. itself (an "autophoto-electric" effect). He found, on thermodynam- ical assumptions, that the photo-electric current from a sub- stance when illuminated by full radiation characteristic of a tem- perature, T, should have the form This equation is identical with that for the ordinary thermionic emission. W. Wilson 3 found that the photo-electric current from sodium potassium alloy obeyed this law as the temperature T 1 P. M., 29, 618 (1915). 2 "Emission of Electricity from Hot Bodies," p. 101. 3 P. R. S., 93, 359 (1917). 1L4 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES of the source of light was varied. (The source was arranged so as to give approximately black body radiation.) Richardson concludes, however, that though the photo-electric current de- pends in the same way on the temperature as the thermionic emission, the photo-electric current forms at the best but a very insignificant fraction of the thermionic current. Langmuir 1 men- tions that the magnitude of the photo-electric current produced by radiation from an incandescent filament was only one-mil- lionth part of the thermionic current from the filament. A de- posit of tungsten was formed on the inside of a bulb by evaporation from a tungsten filament at the center, and a photo-electric current a+oo from the deposit was obtained on making the filament incandes- cent. Souder 2 carried out a number of investigations on K, Na and Li. In Compton and Richardson's experiments, the Na surface was prepared by distillation, while in his experiments the surface was produced by cutting in vacuo. The results are shown in fig. 9. It will be noticed that there is no evidence of a second maximum in Souder's experiments, as would be expected from Compton and Richardson's experiments. In any attempt to build up a theoretical formula for the photo- electric sensitivity of a substance, it will be seen that any effort to check it accurately by experiment will be subject to serious difficulties. The number of electrons which emerge from a surface in any photo-electric experiment will be a complicated function *J.A. C. S., 42, 2190(1920). 1 P. R.. 8. 310 U816). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 115 of the distance to which the light penetrates into the surface and of the absorption coefficient of the photo-electrons by the sub- stance. Hence, the photo-electric sensitivity, as obtained in the experiments quoted, is subject to unknown corrections before what may be called the true photo-electric sensitivity can be ob- tained. In this connection, reference may be made to a paper by O. W. Richardson. 1 NORMAL AND SELECTIVE EFFECTS One of the most puzzling phenomena in the photo-electric effect is the so-called "selective photo-electric effect," which appears in some cases. According to the careful experiments of PohP the photo-electric current per unit of energy absorbed from the light is, in general, independent of the angle of incidence and also of the state of polarization of the incident beam. In certain cases (the alkali metals), however, there is a remarkable departure from this rule, the photo-electric current being many times stronger when the electric vector in the light beam has a component per- pendicular to the surface than when it is wholly parallel to the surface. This effect, the "selec- tive" effect, is restricted to a range / of wave-lengths characteristic of the metal, while the "normal" effect has no such restriction. A typical diagram of the effect is shown in fig. 10, where E|| and E_l_ denote the relation of the electric vector to the plane of in- cidence. Pohl and Pringsheim have obtained a large mass of ex- perimental data as to these effects, which is summarized in the writer's "Photo-Electricity." No satisfactory explanation has yet been given of the phenomena. With unpolarized light, the two effects are superposed showing a well-marked maximum coinciding with the maximum of the selective effect. In the earlier investigations, it was generally assumed that a pronounced maximum in the photo-electric current curve, when the substance was illuminated by light of different wave-lengths, indicated a selective effect. While this is no doubt 1 P. M., 31, 149 (1916). * V. d. D. P. G., 10, 339, 609, 715 (1909). 116 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES true in the case of the alkali metals, many recent investigations show a maximum in the photo-electric current curves plotted against wave-lengths when a selective effect does not exist. The theoretical investigations of Richardson, already referred to, point to a maximum photo-electric current at a certain wave- length, without any reference to the state of polarization of the light, or to the inclination of the beam. It is, therefore, advisable to restrict the term "selective effect" to the narrow limits originally suggested by Pohl and Pringsheim. It should be kept for that occasion when, reckoned for equal amounts of absorbed light, the photo-electric sensitivity is abnormally large when the electric vector in the light beam has a component perpendicular to the surface, as compared to the case when it is entirely parallel to it. A criterion equivalent to this, which in some cases may be more convenient to employ, is an accentuation of the maximum as the angle of incidence is increased. An illustration of this may be taken from Pohl and Pringsheim's work. Na-K alloy and Ca both show a maximum (which is, however, much less pronounced in the case of Ca) in the photo-electric current curves when unpolarized light is used. However, as the angle of incidence is increased, the maximum becomes more and more pronounced in the case of Na-K alloy, but in the case of Ca, it becomes less and finally vanishes. A direct test with polarized light shows that Ca does not possess a selective effect, while Na-K alloy does. It is evident, then, that something more than the mere presence of a maximum in the photo-electric current curves when plotted against the wave-length, is necessary to determine whether a real selective effect is present. It may be remarked that Pohl and Pringsheim assumed that Li had a selective effect because of the presence of a maximum at X 2800, when illuminated by unpolarized light, on the analogy of the maxima obtained for the other alkali metals, for most of which the existence of the selective effect had been directly demonstrated with polarized light. Had they worked on Li, after discovering that the maximum in the case of Ca did not necessarily imply a selective effect, they would no doubt have made further tests using polarized light. Millikan and Souder 1 investigated the photo-electric current from a sodium surface as a function of the wave-length of light using perpendicular incidence. They found a maximum sen- 1 N. A. 5. P., 2, 19 (1916); P. R., 8, 310 (1916). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 117 sitivity at the same wave-length as that at which Pohl and Pring- sheim located the maximum of the selective effect. They also found that the magnitude of the maximum depended very much upon the age of the surface, indeed, for very new surfaces the maximum could scarcely be detected. They are inclined to regard the selective effect as merely the normal effect in the neighborhood of an absorption band, i. e., where one particular frequency pre- dominates. The maximum velocities of the electrons in the region where the selective effect is most marked fall in with Einstein's quantum relation exactly as they do in regions where only the normal effect exists, a result verified by Millikan's accurate experiments on Na and Li in the determination of "h." Hughes 1 found that there was a small difference in the distribu- tion of the electrons emitted in the selective effect as compared with that of those emitted in the normal effect. This difference, however, was quite small compared with the difference between the total currents for the two effects. The difference in the dis- tribution could be attributed to a difference in the direction dis- tribution, or to a difference in the velocity distribution of the photo- electrons, that is, the effects could be explained by assuming that in the selective effect the photo-electrons were somewhat more crowded towards the perpendicular to the surface, or that they were, on the whole, slower than in the normal effect. In a later paper 2 it was shown that a difference in the direction distribution existed, the photo-electrons in the selective effect being emitted, on the whole, in directions nearer to the perpendicular to the surface than in the normal effect. If, over the region in which the selective effect existed, one found that the light polarized in the E|| plane were absorbed in a much thinner layer of the surface than light polarized in the E_]_ plane, one would have a natural explanation for the existence of the selective effect. According to Pohl and Pringsheim 3 the position of the selective effect is that in which the reflecting power is exceptionally high, and high reflecting power, on optical theory, goes with rapid absorption. They did not investigate whether the reflecting power in the region of the selective effect differed markedly for beams polarized in the two principal planes. Re- ' P. M., 31, 100 (1916). 2 P. R., 10, 490 (1917). 3 V. d. D. P. G., 15, 173 (1913). 118 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES cently, some experiments on this point have been carried on by Miss Mabel Frehafer, 1 who investigated the reflection and ab- sorption of light by K and Na. The ratios of the reflecting powers of Na and K surfaces for light polarized in the two principal planes are shown in fig. 11. It will be seen that for both Na and K, a remarkable variation in the ratio takes place in the ultraviolet. For Na, the minimum in the curves is approximately near the position of the selective maximum. Any association between the two is made doubtful, however, by the fact that no such asso- ciation appears for K. Experiments were carried out on the REFZ.ECT/O/V OF POLARIZIO LIGHT LSOO 3000 3600 4-000 4500 SOOO FIG. 11. transparency of thin films of K and Na, but no specially marked effects were noted in the selective region . Should further experiments show definitely that no explanation of the selective effect in terms of unequal absorption of the beams polarized in the two principal planes is possible, it will be necessary to look elsewhere for an explanation. Possibly there are systems in the alkali metals so orientated that they are more easily broken up by light which has a component of electric force perpendicular to the surface than when the electric force is entirely parallel to it. So far as energy relations go, the selective effect appears to be governed by the quantum relation, just as is the normal effect. 1 P. R. t 15, 110 (1920). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 119 CHAPTER VI PHOTO-ELECTRIC PROPERTIES OF THIN FILMS Certain investigations recorded in the writer's "Photo-Electricity" implied that the photo-electrons from thin films had necessarily high emission velocities. Stuhlmann and Compton 1 found that there was no departure from Einstein's law, provided adequate care was taken to prevent the formation of charged layers, ap- parently due to thin films of oil or grease forming on the surfaces investigated. They also found that the photo-electric effect of sputtered platinum was greater than that of ordinary platinum. Stuhlmann 2 took up the question of the maximum velocities of photo-electrons emitted from thin films of Pt on quartz plates. He confirmed Robinson's result that the maximum emission energy of the photo-electrons was about 40 per cent greater when they left the emergence side than when they left the incidence side. At the time these experiments were carried out, the investigations of the emission energies of photo-electrons from ordinary surfaces (i. e., from the incidence side) all led to values of "h" about 10 per cent to 20 per cent too low, and it was suggested therefore, as a result of these velocity measurements on thin films, that the correct value for "h" might be obtained, provided that the velocities were measured for the emergence side. In view of Millikan's proof that "h" can be obtained accurately from experiments on the velocities of photo-electrons from the incidence side, this explana- tion falls to the ground, and indeed renders it exceedingly doubtful whether the photo-electrons can possibly have a greater maximum energy on the emergence side. The differences in the velocities are probably spurious, though it is difficult to say where the experi- mental arrangements are faulty. Robinson 3 measured the photo-electric effect per unit light energy absorbed for thin films as a function of the thickness. The curves had very sharply marked maxima at thicknesses of about 10 ~ 7 cm. In 1919, two independent investigations of a more elaborate type than any of the previous investigations on thin films were published. The results differ in several respects. Stuhlmann 4 1 P. R., 2, 199 (1913); 2, 327 (1913). 2 Ibid., 3, 195 (1914). P. M., 32, 421 (1916). P. R., 13, 109 (1919). 120 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES produced his thin films by a novel method. They were obtained by the evaporation in vacuo from a wire, made white-hot, and placed to one side of a plate of quartz. Thus a deposit of decreasing thickness was obtained on the quartz and the photo-electric effect of any thickness could be studied by directing the light on to the proper part of the deposit. The metals investigated were Ft and Ag. The results for Ft are reproduced in the diagram. It will be seen that the thickness for which the photo-electric current is a maximum decreases with the frequency. Since all the curves cut the abscissa at the origin at a finite angle, the tangent here may be taken to measure the photo-electric effect per unit thickness unaffected by any question as to absorption of light or of electrons in the film. Stuhlmann concludes that for very small thick- nesses (7 X 10~ 7 cm.) the electrons pass through the film colliding according to Rutherford's hypothesis of "single scattering" and little or no energy is lost by the electron. For thicker films, ordinary "compound scatter- ing" conies into play, in which the electrons lose energy, and for still thicker films true absorption of photo-electrons ^8 Thickness FIG. 12. takes place. By comparison of the results for Ft and for Ag, it was concluded that the stopping power of a metal for photo-elec- trons increases as the energy of the electron increases up to a cer- tain limiting value, and is greater, the heavier the atom. Compton and Ross 1 investigated Ft and Au films produced by sputtering. It may be useful to record that three entirely different methods were used for measuring the thicknesses of the films and gave results in very good agreement. The form of the function giving the relation between the photo-electric effect and the thickness was deduced from three different sets of assumptions. 1 P. R., 13, 374 (1919). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 121 These were as follows: (1) That the number of the photo-electrons which retain ability to escape falls off exponentially with the dis- tance moved through perpendicularly to the surface; (2) that the number which retain ability to escape falls off exponentially with the distance moved through in any direction; and (3) that an electron loses energy in proportion to the distance moved through the metal. The thickness of the film giving the maximum photo- electric effect was determined theoretically for each case and com- pared with the experimental results. The results were found to -fit either hypothesis (1) or hypothesis (2), but would not fit hypothesis (3) at all. (2) is probably more plausible than (1), 90[ I I 5456 7_$. 9 10 II Thickness FIG. 13. as the electrons are as likely to start off in any one direction as in any other. One set of curves is reproduced here. The results differ radically from those of Stuhlmann. In the first place, they possess two maxima. The second maximum (the one for the greater thickness) was found to disappear in the course of time, and seems to be associated with the unstable form of newly sput- tered Pt. The single maximum in Stuhlmann's work would imply that the film produced by evaporation did not pass through the temporary state of instability. The maxima in Compton and Ross's work occur at about 4 X 10 ~ 7 cm. for X 2536 and about 2.5 X 10~ 7 cm. for X 2100 and X 2225, while in Stuhlmann's work the maxima are at about 15 X 10~ 7 cm. for X 2536, and about 12 X 10~ 7 cm. for X 2260. Since the experiments are in agree- 122 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES ment with the view that the ability to escape decreases exponentially with the distance travelled, the average distance through which an electron can travel (a sort of mean free path) may be calculated. The values are 2.67 X 10 ~ 7 for Pt, and 5.0 X 10 ~ 7 for Au, which are greater than, but of the same order of magnitude as, the dis- tance between the atomic centers in these metals. The most important result of this investigation perhaps is that the ability to escape is the same for a fast photo-electron (i. e., one produced by the shortest wave-length) as for a slow photo-electron. This can only be accounted for on the view that a photo-electron loses its energy completely at a single collision, and not gradually during a succession of collisions, for in the latter case it is obvious that the faster electron would travel further. A similar conclusion may be drawn from Stuhlmann's work, although in his experiments the "mean free path" of an electron in the metal appears to be greater than indicated by the work of Compton and Ross. CHAPTER VII PHOTO-ELECTRIC EFFECTS OF NON-METALLIC ELEMENTS AND INORGANIC COMPOUNDS Dima 1 investigated the photo-electric effect of numerous in- organic compounds. These were in the form of capsules of com- pressed powder. The light used was that from a quartz mercury lamp, unresolved. As the substances showed fatigue in widely varying amounts, the initial values of the photo-electric effect were taken so as to make a fair comparison. The values of the initial photo-electric current obtained from the various compounds under similar conditions are as follows : HgI 2 10 Hgl 112 CuO 4800 Cu,O 14400 HgCU 2 HgCl 12 CuCl. 10 CuCl 50000 HgO 70 Hg 2 280 Pb0 2 1700 PbO 3200 Hg(C 6 H f ,C0 2 )2 12 Hg(C 6 H & C0 2 ) 18 CrO 3 1 Cr 2 0, 50 SnO 2 24 SnO 1220 BiO 3 70 Bi 2 0, 110 SnS* 186 SnS 1440 Mn0 2 48 Mn 5 O 4 130 Fe 2 s 202 FeO 7200 MnO 500 Fed, 1 FeCl 2 26 It is evident from the table that when a metal can combine with another element in two ways, that compound in which the metal has the lower valency has the bigger photo-electric effect. 1 C. R., 176, 1366 (1913); 177, 590 (1913). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 123 This view was verified by experiments on the photo-electric effects of MnO, Mn 3 O 4 , Mn 2 O 3 , MnO 2 , which were 400, 240, 176 and 37, respectively. It was noticed that the photo-electric fatigue was always greater in a compound in which the metal has a low valency (i. e., in the one with the bigger photo-electric effect) than in the corresponding compound in which it has a higher valency. Thus, the photo-electric effect of PbO decreased by 60 per cent in twenty minutes, while that of PbO 2 remained constant for a period of over three hours. It is probable that the fatigue is associated with the conversion of the surface of the compound, by illumination, into the more stable compound in which the metal has a higher valency and whose photo-electric effect is smaller. In a few cases, e. g., molybdenum trioxide, the photo-electric effect increased with continued illumination. Dima suggested that the light effected a chemical reduction in such cases. The following results were obtained for a number of halides : Chloride Bromide Iodide K 67 320 1200 Pb 31 97 3000 Hg ( ous) 15 19 1400 Hg ( ic) 5 14 230 Ag 200 430 750 Cd 60 24 18 With the single exception of the compounds of Cd, it will be seen that the photo-electric effect increases as a heavier halogen atom is substituted for a lighter. Naccari 1 investigated the effect of light on the transmission of electricity through toluene between a plate and a gauze 1 mm. in front of it. The fact that the increased conductivity due to illumination was independent of the direction of the field seemed to indicate that a volume ionization was produced in the liquid rather than a surface effect at the electrodes. La Rosa and Cavallaro 2 investigated the effect of illumination on the transmission of electricity through a number of liquids. Surface effects and volume effects were generally superposed in varying proportions. Thus, in water, alcohol and acetic ether, the surface effect predominated, while in ethyl ether and methylene bromide, the volume effect predominated. 1 N. Cimento, 4, 232 (1912). 2 Ibid., 6,39 (1913). 124 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES Kelly 1 employed a method particularly suited to the study of the photo-electric effect of insulators. The insulator was atomized in liquid form (either molten or in a solution), and then passed into an apparatus of the type used by Millikan in his determina- tion of "e." The behavior of one of the particles so produced was studied when in between the plates of the condenser. The particle, when initially charged, as was almost always the case,. could be held at rest by an electric field acting in opposition to- gravity. If, now, a beam of ultraviolet light were passed into the apparatus, the loss of photo-electrons by the particle would alter its charge and destroy the equilibrium. Just as in Millikan's. work on "e," the charge lost could be measured. It was found that with sufficiently low light intensities, the emission of elec- tricity from the particles consisted in the emission of electrons one by one. There is no reason to suppose that this does not hold with greater intensities of light, though it is difficult to dem- onstrate that this is so when the electrons are emitted copiously. The method was used to determine the photo-electric thresholds for insulators. They are as follows: Sulphur X 2200. Shellac < X 2200. Oil and Paraffin < X 2150. CHAPTER VIII PHOTO-ELECTRIC EFFECTS OF DYES, FLUORESCENT AND PHOSPHORESCENT SUBSTANCES The only recent work to be recorded in this chapter is that due to Schmidt. 2 He investigated the effect of light on the "phosphor" CaBiNa. In the dark this is a dielectric, under the influence of light it becomes conducting. When a sufficiently thin layer is used, so that the light can penetrate throughout the material down to the metal which acts as its support, a current can be passed through it when illuminated. This is known as the actino- dielectric effect. The curve connecting the conductivity with the frequency of the light had a maximum at X 5800 and a minimum at X4300 (thickness used .01 mm.). If thick layers are used (e. g., .5 mm.) several maxima are obtained. These are determined more by the absorption characteristics of the substances than by the actinodielectric properties, hence the necessity for thin films. 1 P. R., 16, 260 (1920). * A. d. P., 44, 477 (1914). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES llV) Compton and Smyth 1 have shown that fluorescing iodine vapor is more easily ionized than non-fluorescing iodine vapor. Further mention of this paper will be found in the chapter on ionizing potentials. CHAPTER IX POSITIVE RAYS PRODUCED BY LIGHT No further work appears to have been done on this subject. It seems worth while to examine the matter more thoroughly and to determine the nature of the positive carriers should their existence be verified. CHAPTER X SOURCES OF LIGHT USED IN PHOTO-ELECTRIC EXPERIMENTS Among reliable sources of ultraviolet light which have come into regular use in photo-electric work since 1913 may be mentioned the Cooper Hewitt quartz mercury lamp. For constant illumina- tion, such sources are very satisfactory. The shortest wave-length available is X 1849, owing to the absorption of the quartz. Con- siderably shorter wave-lengths can be obtained from arcs between metals in vacuo and from discharges in gases. Lyman 2 has in- vestigated the extent into the ultraviolet of the spectrum of a discharge through helium and hydrogen and other gases. The hydrogen spectrum ends at about X 905, and the helium spectrum at about X 510. Similar results were obtained by a different method by Richardson and Bazzoni. Lyman also investigated the spectra of sparks between various metals in his vacuum spectroscope. In no case did he get the spectrum to extend beyond about X 2000. McLennan and Lang 3 investigated the extent of the spectrum emitted from arcs of various metals. The Hg, Fe and Cu spectra extended to X 1435, X 1427, and X 1925, respectively. Carbon showed a much shorter line, viz., X 584. Millikan and Sawyer 4 found that sparks between metallic elec- 1 Science, 51, 571 (1920). 2 "The vSpectroscopy of the Extreme Ultra-Violet," Longmans; also A. P. J., 43, 89 (1916); Science, 45, 187 (1917). 3 P. R. S., 95, 258 (1919). 4 P. R., 12, 167 (1918); A. P. J., 52, 47 (1920). 126 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES trodes very close together in an extremely good vacuum gave spectra extending far into the ultraviolet. With electrodes of C, Zn, Fe, Ag and Ni, the respective spectra extend to X 360.5, X 317.3, X 271.6, X 260.0, X 202.0. If one wishes to carry out photo-electric experiments with mono- chromatic light of wave-lengths below about X 1850, some type of vacuum spectrometer of the kind used in the investigations just cited must be used. Sabine investigated the velocities of photo- electrons by very short wave-lengths isolated in a vacuum spec- trometer. Such experiments are very troublesome to carry out on account of the necessity of having the source of light, the grating, and the apparatus containing the illuminated electrode, all in the same vacuum. The monochromatic illuminator made by Hilger is useful for isolating any portion of the spectrum from the visible to X 1850. The light which passes through is not absolutely monochromatic on account of a small amount of unavoidable scattering of light by the lenses and the interior. In some special cases, it has been found advisable to use light niters to help to cut out all the light except that of the particular wave-length desired. For many purposes, where an extremely narrow range of wave- lengths is not required, light filters may advantageously take the place of a monochromatic illuminator in the visible region. A greater intensity of light is usually available than with the instru- ment. In addition to the list of light filters given in the writer's "Photo-Electricity" may be mentioned the numerous light niters put out by the Eastman Company. Being made of gelatine or colored glass, they are much more convenient than solutions. One series of niters is particularly adapted for use with the mercury lamp, the range of transparency not including more than one or two lines, and so practically giving monochromatic light. (A filter is made for use with the green line of mercury X 5467 which trans- mits about 50 per cent of this and is quite opaque to all the rest of the mercury spectrum.) It is unfortunate that a similar series of light filters has not as yet been produced for use in the ultra- violet. The Eastman series includes one ultraviolet filter. Methyl alcohol has a sharply defined absorption band beginning at X 2350 according to Hagenow. 1 Kelly 2 found that cobalt chloride, dissolved in methyl alcohol, had a fairly well-defined transmission 1 P. R., 13,415 (1919). 2 Ibid., 16, 260 (1920). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 127 region in the ultraviolet from X 2650 to X 4600 for a path 8 mm. long through a 2-normal solution. The lower limit was extended to X 2400 for a .01 normal solution. Information as to new substances available for use as "windows" in photo-electric work has been obtained recently by Miss Laird. 1 Silver foil .00002 cm. thick transmitted fairly well down to X 1140, and faintly to X 900. Celluloid films (.02 mg./cm. 2 ) transmitted easily to X 900 and faintly to X 600, and there was no direct indi- cation of a lower limit. Miethe and Stenger 2 give a list of the transparency regions in the ultraviolet for solutions of tartrazine, filter yellow, Martin's yellow, fluorescin, eosin, and nitrosodimethylaniline. The short wave-length limits of these substances range from about X 3200 to about X 2600. Several of them give narrow transmission regions when concentrated, e. g., tartrazine transmits a band from X 3000 to X 3080. Lewis 3 finds that benzol transmits down to X 1900. CHAPTER XI IONIZING AND RADIATING POTENTIALS EXPERIMENTAL METHODS Most of the phenomena of photo-electricity, as hitherto con- sidered, deal with the separation of electricity, when matter in some form or another is illuminated by light of a suitable kind. The inverse effect, the production of light by the passage of elec- tricity through matter, is a vast subject, including the whole field of spectroscopy, and will not be considered here. However, there is one part which is so intimately connected with photo-electricity (in its ordinary connotation) and which brings out the quantum relations similar to those underlying photo-electricity so clearly that it is natural to include it here. That part is usually designated as the subject of ionizing and radiating potentials. A remarkable advance has been made in our knowledge in the subject during the past two or three years. When a molecule is struck by a moving electron, the collision may, or may not, be an elastic one. By an elastic collision is 1 P. R., 15, .543 (1920). 2 Zeits. Wiss. Phot., 19, 57 (1919); Sc. Abs., 1920, 981. 3 P. R., 16, 367 (1920). 128 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES meant one in which the electron rebounds with a negligible transfer of energy. (If the mass of the molecule were infinite compared with that of the electron, there would be no transfer of energy.) If there is a transfer of energy to a monatomic molecule, there may be complete ionization as shown by the production of positive and negative ions, or there may be "partial ionization," i. e., a disturbance of the atom, which is not detectable as ionization but is shown by the production of radiation. In the cases of polyatomic molecules, collisions are more or less inelastic, the transferred energy presumably being used up in increased motion of the component atoms relative to each other. 1 The ionizing potential is the least potential through which an electron, starting from rest, must fall, to acquire sufficient kinetic energy to enable it to ionize a normal molecule on impact. Simi- larly a radiating potential measures the least kinetic energy which an electron must have, so that, on impact with a molecule, it may emit a monochromatic radiation characteristic of the molecule. (It is generally agreed that the radiation occurs afterwards as the molecule returns to its normal state.) There may be several radiating and ionizing potentials depending on the type of ionization and radiation produced. Energy in excess of that corresponding to the critical potential is a necessary, but by no means a sufficient, condition that ionization or radiation may occur. There are two principal ways of measuring the ionizing v v and radiating potentials. The "total and partial current" method is as follows. Let F (fig. 14) be a source of electrons (generally an incandescent filament), G a gauze, and P a plate, in a gas at a suitable ' pressure, generally between 1 mm. and .01 mm. If the total current to the gauze and plate combined be measured as a function of the accelerating potential V A , the curve FIG 14 ' ^ ^ e a smooth curve up to a certain point, where its slope will increase abruptly (fig. 15). This indicates that the electrons from F have acquired sufficient energy from the electric field to ionize some of the molecules with which they collide. If now it is arranged so that there is a small field, VR, between the gauze and the plate, retarding the electrons passing through the gauze, the current to the plate is called the 1 Compton and Benade, P. R., 8, 449 (1916). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 129 partial current. Now as long as the collisions are elastic, the electrons in their journey from F to G will acquire kinetic energy determined by their position in the field. Hence, the current received by P will, in general, remain constant or increase regularly as the accelerating potential is increased. If, however, the electrons are accelerated until they make inelastic collisions, then those which get through the gauze will have a smaller velocity than before, and so the field V is able to stop a greater proportion than before inelastic collisions set in. The dips in the curves then tell us when inelastic collisions take place. When the gas pressure is high enough to ensure that most electrons make several collisions between F and G (fig. 16), then a dip will occur, under suitable conditions, at O 1 4-567 Accel. Pot. FIG. 15. 8 9 J I 13 + Accel. Pot FIG. 16. every multiple of the value of the critical potential. Collisions are, of course, inelastic when ionization takes place, for there is a transfer of energy to effect the ionization. If an inelastic collision is indicated by the partial current method, but no ionization is indicated by the total current method, it is concluded that at this point radiation sets in. This is inferred from the good agree- ment between experiments of this kind and those showing directly the presence of radiation. Hence, when it is convenient to use the "total and partial" current method, one may infer radiation from the dips in the partial current curves. The use of the partial current curves is due to Franck and Hertz. 1 It is, of course, possible that other phenomena (e. g., dissociation) besides radiation may be accompanied by inelastic collisions. Should there be 1 V. d. D. P. G., 16, 10 (1914). 130 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES two radiating potentials, as in the case of mercury (4.9 and 6.7 volts corresponding to X 2536 and X 1849), it may well be that the "total and partial" method and the more direct methods do not show up the two critical points equally well. The Lenard method is to accelerate electrons from a source, F, to a gauze, G, after which they pass into a retarding field, VR, which is greater than V A , so that no electrons get across to P (fig. 17). If the electrons on passing through G have acquired sufficient energy to ionize molecules after passing through the gauze, the positive ions so produced are driven into P. The method then consists in noticing (usually by a sensitive electrometer) the point at which positive ions can be detected as V A is increased. Inas- much as this method marks the ionizing potential by noting the \G G G' FIG. 17. FIG. 18. beginning of a current of positive ions, while the total current method marks it by an increase in a negative current already existing, it should, in general, be the more sensitive method. This method, which was the one principally used in earlier investiga- tions, did not distinguish between the ionizing potential and the radiating potential, for the radiation falling on P would produce a photo-electric effect at P and so cause it to emit photo-electrons, leaving it charged positively, which, of course, is exactly what happens when positive Ions are driven to it. Davis and Goucher 1 introduced a method whereby the ionizing potential and radiating potential could be separated. A second gauze, G', was introduced as shown (fig. 18) so that a small field, Vi sufficient, if properly directed, to prevent the emission of photo-electrons, could be thrown on, to the right, or to the left, as one chooses. The essential 1 P. R., 10, 101 (1917). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 131 Mei~cun >*; idea of the method is this. P can only emit photo-electrons, and, therefore, acquire a positive charge, when the field is such as to accelerate them away from it. When the field is reversed these photo-electrons cannot escape. On the other hand, the photo-electrons produced by the radiation reaching the right- hand side of the gauze G' (by reflection or otherwise) are now enabled to pass over to P and so to give it a negative charge. Con- sequently, the charge acquired by P changes sign as the direction of Vi is changed so far as radiation is concerned. Provided that the pressure is not too large, and that the difference of potential Vi between G' and P is considerably less than V R , the positive ions pro- duced between G and G' will reach P whether the field between G' and P helps or hinders them, for their velocity on reaching G' is sufficient to overcome the effect of the field. Hence, the charge acquired by P does not change sign as the direction of Vi is changed, so far as the posi- + tive ions are concerned. A typical curve is shown in fig. 19. Theradiat- c ing potential corresponds to the point - where the (I + R) and (I - - R) g curves diverge, while the ionizing potential corresponds to the points ' where there is an abrupt upward deflection in both curves. A second modification of the Lenard method, to distinguish radiating potentials and ionizing potentials is due to Compton. 1 The collecting electrode is a cylinder closed at one end by a gauze, G', and at the other by a plate, P (fig. 20). This cylinder can be rotated about an axis as shown, so that G' or P faces the gauze G. The essential idea of this method lies in the fact that the positive ions will contribute the same charge to the cylinder, whether the gauze end or the plate end faces the gauze G. Radiation pro- duces a larger effect when P faces G than when G' faces G, for 1 P. M., 40, 553 (1920). TB1 FIG. 19. 132 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES /.xij of Rotation \ I I 'G' FIG. 20. in the latter case much of the radiation passes through the gauze and the photo-electrons produced inside do not escape and so contribute nothing to the charge acquired by the cylinder. Typical curves for He are shown in fig. 21. A represents the current to the cylinder (P facing G) as a function of the accelerating potential. B represents the current when G' faces G. R is the ratio of the currents when P faces G to the case when G ' faces G. From 20 to 25 volts the ratio is constant, while from 25 volts onwards it falls rapidly correspond- ing to the increasing effect of ioniza- tion after the ionizing potential is passed. x It should be mentioned that the constant value for R between 20 and 25 volts does not imply the absence of ionization (it implies a constant ratio between the amounts of ionization and radiation), indeed the method was devised for investigation of the ionization produced as a secondary effect of radiation under special conditions. This will be taken up again later. To determine the exact maximum energy of the electrons used in any determination of a critical potential, it is usual to take a "velocity distribution" curve at the same time. This is done by accelerating the electrons from F to G (figs. 14, 17, 18, 20) and measuring the number reaching P as a func- tion of a retarding field be- tween G and P. The differ- ence between the applied ac- celerating potential and the retarding potential required to stop all the electrons gives the necessary correction to the applied potential. The more homogeneous in veloc- ity the electron stream, the sharper are the discontinuities in the curves showing the critical potentials. To secure homogeneous electron streams, some investigators have avoided the variation 600 500 I R f-60 1 40 I 20 I.OO / // A i I i 100 // ^ ft ^ ^ -. 26 28 JO- Jt Vo/rs FIG. 21. REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 133 in speed arising from the fall of potential along the filament by means of a rapidly rotating commutator which cuts off the heating current when the accelerating potential is applied and vice versa. Thus the filament is an equipotential surface, while electrons are being accelerated away by the field. Other investigators (e. g., Davis and Goucher) have used an equipotential platinum tube (coated with lime) heated internally by a resistance coil. An idea of the energy distribution among the electrons as they are emitted from an incandescent filament may be obtained from the following table due to Langmuir. Filament temperature 2400 K 1200 K 90 per cent have energy exceeding 0.022 volt 0.011 volt 75 per cent have energy exceeding 0.059 volt 0.030 volt 50 per cent have energy exceeding 0.143 volt 0.071 volt 25 per cent have energy exceeding 0.29 volt 0.145 volt 10 per cent have energy exceeding 0.48 volt 0.24 volt 1 per cent has energy exceeding 0.95 volt 0.47 volt . 1 per cent has energy exceeding 1.42 volts 0.71 volt 0.0001 per cent has energy exceeding 2.85 volts 1.45 volts Thus, a more homogeneous stream of electrons is obtainable with a low temperature source. If we infer the energy of the fastest electrons in an electron stream from the point where the velocity distribution curve cuts the potential axes, and assume that this energy is that of the elec- trons which are responsible for the first observable ionization (or radiation) the critical potentials so deduced will not be exactly correct. The reason for this is that the proportion of collisions resulting in ionization (or radiation) is very small just above a critical potential, and even if it amounted to its maximum possible value, the geometrical arrangement of the apparatus, and the fact that at low pressures the electrons do not all collide with molecules in the space where they have kinetic energy above the critical value, would prevent its full value being registered. Hence it is that the fastest electrons which are present in only just suffi- cient numbers to give an indication on the velocity distribution curve, cannot give a measurable ionization or radiation if the same indicating instrument be used for both measurements. This was first pointed out by Smyth 1 who worked out a meChod of 1 P. R., 14, 409 (1919). 134 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES correcting for it. For details, reference must be made to the paper. In several subsequent investigations efforts have been made to overcome the error pointed out by Smyth, but they do not in all cases appear to be adequate. An exceedingly simple and accurate method of correcting for initial velocities is due to Franck and his collaborators. In the partial current curve showing inelastic collisions (fig. 16) the distance between two corresponding dips gives the exact value of the critical potential, and by comparing this with the distance between the first dip and the origin, the necessary correction to the applied potential is obtained. COLLECTED RESULTS The experimental results are given on the following pages. In cases where the same gas has been investigated by several observers independently, the results are given under each gas to facilitate comparison. In other cases, where a number of gases have been examined under identical conditions, it was thought advisable to collect them together, e. g., the results of Foote, Mohler, and collaborators, and of Hughes and Dixon. It should be added that in many cases, investigators have used the Lenard method and have not distinguished between the ionizing potentials and the radiating potentials. Though in many of the earlier researches, the critical potentials observed have been called ionizing potentials, in view of our recent knowledge it is possible that some of them may be radiating potentials and they are, there- fore, marked with an *, unless the investigator definitely decided against radiation as an adequate explanation. Hydrogen R. P. I. P. Franck and Hertz (V. d. D. P. G., 15, 34 (1913)) 11* Goucher (P. R.. 7, 561 (1916)) 10.25* Hughes and Dixon (P. R., 10, 495 (1917)) 10.2* Bishop (P. R., 9, 567 (1917)) 11* 15.8 Davis and Goucher (P. R., 10, 101 (1917)) 11 11 (. e., both at 11 volts) 15. 8 2nd type 13.6 .. 2nd type Horton and Davies (P. P.. S., 97, 23 (1920)) 10. 5 14 .4 (atom) 13.9 16.9 (molecule) Mohler and Foote (/. 0. S. A, 4, 49 (1920); 10.4 13.3 (atom) Bur. Stan., 1920, 670) 12.22 16.5 (molecule) Stead and Gossling](P. M., 40, 413 (1920)) . . 15 Franck, Knipping and Kriiger (V. d. D. P. G., 21, 728 (1919)) . . 11.5 (molecule) REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 135 R.P. 13.6 Found (P. R., 16, 41 (1920)) Compton and Olmstead (P. R., 17, 45 (1921)) 10.8 13.4 Helium Franck and Hertz (V. d. D. P. G., 15, 34 (1913)) 21* Bazzorii (P. M., 32, 566 (1916)) Rentschler (P. R., 14, 503 (1919)) none Franck and Knipping (P. Z., 20, 481 (1919) ; V. d. D. P. G., 20, 181 (1919)) 20.5 Franck and Knipping (Z. /. P., i, 320 (1920)) Horton and Davies (P. R. S., 95, 408 (1919); P.M., 39, 592 (1920)) (20.45) 21.25 20.5 41 Found (P. R., 16, 41 (1920)) Stead and Gossling (P. M., 14, 413 ( 1920)) Compton (P. M., 40, 553 (1920)) 20.2 Argon Franck and Hertz (V. d. D. P. G., 15, 34 (1913)) 12* Rentschler (P. R., 14, 503 (1919)) 12 Horton and Davies (P. R. S., 97, 1 (1920)) 11.5 Found (P. R., 16, 41 (1920)) Stead and Gossling (P. M., 40, 413 (1920)) Neon Franck and Hertz (V. d. D. P. G., 15, 34 (1913)) 16* Rentschler (P. R., 14, 503 (1919)) none Horton and Davies (P. R. S., 98, 121 (1920)) (11.8 (17.8 Nitrogen Franck and Hertz (V. d. D. P. G., 15, 34 (1913)) 7.5* Goucher (P. R., 7, 561 (1916)) 7.4* Hughes and Dixon (P. R., 10, 495 (1917)) 7.7* /. P. . . (dissociation and radiation) 17.1 (dissociation and ionization) 30.45 (dissociation and double ionization) 15.1 10.8 15.9 20 27 25. 4 (normal atom) 79.5 (double ionization) (not present in the pur- est He) 25.3 25.7 (normal atom) 55 (charged atom) 80 (double ionization) 20.5 20.8 25.5 17 15.1 15.6 12.5 21 16.7) 20.0^ 22.8) 136 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES I. P. Bishop (P. R., 9, 567 (1917)) Davis and Goucher (P. R., 13, 1 (1919)) Karrer (P. R., 13, 297 (1919)) Smyth (P. R., 14, 409 (1919)) Sodium Hebb (P. R., 12, 482 (1918)) Wood and Okano (P. M., 34, 177 (1917)) Tate and Foote (P. M., 36, 64 (1918)) Iodine Found (P. R., 16, 41 (1920)) Mohler and Foote (P. R., 15, 321 (1920)) 18 Found (P. R., 16, 41 (1920)) Mohler and Foote ( J. O.S.A.,4, 49 (1920)) Stead and Gossling (P. M., 40, 413 (1920)) Nitrous Oxide Bishop (P. R., 9, 567 (1917)) Oxygen Franck and Hertz (V. d. D. P. G., 13, 34 (1913)) Hughes and Dixon (P. R., 10, 495 (1917)) Bishop (P. R., 10, 244 (1917)) Mohler and Foote ( J. O.S.A.,4, 49 (1920)) Mercury Goucher (P. R., 8, 561 (1916)) Bishop (P. R., 10, 244 (1917)) Hughes and Dixon (P. R., 10, 495 (1917)) Tate (P. R., 10, 81 (1917)) Davis and Goucher (P.R., 10, 101 (1917)) Hebb (P. R., u, 170 (1918)) Hebb (P. R., 15, 130 (1920)) Found (P. R., 15, 132 (1920)) Kingdom (in print) Stead and Gossling (P. R., 40, 413 (1920)) Franck and Einsporn (Z. /. P., 2, 18 (1920)) R. P. 7.5* 7.5 9.0 No. I. P. < 10 volts 8.29 (strong) 18 7.3 (doubtful) ... 6.29 (strong, low . . pressures) 15.8 8.18 16.9 17.2- 7.5< 9.0* 9.2* .. 9.0* 7.91 15.5 4.9 4.9 6.7 4.9 (4.68 (4.9 (5.32 (5.76 (6.04 (6.30 (6.73) 1.0 0.5 2.12 2.34 7.12) 7.46) 7.73) 8.35) 8.64) 8.86) 10.27 10.2 10.3 10.4 4.9 3.2 10.1 4.9 10.8 10.38 2.5 .13 8.5 10.1 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 137 Compton and Smyth (Science, 51, 571 (1920)) (P. R. 16, .501 (1920)) Carbon Monoxide Hughes and Dixon (P.R., io,495 (1917)) 7.2* Found (P. R., 16, 41 (1920)) Stead and Gossling (P. M., 40, 413 (1920)) . . Hydrochloric Acid Gas Hughes and Dixon (P. R., 10, 495 (1917)) 9 . 5' Foote and Mohler (J. A. C. S., 49, 1821 (1920)) 8.0 (atom) 9.4 (molecule) 6.8 (fluorescing mole- cule) 15.0 15.0 13.7 The following is a table of experimental results obtained for a large number of metallic vapors (and some non-metallic vapors also) by Foote, Mohler, Tate, Stimson, Meggers, and Rognley. They were obtained chiefly by the total and partial current method, and in view of the fact that the experiments were carried out on a systematic and connected plan, it was thought desirable to collect them together. Na Tate and Foote (P. M., 36, 64 (1918)) K Tate and Foote (P. M., 36, 64 (1918)) Cd Tate and Foote (P. M., 36, 64 (1918)) Cd Mohler, Foote and Meggers (Bur. Stan., 1920, 734) Zn Tate and Foote (P. M., 36, 64 (1918)) Zn Mohler, Foote and Meggers (Bur. Stan., 1920, 734) Mg Foote and Mohler (P. M., 37, 33 (1919)) Mg Mohler, Foote and Meggers (Bur. Stan., 1920, 734) Hg Tate (P. R., 10, 81 (1917)) Hg Mohler, Foote and Meggers (Bur. Stan., 1920, 734) Ca Mohler, Foote and Stimson (Bur. Stan., 1920, 368) Tl Foote and Mohler (P. M., 37, 33 (1919)) Pb Mohler, Foote and Stimson (Bur. Stan., 1920, 368) Rb Foote, Rognley and Mohler (P. R., 13, 61 (1919)) Cs Foote, Rognley and Mohler (P. R., 13, 61 (1919)) As Foote, Rognley and Mohler (P. R., 13, 61 (1919)) N 2 Mohler and Foote (/. O. S. A., 4, 49 (1920); Bur. Stan., 1920, 670) Oa Mohler and Foote (J. O. S. A., 4, 49 (1920); Bur. Stan., 1920, 670) H 2 Mohler and Foote (/. 0. S. A., 4, 49 (1920); Bur. Stan., 1920, 670) R. P. 1. P. 2.12 5.13 1.55 4.1 3.88 8.92 f 3.95 \5.35 9.0 4.1 9.5 J4.18 9.3 \ 5.65 2.65 7.75 f 2.65 8.0 \ 4.42 4.9 10.3 4.76 10.2 6.45 1.90 6.01 2.85 1.07 7.3 1.26 7.93 1.6 4.1 1.48 3.9 4.7 11.5. 8.18 7.91 10.4 12.22 16.9 15.5 13.3 16.5 138 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES P Mohler and Foote (P. R., 15, 321 (1920); Bur. Stan., 1920, 670) 5.80 13.3 I Mohler and Foote (P. R., 15, 321 (1920); Bur. Stan., 1920,670) 2.34 10.1 S Mohler and Foote (P. R., 15, 321 (1920); Bur. Stan., 1920, 670) 4.78 12.2 The following are values obtained by Hughes and Dixon 1 for the first critical potentials for a number of gases. In view of our recent knowledge, it is probable that in most cases they refer to radiating potentials and not to ionizing potentials. H 2 10.2 volts CO 7.2 volts 2 9.2 volts C0 2 10.0 volts Ni 7.7 volts NO 9.3 volts Cl 8.2 volts CH< 9 . 5 volts Hg 10.4 volts C 2 H 6 10.0 volts HC1 9 . 5 volts C 2 H 4 9.9 volts Br 10.0 volts C 2 H 2 9.9 volts Richardson and Bazzoni 2 carried out an interesting investiga- tion on the extreme ultraviolet radiation emitted by He, H 2 and Hg, when electrons of velocities up to 800 volts were driven through them. The radiation produced was allowed to fall on a metal surface and the velocities of the photo-electrons emitted were measured by a magnetic method. The fastest photo-electrons in each case gave the shortest wave-length in the radiation. It was found that the shortest wave-length in the radiation was determined by the nature of the gas and was quite independent of the energy of the electrons up to 800 volts (provided, of course, that the mini- mum energy corresponding to the radiation was present). These results are not directly comparable with those on the radiating potential, for the latter indicate the first line in the series, while these experiments indicate the shortest line of appreciable intensity in the series. The results are as follows : Shortest wave-length Corresponding voltage H 2 He Hg > X830 < X950 > X420 < X470 >X1000 13.0 <29.4 >25.7 <12.4 >10.2 It will be seen that a large amount of evidence as to the ionizing and radiating potentials has been amassed. It seems certain ' P. R., 10, 495 (1917). * P. M., 34, 285 (1917). REPORT ON PHOTO-ELECTRICITY; A. LL. HUGHES 139 that each gas or vapor has at least one radiating potential and one ionizing potential, both clearly marked. Cases in which ionization occurs at the radiating potentials are ignored as they will be con- sidered later as examples of "cumulative effects." It remains to be seen how these results can be accounted for theoretically. The most reliable values for the critical potentials can be associated with certain important features in the spectra of the gases in the majority of cases. For hydrogen and helium, Bohr's theory is found to link up most of the results satisfactorily, while in the case of the metals for which series are known in some detail, con- sistent relations can be found. BOHR'S THEORY FOR HYDROGEN AND HELIUM Hydrogen. According to Bohr, the negative energy of the hy- drogen atom pictured as a simple system of one positive nucleus with one electron rotating around it in a circular orbit, is where W is the negative energy of the innermost possible orbit (for which r = 1) and r is an integer. (Only those orbits for which T is an integer are possible, on Bohr's theory.) The innermost possi- ble orbit (r = 1) corresponds to the normal hydrogen atom. When- ever the atom undergoes a change in configuration, such that the atom passes from orbit r 2 to r l (T^T^ an amount of energy is liberated amounting to An essential feature of Bohr's theory is that this liberation of energy determines the frequency of the monochromatic light emitted during the change, through the quantum relation or where K is known as Rydberg's number which equals 3.290 X 10 15 . To each pair of integral values for T X and r 2 corresponds a line, 140 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES and these lines fall into series, each series having different values for T! and each member in a series having a different value for T 2 . Spectroscopic identification of the various series has led to a determination of K', in the corresponding "wave number" equation as 109678.3 correct to about one part in a million. 1 Our K (Ryd- berg's constant) is related to K' through the velocity of light (K = cK') and is therefore known to the same degree of accuracy as the velocity of light is known. Its value is 3.290 X 10 15 as far as the first four significant figures. We shall now proceed to calculate the potential corresponding to the limit (X ) of the shortest wave-length series in hydrogen (T! = 1 , T 2 = oo ). This frequency V Q is important, in that through the quantum relation we have at once the energy re- quired to remove the electron from the innermost orbit (i. e., from a normal hydrogen atom) to infinity, in other words, the energy necessary to ionize the hydrogen atom. V e = hv Q = W = Kh Hence V = K - e - 3.290 X 10 X - 547 X 10 " 27 X 2 "' 86 volts 4.774 X 10- 10 = 13.52 4 volts. The corresponding X is 911.74 Angstrom units (which may also be identified directly as the reciprocal of Curtis's value for the hydrogen spectrum constant). According to Bohr, the negative energies of the hydrogen atom, the hydrogen molecule (i. e., two separated positive nuclei with two electrons rotating symmetrically around the line joining them as axis) and the charged hydrogen molecule (i. e., the hydrogen molecule just referred to with one electron removed) are as follows : Neutral hydrogen atom W Neutral hydrogen molecule 2 . 20 W Charged hydrogen molecule 0.88 W The following energies are required to effect the changes indicated : 1 Curtis, P. R. S., 96, 147 (1919). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 141 Corresponding volts Neutral atom > Charged atom.. .. W W (13.52) Neutral molecule > charged mole- cule 2.20 W 0.88 W = 1.32 W (17.85) (Bohr considers this less likely to represent ionization of the molecule than the following) Neutral molecule > Neutral atom and charged atom 2.20 W W = 1.20W (16.22) Neutral molecule > Two neutral atoms (dissociation) 2.20 W 2 W = 0.20 W (2.70) Neutral atom > Neutral atom W J : = 0.75 W (10.14) (Electron shifts orbit 1 > orbit 2 giving first radiating potential) Of the experimental results which distinguish between radiating and ionizing potentials, those by Mohler and Foote, and by Horton and Davies, are put forward as being in fair accord with the pre- dictions of Bohr's theory. It is by no means clear, however, why experimental investigations on ordinary diatomic hydrogen should give the critical potentials predicted for atomic hydrogen. Al- though it is known that an incandescent filament dissociates molecular hydrogen, and that atomic hydrogen is present with intense electron currents, the amount of atomic hydrogen in most experiments will be, at the most, but a small fraction of the un- dissociated hydrogen. Hence it is not clear why critical values associated with simple collisions between atomic hydrogen and electrons should be found. An exceedingly suggestive point of view has been put forward by Franck, Knipping and Kriiger in support of their experimental results. According to them, the first critical potential at 11.5 volts is an ionizing potential and not a radiating potential, and it is maintained that this corresponds to the detachment of an electron from a hydrogen molecule. That a positively charged hydrogen molecule can exist is shown clearly by Sir J. J. Thomson's work on positive rays 1 from which he deduced the ionizing potential of the hydrogen molecule to be 11 volts, a valuable result in that the method is entirely different from the methods here considered. (From energy considerations, Bohr considers that the formation of the positively charged hydrogen molecule is less likely to happen than the breaking up of the molecule into a neutral atom and a charged atom. This gives 16.22 volts instead of 17.85 volts (see table above). This means that, on Bohr's theory, no positively charged hydrogen molecule could exist.) Franck, Knipping and 1 "Rays of Positive Electricity," p. 36, Longmans. 142 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES Kriiger explain the radiating potential which they obtained at 13.6 volts as the simultaneous dissociation of the molecule into atoms and the displacement of the electron in one of them to the second Bohr orbit, so that it is in a position to give out radiation of frequency K [l/l 2 1/2 2 ] corresponding to 10.14 volts. They consider 3.5 volts to represent the best experimental value of the work of dis- sociating a hydrogen molecule. (Langmuir's value is 3.6 volts.) Thus the radiating potential at 13.6 volts is taken to correspond to dissociation plus radiation from an atom (theoretically = 3.5 -f- 10.14 = 13.6 volts). Similarly the ionizing potential at 17.1 volts is accounted for by dissociation plus ionization of an atom (theoretically = 3.5 + 13.52 = 17.0 volts). Again the ionizing potential at 30.4 volts is accounted for by dissociation plus ioniza- tion of both atoms (theoretically = 3. 5 + 2 X 13. 52 = 30. 5 volts). (On the same lines we might expect a radiating potential at 3.5 + 2 X 10.14 = 23.8 volts, but none is recorded.) In support of these results, Compton and Olmstead 1 find ionizing potentials at 10.8 volts and at 15.9 volts, and a radiating potential at 13.4 volts. They find, however, radiation at 10.8 volts, contrary to Franck, Knipping and Kriiger. The proportion of radiation to ionization between 10.8 and 15.9 volts depends largely on conditions such as gas pressure and electron current. It may be that cumu- lative effects occur and that in some cases atomic hydrogen is present in sufficient amount to show its own critical potentials. It will be noticed that the earlier results of Davis and Goucher tend to support the view of Franck, Knipping and Kriiger rather than the other. The distinction between the two views as to the interpretation of the experimental results is of fundamental im- portance in the theory of the hydrogen molecule. Even if we restrict ourselves to the results published during the last two years, it will be seen that there is lack of agreement as to the critical potentials of hydrogen, and still more, as to their interpretation. While Bohr's theory gives a wonderfully accurate picture of the hydrogen atom, it gives an inexact picture of the hydrogen mole- cule. The calculated work of dissociation corresponds to 2.70 volts, while Langmuir's experimental work yields 3.6 volts. On Bohr's theory, an impact between an electron and a hydrogen molecule will result in the formation of a neutral atom and a charged atom, and not in the formation of a positively charged 1 P. R., 17, 45 (1921). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 143 molecule, but the evidence discussed in the last paragraph shows that it is produced. Langmuir 1 has suggested a new model for the hydrogen molecule. Each electron keeps to its own path, the paths being arcs of curves situated in a plane perpendicular to the line joining the two nuclei. The electron in the path ab always keeps the same distance from a, as the other electron does from a', fig 22. Langmuir makes use of the known work of dissociation to calculate the magnitude of the orbits and the negative energy of the system. For the positively charged molecule, he assumes that the single electron oscillates along a straight line perpendicular to the line joining the nuclei, fig. 23. Two ways of applying the t Fir,. 22 FIG 13. quantum theory are suggested to determine the dimensions of the system . On taking the difference between the energies of the neutral molecule and of the charged molecule, Langmuir obtained 10.15 and 14.10 volts as the ionizing potential, according as to which of the two ways he deduced the energy of the charged molecule. The model at least offers a possible explanation of the stable charged molecule and of an ionizing potential near to 1 1 volts. Helium. According to Bohr, the negative energy of the helium atom, pictured as a system of two electrons rotating around a doubly charged nucleus, is W = 6.13 W 1 Science, 52, 433 (1920). 144 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES The negative energy of the positively charged helium atom, *. e., with only one electron rotating around the same nucleus is W = 4.00 W . This positively charged atom is very similar to the hydrogen atom, except for the charge on the nucleus, and should give out series of lines represented by 4W l Several lines (called "enhanced" lines) belonging to these series have been identified by Evans, Paschen and Fowler in the spectrum of the disruptive discharge through helium, where we may suppose that the flow of current is so intense, momentarily, that many helium atoms ionized by one impact are again struck before they return to the normal state. Compton and Lilly 1 have obtained the enhanced line X 4868 in an intense helium arc. The following energies are required to effect the changes indi- cated : A. Neutral atom - > positively volts charged atom ............... . 6. 13 W 4.00 W = 2. 13 W (28.81) B. Neutral atom - > doubly charged atom ................ 6.13W =6.13W (82.90) C. Charged atom - > doubly charged atom ................ 4.00 W = 4.00 W (54.10) D. Pos. atom - > pos. atom ....... 4.00W - -- 1 = 3.00 W (40.57) (State 1) (State 2) (Giving first R. P. of charged atom) No theoretical value for the radiating potential of the neutral helium atom has been given. It will be seen that the experimental ionizing potentials for the helium atom are around 25.5 volts, which differs from the theoretical value (28.86 volts) by more than errors of observation. The agreement between the experimental results for helium seems to be better than in the case of other gases. Excepting the values of 20 volts for ionization, which can easily be explained as "cumulative" effects, as will be seen later, it appears to be well established that the ionizing potential for the normal atom is close to 25.5 volts and the radiating potential to 20.4 volts. The former value is definitely lower than Bohr's theoretical values which yield 28.81 volts, by about 3.3 volts. Both Horton and Davies, and Franck and Knipping found that a second type of 1 A. P. J., 52, 1 (1920). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 145 ionization occurred at about 79.7 volts, again about 3.3 volts lower than Bohr's theoretical value 82.90 volts, corresponding to the simultaneous removal of both electrons. If, however, we calculate the ionizing potential for a charged helium atom, i. e., the helium nucleus with one electron, by subtracting the first ionizing potential from the second, we get according to Horton and Davies 80.0 25.7 = 54.3 volts, and according to Franck and Knipping, 79.5 25.4 = 54.1 volts. These values are ex- ceedingly close to the theoretical value for the charged helium atom, viz., 54.10 volts. We find here, therefore, a close parallel to the case of hydrogen. Bohr's theory is quantitatively exact when we have to deal with a nucleus and one electron, whether that nucleus is singly or doubly charged as in the case of the hydro- gen atom and the (charged) helium atom, respectively. Bohr's model is not quantitatively exact when there are two electrons outside the nucleus to be considered as in the case of the hydrogen molecule and the normal helium atom. By a skilful variation in the choice of the pressure and the electron current, Horton and Davies were able to show in the same apparatus the radiating and ionizing potentials of the normal helium atom, the ionizing potential corresponding to the removal of both electrons from the normal atom, and the radiating and ionizing potentials for the charged helium atom. The latter were obtained by using a very intense stream of electrons so that there was opportunity for electrons to hit helium atoms already ionized. Franck and Knipping 1 discovered the existence of a second radiating potential for helium by means of an ingenious applica- tion of the Schuster-Rydberg principle. According to this prin- ciple, the difference between the frequency of the first line of a series and the frequency of the limit of the same series is also the frequency of the limit of another series. Now these frequencies can be replaced by their corresponding critical potentials. The radiating potential, 20.5 volts, corresponds to the first line (IX 1Y) of a series, and the ionizing potential, 25.4 volts, corresponds to the limit (IX) of the same series. (This notation will be explained in a later section.) Hence the difference of these values should correspond to the limit of another series. IX 1Y = 20.5 volts IX = 25.4 volts Hence 1Y = 4.9 volts :' P. Z., 20,487 (1919). 146 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES The frequency of the limit (1Y) of this other series, corresponding to 4.9 volts, should be X 2520. Franck and Knipping recognized that this was close to the limits of the two principal series for helium X 2600 (4.74 volts) and X 3122 (3.95 volts), respectively. (These series are sometimes referred to as the helium and parhelium series.) The natural assumption was then made that the limit 1Y of the hypothetical series was identical with the known limit, Is (X 2600), of the principal series, Is-mp, for helium. The exis- tence of the limit, IS (X 3122), of the parhelium series suggested that there should be another radiating potential. The two radiat- ing potentials should therefore be 25.4 4.74 = 20.66 volts 25.4 3.95 = 21.45 volts i. e., there should be two radiating potentials, separated by .8 volt. Careful observations on pure helium showed this to be the case. In a second paper on helium, Franck and Knipping 1 make further important contributions to our knowledge of the radiating po- tentials of helium. They start with the view that in the normal helium atom the two electrons cannot exist in co-planar orbits but that they are to be found in crossed orbits, i. e., the planes of the orbits are inclined to each other at right angles. (Land6 has discussed such orbits from a theoretical standpoint.) Thus the only possible 1 -quantum state for the helium atom is that in which the orbits are at right angles (crossed orbits). The next state, the 2-quantum state, can exist in two forms, i. e., with crossed orbits and with co-planar orbits. Now the transition from the 1 -quantum state to the 2-quantum state requires different amounts of energy according as to whether the second state is that of crossed orbits, or that of co-planar orbits. According to Lande, the two possible forms of the second state (crossed and co-planar orbits) for the atom are taken to account for the two principal series (helium and parhelium). Lines of helium principal series (and other series having the same limit) are given out when an electron falls back to give the 2-quantum, co-planar state, and lines of the parhelium principal series (and other series having the same limit) are given out on return to the 2-quantum crossed state. The two radiating potentials, previously found, and differing by .8 volt, correspond to displacements of an electron from the 1 -quantum Z./. P., 1,320(1920). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 147 state to the 2-quantum crossed state and to the 2-quantum co- planar state, respectively. Fig. 24 represents diagrammatically the two radiating potentials and the origin of the helium and parhelium series. Franck and Knipping investigated the radiating potentials by measuring the photo-electric effect of the radiation, much as in the Lenard method. The electrons were accelerated through a gauze, then were allowed to collide with molecules in a com- paratively large volume bounded by a second gauze, beyond which was the photo-electric plate. The most important result of this paper is that in the very purest helium, no trace of the 20.45 volt radiating potential could be obtained. In such a case, the 21.25 volt effect shows up alone. However, the slightest amount of /Vormai Helium- Atom (u.u>uM Statt, State, 1 Z -10' -4-0 -> FIG. 24. impurity (such as can be obtained by warming up the charcoal on slightly lowering the liquid air for a short time) causes the 20.45 volt to appear again. The amount of impurity was considered too small to show any ionization or radiation by itself. It is suggested by the authors that the 2-quantum co-planar state is a stable state, and that the helium atom will not spontaneously return from it to the normal 1-quantum state. Thus they account for the non- appearance of radiation in the purest helium at 20.45 volts. (It is to be presumed that they showed that, although there was an absence of radiation, there were inelastic collisions at this accelerat- ing voltage in the purest helium.) The 2-quantum co-planar helium atom resembles the hydrogen atom to some extent, and may possibly be capable of entering into transient combination with atoms having electron affinities, such as oxygen. The ap- pearance of the 20.45 volt radiating potential is explained on the 148 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES assumption that the short-lived compounds formed with 2-quan- tum co-planar helium atoms break up with the emission of radiation corresponding to 20.45 volts. The impurities thus have a sort of catalytic effect in promoting the return of the 2-quantum co- planar helium atom to the normal 1 -quantum state. The idea of electrons displaced to certain orbits outside the normal 1 -quantum orbit being unable to return, and so giving rise to a "metastable" state has been suggested by several facts in spec- troscopy. Thus the resonance line of mercury X 2536, IS Ipz, is called out strongly by collisions of mercury atoms with electrons having energy above 4.9 volts, while the other lines of the triplet of which X 2536 is the middle member, are so feeble that they have not been detected until recently. It would appear that the falling back from the \p\ and the Ips orbit in mercury to the normal IS orbit does not occur nearly so easily as from the lp% orbit. The conception of helium atoms in a metastable state makes the explanation of ionization below the ionizing potential by the action of the cumulative effects of successive collisions much more plausible than before (see later section on cumulative effects, particularly the part dealing with Compton's work on helium). The absence of radiation when pure helium is bombarded by 20.45 volt electrons and presumably being changed continuously into the metastable state, opens up interesting possibilities as to the physical and chemical properties of this new type of helium. Franck and Knipping found several discontinuities in their curves showing the photo-electric effect of radiation, between the first radiating potential and the ionizing potential. They attributed these to emission lines in the spectrum of helium. The values of all the radiating potentials found are given in the following table. The first value (20.45 volts) is enclosed in brackets to show that it does not appear in the purest helium. Observed X from h v = Ve Calculated (20.45) 21.25 21.9 23.6 (610) 585 569 523 (20.45) 21.25 21.85 23.7 25.3 493 25.23 A series formula of the form REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 149 N N (i + *) (m + yY was used to calculate the values in the last column. The constants were determined from the experimental values of two of the po- tentials, and the remaining potentials calculated. It will be seen that the results are in good agreement with the formula. Professor Lyman in a letter to the writer states that he has found, spectroscopically, a strong line in helium at X 585. This value corresponds to the second radiating potential, 21.25 volts. No line appeared to correspond to the commonly found radiating potential at 20.45 volts. It is unlikely that Lyman's helium (in a vacuum spectroscope) could be so free from impurities as that for which Franck and Knipping found the 20.45 volt effect to vanish. Radiation corresponding to this at about X 605 might well have been expected. Other Gases. As we have no satisfactory atomic models of the molecules of other gases, we are compelled to look in another per- haps less fundamental direction and see if we can establish some correlations. On the analogy of satisfactory correlations in the case of metallic vapors, attempts have been made to apply the same results to gases. Unfortunately our knowledge of the spectrum in the extreme ultraviolet, and especially of series there, is not nearly so complete as in the region X 7000-X 2000. Smyth and Mohler and Foote identified their values (8.29 and 8.18, respectively) with the nitrogen doublet X 1492.8 and X 1494.8, which would correspond to 8.26 volts. According to Lyman, oxygen has an absorption band in the ultraviolet, with its center about X 1400. This expressed in volts is 8.8 volts, and possibly corresponds to the experimental critical potentials about 9.0 volts. No other corre- lation, outside metallic vapors, appears to have been made. Further work on the emission and absorption spectra of gases and vapors in the extreme ultraviolet is much to be desired to supply data for comparison with critical potentials. In this section we may refer to Compton and Smyth's work 1 on iodine. They found two ionizing potentials, in iodine vapor, one corresponding to the molecule (10.0 volts) and the other to the atom (8.5 volts). As the temperature is raised, and the amount of dissociation increased, the intensity of the 8.5 volt effect increases with respect to the 10.0 volt effect, as would be expected. The 1 Science, 51, 571 (1920); P. R., 16, 501 (1920). 150 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES difference corresponds to the value of the work necessary to dis- sociate iodine as determined by the methods of physical chemistry. SPECTRAL SERIES NOTATION* The spectra of many metals, particularly those of the alkali metals and of the alkaline earths, can be resolved into a number of series. The frequencies of lines in a series of lines in the spectrum of an element may be represented as the difference of two terms, such as N N which represents the Principal Series. To a first approximation, the functions are such that the difference may be written N N ~ (1 + S) 2 (m + P) 2 "N" is a universal constant. "S" and "P" are constants for this particular series, and m takes successive integral values up to infinity, each value corresponding to a line in the series. It will be seen that as m increases indefinitely, the frequencies become closer and closer together, and when m = < , we have only the first term left which is called the "convergence frequency" or "limit" of the series. The first line of the series is usually that for which m is the smallest integer which will make the expression positive. For example, the first line of the Principal Series is N N ~ (1 + S) 2 (1 + P) 2 A short notation for this series is v = IS mP The chief series are as follows: Principal IS mP Sharp IP mS Diffuse IP wD Fundamental 2D wF m = 1,2, 3, 4. m = 2, 3, 4, 5. m = 2,3, 4, 5. m = 2, 3, 4, 5. * The author wishes to thank Professors A. Fowler and F. A. Saunders for valu able information in this connection. The author has adopted their notation, which is that generally used by American and English spectroscopists, in preference to that often followed by writers on ionizing and radiating potentials, as it seems desirable for the sake of uniformity that all should follow the same notation. The notation will be used by Professor Fowler in his forthcoming book "Report on Series Spectra" to be published by the London Physical Society. Much information on series notation -.ill be found in papers by Professor Saunders (.4. P. J., 41, 323 (1915); 50, 151 (1919); 51, 23 (1920)). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 151 "Combination" series are also found, formed by taking terms from two different series of those mentioned above; e. g., IS wD is a combination series. The radiating and ionizing potentials of the alkali metals are found to be related to the "principal series of doublets," for which the above notation is used, except that Greek letters are used instead of capitals. When it is necessary to distinguish between the two lines of a doublet, suffixes are used. Thus the two lines forming the familiar doublet of sodium X 5890 and X 5896 are la- l7r b and Iff l7T 2 , respectively. When we deal with the radiating and ionizing potentials of the metals of the second column in the periodic table, we have to do with the "principal series of single lines" or "singlets," as they are frequently called, IS mP, and also with a combination series formed by subtracting from the convergence frequency of the principal series of singlets IS, the terms mp z of the principal series of triplets (middle terms only). The combination series referred to is IS mp z . Thus the first line of the principal series of singlets IS mP (m = 1) for mercury is X 1849, the first line of the combination series IS mpz (m 1) is X 2537, while the limit for both series, IS, is at X 1188. A method of visualizing the origin of series in the Hg spectrum is shown in fig. 25. The various vertical lines IS, IP, 2S, etc., and lp lt lp z , Ipa, etc., represent some of the possible stationary states of the atom, in Bohr's sense. We have no means of know- ing the actual spacing between the orbits, or how they are arranged in such a complex .system as the Hg atom, but we do know the wave-length of the lines given out as an electron falls back from one of the outer stationary orbits to one of the inner orbits. Since the emission of lines is supposed to be governed by the quantum relation Ve = hv, we know the energy emitted by the atom for each line, and hence the diagram is plotted in terms of energy (expressed in volts) below and in the equivalent wave numbers (= i/\ = v/3(10) 10 ) above. The diagram therefore shows the energy of the atomic system according to which of the stationary- orbits is occupied by the outermost electron, and the wave num- ber of the line emitted as the electron falls from an outer orbit to an inner orbit. (Possibly it would be more correct to say that the vertical lines specify the energy of the various possible con- figurations of the atom, and that it is conjectured that these con- 152 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES figurations correspond to the atom with its outermost electron in one or other of the possible stationary orbits. The energy re- lations are definite, the way in which they are accounted for is still to be settled.) Thus the energy given out by the atom when the line X 2536 is emitted is measured by 4.9 volts, and the energy given out when the electron is completely removed is 10.4 volts. In the normal unexcited atom, the orbits outside the IS orbit are supposed to be unoccupied. Evidence for this will be given later. In the upper part of the diagram will be found a few of the spectral lines (shown by horizontal lines) in three singlet series, viz., the Principal, IS mP, the Sharp IP mS, and the Diffuse IP mD. In the lower part, one series of triplets, Ip ms, is illus- trated. In the middle, several lines belonging to combination series are shown, e. g., ISmp. Fig. 25 is shown to illustrate Franck and Einsporn's paper which will be considered later. The full horizontal lines are those which they identified through radiat- ing potentials. For the present discussion, the distinction be- tween full and dotted horizontal lines is to be ignored. The following table shows the correspondence between the notation used by Ritz and Paschen and that used here, following Fowler and Saunders. Ritz and Paschen Fowler and Saunders 1 5S 2 5S 3 5S IS 2S 3S 2P, 3P IP 2P - 3D, 4D 2D 3D 1 . 5S mP, m = 2,3 IS mP,. m = 1,2 2P mS, m = 2.5,3.5. IP mS, m = 2,3 2P wD, m = 3,4 IP mD, m = 2,3 1 . 5S mp2, m = 2,3 IS m/> 2 , m = 1,2 Metallic Vapors. It was noticed by Davis and Goucher that in the case of mercury vapor there were radiating potentials corre- sponding to the lines X 2536 and X 1849 and an ionizing potential corresponding to X 1188, the limit of both the series to which X 2536 and X 1849 belong. This gave the clue to another method of deter- mining the ionizing potentials and was used very successfully by Mc- Lennan and Young. 1 It was assumed that any method which would give the convergence frequency of the principal series would at the same time give the ionizing potential through the quantum 1 P. R. S., 95, 273 (1918). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 153 relation. McLennan and Young made use of the property that lines in the principal series are strongly marked absorption lines when light is passed through the vapor of the element. In this way they were able to pick out the lines in the principal series of a number of elements, and so to calculate the convergence frequency and the ionizing potential. They gave the following values of the ionizing potentials calculated by this method : Hg 10. 45 volts Zn 9.4 volts Cd 9.0 volts Mg 7. 65 volts Ca 6. 12 volts Sn 5. 70 volts Ba 5.21 volts Na 5. 13 volts Ka 4. 32 volts Mohler and Foote, and others, have made extensive deter- minations of radiating and ionizing potentials of a large num- ber of metals. They conclude that in the case of the elements Na, K, Rb and Cs (Group I of the Periodic Table) the ionizing potential corresponds to la and the radiating potential to the shorter line l 2 and the ionizing potential to IS. There is, however, in Ca a radiating potential at IS IP as well, and possibly a corresponding one would be found for Mg. Davis and Goucher have shown that a second radiating potential exists for Hg corresponding to IS IP. It seems safe to generalize that IS corresponds to the ionizing potentials of the elements in this group, and that IS Ipz and IS IP correspond to radiat- ing potentials, although the latter may often be masked by the former. * Not enough is known about series in the spectra of ele- ments outside the first two groups. Mohler and Foote have indeed suggested that the experimental values of the ionizing and radiating potentials for such elements may be used as starting points in the search for series relations. SINGLE AND MULTIPLE LINE SPECTRA The work of Franck and Hertz 1 in 1914 must be regarded as the * In a recent paper, Foote. Mohler and Meggers (Bur. Stan., 1920, 725) find that every element in Group II, which they tried, viz., Zn, Cd, Hg, Mg and Ca, has two radiating potentials, corresponding to IS Ipt and IS IP. 1 V. d. D. P. G., 16, 512 (1914). 154 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES starting point of all work on radiating potentials, for they were the first to demonstrate that by bombarding the atoms of mercury vapor with electrons, the single line X 2536 appeared as soon as the energy of the electrons exceeded 4.9 volts. This was the first proof of the quantum relation as applied to the direct production of radiation by electron impacts, and, moreover, served to identify the radiation associated with the radiating potential as mono- chromatic light whose wave-length is that of the first line in an important series of the spectrum. McLennan and Henderson 1 and McLennan 2 extended this investigation to other metals. It was found that for Hg, Zn, Cd and Mg, electrons must possess a certain characteristic minimum velocity before the single lined spectrum of these elements could be called out, and that this minimum velocity agreed well with the value deduced by the quantum relation from the corresponding wave-lengths. These wave-lengths X 2536 for Hg, X 3076 for Zn, X 3260 for Cd, and X 2852 for Mg are all first lines of important series. The first three lines belong to the combination series, being IS lp z , while the Mg line is the IS IP line. It should be noted here that Mohler and Foote in their experiments on radiating potentials found that the frequency IS lp z could be excited with Mg just as with the other elements. They state that their method really measures the points of inelastic impact hitherto taken to indicate the wave-length of the principal radiation. For Mg and Ca (and presumably Ba and Sr) the inelastic collision method emphasizes the frequencies IS Ifa and indicates the presence of IS IP while the spectroscopic evidence emphasizes IS IP. McLennan and Henderson found that as the speed of the elec- tron was increased nothing but the single line was obtained until the speed corresponding to the head of the series was reached, at which point all the lines of the series appeared together. It would seem natural to expect that shorter lines than the single lines would appear when electrons of the proper velocity were driven through the vapor. McLennan and Ireton 3 investigated this carefully and found that in the case of Zn and Cd two lines, and only two lines, could be called out by electrons having just sufficient energy to call out the shorter. (If the conditions are not such as to render cumulative effects negligible, other lines may 1 P. R. S., 91, 485 (1915). 2 Ibid., 92, 305 (1916). ' P. M., 36, 46 (1918). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 155 appear as well.) The first line called out in each case is the first line of the combination series IS Ifa and the second line is the first line of the principal series IS IP. Foote and Meggers 1 have made some very interesting investi- gations on the spectrum of Cs excited by slow electrons. Measure- ments of the energy in the various spectral lines were made for different values of the accelerating potential. It was concluded that the two lines of the doublet la ITT were called out simulta- neously, and that they were called out only when the electrons had energy in excess of that corresponding to the shorter of the two lines forming the doublet. No other lines were called out until the ionizing potential was reached. This result seems to be in conflict with that of Franck and Knipping on helium, and with that of Franck and Einsporn on mercury. There seems to be no a priori reason for supposing that the lines of higher fre- quencies in either the IS- IP or the IS \p series cannot be ex- cited, with electrons of appropriate energy, if the first lines can be excited. One would expect the mechanism to be the same for all lines of the series, the only difference being the amount of energy involved. Possibly the intensities of the lines -of higher fre- quencies are too feeble, under most experimental conditions, to allow them to be detected. Franck and Einsporn 2 have recently made a very important contribution to our knowledge of the radiating potentials of mer- cury. They measured the photo-electric effect produced by elec- tron impacts with mercury atoms, as a function of the potential accelerating the electrons. By taking great care to use pure mercury vapor, to secure absolutely steady electron emission from the source and to increase the accelerating potential by very small steps, they found no less than eighteen discontinuities in their curves between the first radiating potential and the ionizing potential. (They used a modified Lenard method.) The results are given in the table. They were able to identify a number of the discontinuities with known lines. In some cases, e. g., No. 17 and 17', the theoretical values corresponding to possible interpreta- tions of the breaks in the curves are so close that it is not possible to decide which interpretation is correct. Corresponding to No. 3 there is the known optically absorbing region from X 2313 to X 2338, which does not fit into any known series rela- 1 P. M., 40, 80 (1920). 2 Z./. P., 2, 18(1920). 156 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES tions. There are some breaks, e. g., 6, 7, 9, .... which do not correspond to any spectrum lines known at present. The break, No. 5, is of interest in that Franck and Einsporn suggest that it is due to the complete expulsion of an electron from the orbit Ip 3 , i. e., to ionization of an abnormal Hg atom, abnormal in the sense that in the normal Hg atom, no electrons exist outside the orbit IS. It is well known that in the triplet IS Ip the No. Observed V. Strength s*strong, = mediuin, X Series Calculated V. 1 4.68 W 2656 . 5 IS 1ft 4.66 2 4.9 very s, especially at 2537 IS 1ft 4.86 high pressures 2338 5.28 3 5.32 w 2331 5.34 4 5.47 } w at mean pressures If 2270.6 IS 1ft 5.43 5 5.76 s 2150 1ft 5.73 6 6.04 w 2043 7 6.30 w (1956) 8 6.73 m 1849-6 IS IP 6.67 9 7.12 / s high pressures \ w low pressures (1733) 10 7.46 m 1656 11 7.72 m 1603.9 IS Is 7.69 12 8.35 IV (1447) 13 8.64 s 1435.6 IS 2ft 8.58 14 \ ( m 1402.7 IS 2P 8.79 14' } 8.86 \m 1400 IS 2d 8.81 15 907 2656.5 2 X 4.66 . o/ w 2656.5 = 9.32 16 w 1307.8 IS 3ft 9.44 4 9.60 w 2656.5 4.66 + 4.86 J 2537 - 9.52 17 9.7 m 2537 2 X 4.86 17' 1 2537 IS 3P = 9.72 J m 1268.9 9.73 18 10.38 s low pressures 1187.9 IS 10.39 w high pressures middle term IS Ip? is far more intense than either IS \p\ or IS \p%. The electrons apparently have much more difficulty in dropping back from orbits I pi or Ip 3 to IS than from lp t to IS, consequently it has been suggested that a mercury atom with electrons in these orbits is in a semi-stable state. Hence there is a REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES L57 far greater probability of impinging electrons hitting an atom in these states, and so causing ionization or radiation, of another- type, than for the case of an atom with an electron in the orbit Ipz. The diagram (based on those given in Franck and Einsporn's paper) shows some of the possible orbits (in Bohr's sense) in the mercury atom (fig. 25). They are so spaced as to measure the FIG. 25. frequency difference between each orbit and consequently the energy necessary to shift an electron from any orbit to any other. The left boundary of the diagram is in the IS orbit, the outermost orbit occupied by an electron in the normal atom. The right boundary corresponds to an orbit at infinity, and the distance 158 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES between the boundaries measures the ionizing potential. The horizontal lines (whose length is a wave number) correspond to spec- tral lines denoted by the position of their ends. Thus four lines in the principal series IS mP are shown. The full lines shown in the diagram are those which probably correspond to the radiating potentials found by Franck and Einsporn. (The dotted lines illustrate other lines belonging to a few of the well-known series of mercury.) CUMULATIVE EFFECTS LOW VOLTAGE ARCS In this section will be considered a group of investigations on the phenomena which only become evident when sufficiently dense electron streams are passed through a gas, and whose density is usually appreciable. In the foregoing sections, attention has chiefly been paid to those results which bear out the quantum relations. But certain investigations have been published, which, at first sight, seem in conflict with the results expected from theory, and confirmed by other researches. In general, these effects come into play only when the impact of an electron on a molecule can no longer be regarded as an isolated event, that is, when the result of an impact is not influenced by the radiation to which the molecule is exposed from other impacts in its vicinity, and by impacts which it may have experienced a short time previously. As the supply of electrons is increased when electrons are driven through a gas at a low voltage (provided pressure and voltage are suitable) a more or less visible discharge (an arc) may suddenly set in, a large increase in the current occurring simultaneously. This increase in current indicates ionization. It became evident, however, that a voltage much less than the ionizing potential would produce an arc under favorable conditions, thus throwing doubt on whether any fundamental importance could be attached to the so-called ionizing potentials. Richardson and Bazzoni 1 found that with an intense electron stream through helium, an arc could be made to strike when the potential accelerating the electrons was as low as 22.5 volts and mentioned that there were indications that the striking potential tended to still smaller values. Hebb 2 found that a mercury arc could be started with electrons whose velocities correspond to 4.7 volts only, and that it could be maintained at 3.2 volts. Later, 3 1 Nature, 48, 5 (1916). 2 P. R., 9, 371 (1917). Ibid., ii, 170 (1918X. REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 159 he maintained that Hg was directly ionized by electrons of 4.9 volts velocity and that no explanation of the effect as a secondary phenomenon could be considered adequate. In another research 1 on arcs in mixtures of K and Hg and of Na and Hg, he found that an arc could be struck in the former at 1.6 volts and in the latter at 2.5 volts and maintained in them at .5 and 1.4 volts, respectively. In the case of the arc in the mixture of Hg and K vapors, it is interesting to note that even when the electrons had a velocity of only .5 volt, the mercury spectrum was visible. This value, .5 volt, as well as the 1.4 volt, at which the arc was started, is, of course, much below the ionizing and radiating potentials of Hg. Very recently Hebb 2 concluded that ionization of Hg vapor can take place at potentials as low as 3.2 volts, with very intense electron streams, and that the previous limit given for ionization, viz., 4.9 volts, has no special significance. Tate 3 showed that with large electron streams through Hg vapor, arcs could be started with velocities down to 7.3 volts. McLennan 4 found that the potentials at which arcs would strike depended very much on the density of the electron stream, and the density of the vapor, de- creasing as these factors increased. With Hg and Cd, the lowest potentials, under the most favorable conditions at which arcs could be struck, were 4.75 and 5 volts, respectively, while they could be maintained at potentials as low as 2.84 and 2.0 volts. It was recorded that close to these minimum striking potentials the arc took an appreciable time to start, after the potential was applied. It should be noted that time lag effects, of which this is one, have frequently been observed in connection with phe- nomena associated with intense electron currents through gases. 5 As will be seen from the table, several investigators have found strong ionization in helium and argon at potentials at which other experimenters find radiation only. (In the experiments showing ionization, dense electron currents and appreciable pressures were used.) Compton, Olmstead, and Lilly 6 found that with a suffi- ciently dense stream of electrons in He at a suitable pressure, arcs could be started at a potential as low as 20.2 volts, i. e. t the radiating ip. R., 12,486 (1918). *Ibid., 15, 130 (1920). 3 Ibid., io,81 (1917). * P.L.P. 5., 31, 1 (1918). 5 Richardson and Bazzoni, P. M., 32, 426 (1916); Tate, P. R., 10, 81 (1917); Comp- ton, Lilly and Olmstead, Ibid.. 15, ->ir> (1920). 6 P. R., 15, 545 (1920). 160 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES potential, and could be maintained at still lowe'r potentials, 8 volts being the lowest. However, no further decrease in the striking potential (20.2 volts) could be obtained by any further increase in the density of the electron stream or change in pressure. It would appear that these experiments definitely prove that the radiating potential in helium is the lowest potential at which ionization can be started, even under the most favorable conditions, a result which is in agreement with McLennan's results, and with Hebb's earlier results. Various explanations of these apparently anomalous effects with dense electron streams through gases at appreciable pres- sures have been offered. Richardson and Bazzoni 1 suggest that ionization at potentials lower than those corresponding to the theoretical ionizing potential is the result of successive impacts, or results from impacts on atoms in an abnormal condition caused by the absorption of radiation generated in other atoms. Millikan 2 suggested that just above the radiating potential (this term was not in use at the time), 4.9 volts for Hg, the radia- tion (X 2536) produced might act photo-electrically on the neigh- boring atoms, certainly those of the cathode and possibly those of the surrounding vapor, and thus add more electrons to the original stream. The electrons so produced would be accelerated by the field and might produce, if favorably placed, more radiation, which, in turn, would release more electrons photo-electrically. In this way, it could be readily seen how an indefinite multiplica- tion of the original electron current might occur, and how ioniza- tion would accompany the radiation produced at 4.9 volts. The most important feature of Millikan's theory is that charged atoms resulting from the expulsion of the photo-electrons would permit (1) ionization, (2) the many-lined spectrum, and (3) the low voltage arc, all below the accepted ionizing potential 10.4 volts. This follows from the fact that an atom which had lost an electron photo-electrically would sometime later regain an electron, and this would pass from one Bohr orbit to another on its way to the orbit which it would finally occupy in the normal atom. In jump- ing from one orbit to another radiation of the corresponding wave- length would be given out, and thus the production of the many- lined spectrum could easily be accounted for. Also, it would need but very little energy on the part of an impinging electron to 1 Nature, 48, 5 (1916). 3 P. R., 9, 378 (1917). REPORT 0.\ PHOTO-ELECTRICITY: A. LL. HUGHES 161 ionize an atom in which the electron happened to be in one of the outer orbits while on its way inwards to the orbit normally occupied. Thus ionization of abnormal atoms was accounted for hence low voltage arcs. Atoms in all stages of recovery to normal would be found, and the effects (1), (2) and (3) would be weak or strong according as few or many of the outer orbits were occupied by electrons. The minimum striking potential for the arc (4.9 volts) was taken to show that the outermost orbit occupied in the normal Hg atom was the one corresponding to the limit of the series of which X 2536 is the first member, for, unless the impinging electron has sufficient energy to displace an electron to the next possible orbit, no radiation (and ionization resulting therefrom) can possibly occur. Though it is now evident that X 2536 cannot ionize Hg vapor, Millikan's theory would still hold if, for the complete ionization of the Hg atom photo-electrically, one sub- stituted partial ionization, i. e., the displacement of the electron from the normal orbit IS to the l 2 orbit, in which state the atom could be ionized by subsequent impacts of electrons with energy well below 10.4 volts. Van der Bijl 1 suggested that the production of ionization, the many-lined spectrum, and arcs with potentials below the ionizing potential, could be accounted for on the assumption that with dense electron streams an atom would frequently experience a second collision while still in an abnormal state resulting from a previous collision. Thus if we have a stream of electrons with energy corresponding to 5 or 6 volts passing through Hg vapor, the first collision with an atom will displace an electron from the normal orbit to the orbit Ifa. The atom is now in a state that needs but 10.4 4.9 = 5.5 volts to remove the electron, so that a second collision may effect complete ionization. The idea here involved is frequently referred to as that of successive impacts. McLennan 2 attributed the production of "faint arcs" showing the many-lined spectrum, at potentials much below the ionizing potential, to the presence of electrons of abnormal velocities among those emitted by the incandescent source. In one case, electrons of initial energy corresponding to 5 volts would have to be assumed. For these arcs, it was suggested that the direct action of the few high speed electrons would be sufficient. (A calculation of the proportion of electrons having such high speeds from any thermal source, 1 P. R., 10, 546 (1917). *P. L. P. S., 31, 1 (1918). 162 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES however, shows it to be negligibly small.) In the case of "brilliant arcs" McLennan considers that some other consideration (e. g., the part played by positive ions) must be taken into account, high speed electrons alone being inadequate to account for the results. Compton 1 investigated whether the theory of successive impacts was adequate to explain the experimental effects. His calcula- tions hinge upon the average time interval in which an atom can remain in an abnormal state. Unfortunately, this is not known for Hg, but use was made of Stark's value, 6 X 10 ~ 7 sec. for hydro- gen, as a tentative assumption. His conclusion is that the effect of successive impacts may in some cases be sufficient to account for the phenomena, but that, in general, it is insufficient to account for all the observed phenomena. The cumulative effect of ab- sorbed radiation and direct impact may be more important.* Compton 2 has recently made quantitative measurements of the amount of ionization which accompanies radiation in He produced by electrons whose energy is between the radiating potential and the ionizing potential. These experiments are of importance in that they indicate clearly the source of conflicting results on the presence of ionization below the ionizing potential. The arrange- ment shown in fig. 20 was used. From the ratios of the effects obtained when the gauze side and when the plate side of the cylinder faced the electron stream the ratio of the ionization effect to the radiation effect could be obtained. From 20 volts to 25 volts (fig. 21) this ratio was a constant in any particular case and de- creased abruptly at 25 volts, because direct ionization set in. The following are the results obtained at various pressures with electron currents of the order of 10 ~ 6 amperes: Pressure Mm. Ionization P.) Radiation v "' 0.0005 0.055 0:003 0.176 0.082 0.333 0.044 0.410 1.0 2.22 25.0 11.4 I P. R., 15, 130, 476 (1920). * In view of the suggestion (discussed above), put forward by Franck and Knipping, that atoms in pure helium can be put into an abnormal state by collision with 20.45 volt electrons, and show no tendency to return to the normal state, it may be that "successive impacts" will account for much more of the cumulative effects than would be expected on the assumption that the abnormal state lasts only about 6 X 10 ~ 7 sec. II P.M., 40, 553 (1920). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 163 It will be seen that at higher pressures ionization predominates, and is present even at the lowest pressure. When the pressure is constant and the electric current is increased the ratio also increases but not much, showing that the cumulative effect of successive impacts is not so important as the cumulative effect of absorption of radiation and a direct impact. Horton and Bailey 1 believe that their results show that when there is ionization in helium below 25 volts, it is due to ionization of the impurities (e. g., Hg) by the 20 volt radiation from the helium. While their experiments certainly show that apparently negligible traces of impurities produce large ionization effects, Compton's results cannot be accounted for as an impurity effect. Reasons in support of this are given in Compton's paper. Perhaps the clearest proof of the existence of ionization in helium below 25.5 volts is the striking of an arc in the purest helium at 20.2 volts, an effect which is too big to be accounted for by traces of impurities. Compton's view of the cumulative effect of absorbed radiation and direct impact has been strengthened by some very recent experiments of Compton and Smyth 2 in which it was found that the ionizing potential of fluorescing iodine vapor was lower than that of normal iodine vapor. For normal iodine vapor, the value was about 10 volts, while for iodine vapor made fluorescent by green light (corresponding potential 2.3 volts), the value was 7.5 volts, just 2.5 volts less. An interesting illustration of cumulative effects appears in the recent work of Compton, Lilly and Olmstead. 3 They found that in the helium arc all the ordinary lines appear simultaneously when the arc strikes. As we have seen, in order that the arc shall strike, there must be ionization, and when this takes place below 25.5 volts, it cannot be anything else but a cumulative effect. There will be electrons in all the outer orbits dropping step by step towards the inner orbits thus emitting lines in the visible as well as the ultraviolet region. When the arc is intense, few of them will get to the normal position as the frequent electron collisions re-eject them. The enhanced line X 4686 belongs to the charged helium atom, and is denoted in Bohr's notation as 4N { 1/3 2 l/4 2 j . It cannot be produced until a helium atom already 1 P. M., 40, 440 (1920). 2 Science, 51, 571, June 4 (1920). 3 P. R., 16, 282 (1920). 104 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES deprived of one electron is disturbed. As the formula shows it is emitted when the electron drops back from orbit 4 to orbit 3, in the charged atom (fig. 26) . It can be produced in two different ways, the first being found when the electrons in the electron stream have sufficient energy to detach both electrons from the atom. This corresponds to the ionizing potential 80 volts (see table on p. 142) . In agreement with this, the authors found that the enhanced line X 4686 suddenly appeared when the potential across the arc was raised to 80 volts (the gas pressure being low and the thermionic current not intense). With higher pressure and more intense currents, the line becomes visible at 55 volts. This was explained as a cumulative effect, one example of which is the removal of the first electron from the atom and a collision between the charged atom and an electron of sufficient energy to lift the remaining He,l>iu,m, Atom, Ot-Jb/T Numbers > y 45o II 1 Energy **, v ,. \4686/ PT ^ 5/5.73 J -* AA T/7 *n 'TO JU -^ 4n- 67 - electron from orbit 1 to orbit 4 or any orbit beyond. (The dia- gram shows that the minimum energy required is 50.72 volts.) Other combinations forming cumulative effects are given in the paper. Compton's results on ionization in helium between the ionizing and radiating potentials open up interesting considerations as to similar phenomena in other gases. To account for ionization of helium atoms just above the radiating potential, the ionization must be a second "event" in the recent history of the atom, the first being the absorption of energy, 20.4 volts, to make the atom an abnormal one, either through an earlier impact, or, more prob- ably, through the absorption of radiant energy from neighboring atoms. As the energy required to remove the electron completely from an abnormal atom is 25.3 20.4 = 4.9 volts, there is energy to spare in the 20.4 volt electron stream for the "second event." But this is not the case for elements in which the ionizing potential REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 165 is more than twice the radiating potential. We may cite mercury vapor (4.9 volts and 10.4 volts), and still more to the point, iodine vapor (2.34 volts and 10.1 volts). Electrons of velocity just above 4.9 volts would be unable to ionize a mercury atom as a result of two "events;" it would appear that three "events" would be necessary. Presumably a mercury atom cannot acquire any more energy than 4.9 volts, through the absorption of 4.9 volt radiation (X 2536) no matter how intense (as radiation of different wave-length corresponds to removal from 2nd to 3rd orbit), hence, it would appear that the second and third events would have to be successive impacts. Investigation on these lines should lead to important results. COMPOUND GASES Investigations of the critical potentials for compounds should furnish evidence as to the mode in which atoms are bound together in a compound. The first critical potentials for a number of compounds were determined by Hughes and Dixon 1 (see table p. 136.) No general conclusions could be drawn, except that the first critical potentials for CH 4 , CaHe, CoH 4 , C2H 2 were very much the same. It is desirable that these compounds be re-examined on the lines of the best of the recent investigations to obtain accurate values for the radiating and ionizing potentials and to distinguish between them. Several important theoretical papers by Born 2 and Fajans 8 have appeared on the affinity of the halogen atoms for electrons, and on the ionizing potentials of HC1, HBr and HI. The line of argument may very briefly be outlined as follows. Born regards a crystal such as KC1 as being held together by forces between the positive K ion and the negative Cl ion. His theory gives a value, UKCI for the work necessary to dissociate the crystal into gaseous K+ and Cl- ions. This final stage can be arrived at in another way. Imagine the crystal to be dissociated into potassium (metal) and chlorine gas, and then the potassium to be vaporized and afterwards ionized into K+ and free electrons, while the chlorine gas C1 2 is dissociated into atomic Cl and afterwards the Cl atoms unite with the free electrons to form Cl- ions. The energies re- quired are, respectively, QKCI the ordinary heat of combination; D K , the heat of vaporization of K; J K , the work of ionization of K; > P. R., 10, 495 (1917). 2 V. d. D. P. G., 21, 13, 679 (1919). 3 Ibid., 21, 714 (1919). 166 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES DC*, the heat of dissociation of C1 2 ; and Eci, the energy given out when a Cl atom combines with an electron. UKCI = QKCI + D K + J K -f- D cl E C i All the values are known except E C i which turns out to be 5.16 volts. This means that energy is given out when a negative Cl ion is formed, i. e., Cl- is stable. If the ionization of HC1 is merely the disruption of the molecule into H+ and CL-, the energy necessary to effect it, JHCI> can be predicted according to Born and Fajans. The decomposition of HC1 into Hg and CU requires an amount of energy QHCI (the ordinary heat of formation) ; the dissociation of H 2 into H requires an amount of energy D H ; of Cl2 into Cl, an amount D C i; the ionization of atomic H into H-f- and free electrons requires an amount J H (given by the ionizing potential 13.52 volts) ; and the union of Cl with a free electron corresponds to the evolution of heat ECI- Hence, JHCI = QHCI + D H + D C1 + J H E C i If ECI be taken to be 5.16 volts, then J H ci will have a value of 13.9 volts. This was tested experimentally by Foote and Mohler 1 who obtained a value of 13.7 volts for the ionizing potential of HC1. This, then, may be regarded as confirming the view of Born and Fajans as to the mechanism of ionization of HC1. It should be mentioned that Hughes and Dixon 2 found a critical potential in HC1 at about 9.5 volts (together with evidence of a much stronger ionization at about 13 volts) which may, of course, be a radiating potential. Foote and Mohler, however, state that they have found no evidence of a radiating potential. A useful discussion of the values of the ionizing potential to be expected for certain compounds has been given by Foote and Mohler. 3 The compounds considered are those for which there is reason to believe that ionization takes the form of splitting up the molecule into positively and negatively charged atoms, and for which there is sufficient thermochemical data to predict the value of the ionizing potential. The results are arrived at on lines more pr less similar to those used in the last paragraph for HC1. 1 J. A. C. S., 42, 1832 (1920). 2 P. R., io,495 (1917). 1 Jour. Wash. Acad. Sci., 10, 435 (1920). REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 167 APPENDIX (Added after the report was in type March, 1921) I. The principal critical potentials for many metallic vapors have now been shown to be associated, through the quantum relation (Ve = hv~) with lines in certain series in their spectra. These lines are IS- Ifr, IS- IP, and the limit IS, for the metals of the second column of the periodic table, and the doublet la ITT and the cor- responding limit la for the alkali metals. Once the identity of this relation is admitted, much more accurate values of the critical potentials can be calculated from spectroscopic data than can be obtained by direct experiment. Professor F. A. Saunders kindly furnished the data for the alkali metals. The data for the other metals were obtained from a recent paper by Mohler, Foote, and Meggers (Bur. Standards, Sci. Papers No. 403, 1920). It should be mentioned that, in this paper, it has been shown experimentally that each element (in the second column of the periodic table) in- vestigated, has two well-marked radiating potentials and one ion- izing potential. This result is valuable in showing the uniformity of the elements in this group with respect to the critical potentials, a result which was expected, but had not been demonstrated. (The calculated potentials are given to four significant figures only, as h and e are not known to less than 1 in 1000.) Metal Series Wave-length Calculated Potential loiizing Radiating Mercury IS IS Ip, IS IP 1187.96 2537 .48 1849.60 10.392 4.865 6.674 Cadmium IS IS Ip, IS IP 1378 .69 3262.09 2288.79 8.954 3.784 5.394 Zinc IS IS Ip, IS IP 1319.98 3076.88 2139.33 9.352 4.012 5.770 Magnesium IS IS Ip, IS IP 1621.72 4572.65 2853.06 7.612 2.700 4.827 Barium IS is IP, IS IP 2379.28 7913.52 5537.04 5.188 1.560 2.229 Strontium IS IS Ip, IS IP 2176.94 6894.45 4608.61 5.671 1 791 2.679 168 REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES Calcium IS IS IP 2028.20 6574.59 4227.91 6.087 1.878 2.920 Caesium la la ITT 3191.37 f 8523 .33 \ 8945.82 3.873 f 1.447 \ 1 .378 Rubidium la la IT 2968.70 f 7802.39 \ 7949.76 4.154 f 1 .580 \ 1.551 Potassium Iff la lir 2856.76 f 7666.95 \ 7701 . 13 4.317 f 1 .608 \ 1.601 Sodium la lalv 2412.84 f 5891.78 ] 5897.76 5.111 f 2.093 \ 2.091 Lithium la lalw 2299.67 f 6709.94 \6710.08 5.362 j 1.838 \ 1.838 Zinc ethyl Zinc chloride Mercuric chloride . Carbon monoxide. R. P. 7 volts II. Attention is drawn to the following recent papers bearing on "Photo-Electricity." Foote and Mohler, P. R., 17, 394(1921), find the following ionizing and radiating potentials. i. P. 12 volts 12.9 volts 12.1 volts 10 . 1 volts 14.3 volts Joly, P. M., 41, 289 (1921) and Poole, P. M., 41, 347 (1921), dis- cuss the application of photo-electricity and the quantum theory to vision. Experiments on the photo-electric effect of the active materials of the retina are described. Stebbins, A. P. J., 53, 105 (1921), employed a photo-electric cell to study the fluctuations of the star Algol. Angerer, P. Z., 22, 98 (1921), made use of a photo-electric cell to study the after glow in active nitrogen. In connection with the point raised by Elster and Geitel (p. 107), Angerer was able to study rather rapid changes of light intensity satisfactorily. Sir J. J. Thomson, P. M., 41, 526 (1921), calculated the ionizing potentials for a number of elements on the basis of his theory of the atom. The ratio of the ionizing potential for Li to that for H is of the right order; the ionizing potential for Na appears to be considerably smaller than the experimental value. The theoretical REPORT ON PHOTO-ELECTRICITY: A. LL. HUGHES 169 values for O and N cannot be compared directly with the experi- mental values which relate to O 2 and N 2 . A good discussion of critical potentials with reference to the Bohr type of atom will be found in Sommerf eld's "Atombau und Spektrallinien" (2nd edition). (The writer was unable to get a copy in time to make use of it in drawing up the report.) A full account of the work on hydrogen referred to on p. 139 under the names of Franck, Knipping and Kruger will be found in an article by Kruger in A. d. P., 64, 288 (1921). Hodgman, P. R., 17, 246 (1921), gives a list of color filters (dyes in gelatine) with transmission ranges (mainly in the visible), which may prove useful in photo-electric experiments. Data regarding the transmission of various colored glasses for use in securing mono- chromatic light when used in conjunction with the mercury arc, or the discharge in hydrogen or helium will be found in Technologic Paper No. 148, 1920 of the Bureau of Standards. Bulletin of the National Research Council Volume i Number I. The national importance of scientific and industrial re- search. By George Ellery Hale and others. October, 1919. Pages 43. Price 50 cents. Number 2. Research laboratories in industrial establishments of the United States of America. Compiled by Alfred D. Flinn. March, 1920. Pages 85. Price $1.00. Number 3. Periodical bibliographies and abstracts for the scientific and technological journals of the world. Compiled by R. Cobb. June, 1920. Pages 24. Price 40 cents. Number 4. North American forest research. Compiled by the Com- mittee on American Forest Research, Society of American Foresters. August, 1920. Pages 146. Price $2.00. Number 5. The quantum theory. By Edwin P. Adams. October, 1920. Pages 81. Price $1.00. Number 6. Data relating to X-ray spectra. By William Duane. November, 1920. Pages 26. Price 50 cents. Number 7. Intensity of emission of X-rays and their reflection from crystals. By Bergen Davis. Problems of X-ray emission. By David L. Webster. December, 1920. Pages 47. Price 60 cents. Number 8. Intellectual and educational status of the medical pro- fession as represented in the United States Army. By Margaret V. Cobb and Robert M. Yerkes. February, 1921. Pages 76. Price $1.00. Volume 2 Number 9. Funds available in 1920 in the United States of America for the encouragement of scientific research. Compiled by Callie Hull. March, 1921. Pages 81. Price $1.00. Number 10. Report on photo-electricity including ionizing and radiating potentials and related effects. By Arthur Llewelyn Hughes. April, 1921. Pages 87. Price $1.00. UNIVERSITY OF CALIFORNIA LIBRARY Los Angeles This book is DUE on the last date stamped below. APR 2 5 1958 N, JUN 9 1961 AUG 19 2 3 1969 ore 2i 0C 2 ROT 30N 4 1972 JUN 4 fctt MAY 3 1973 MAR 2 199! Form L9 100m-9,'52 (A3105)444 uic sscivtv*; wi me . April, 1920. Pages 45. Price 50 cents. Number 10. Report on the organization of the International Astronomi- cal Union. Presented for the American Section, International Astronomical Union, by W. W. Campbell, Chairman, and Joel Stebbins, Secretary. June, 1920. Pages 48. Price 50 cents. Number n. A survey of research problems in geophysics. Prepared by Chairmen of Sections of the American Geophysical Union. October, 1920. Pages 57. Price 60 cents. Number xa. Doctorates conferred in the sciences in 1920 by American universities. Compiled by Gallic Hull. November, 1920. Pages 9. Price 20 cents. Number 13. Research problems in colloid chemistry. By Wilder D. Bancroft. (In press.) Number 14. The relation of pure science to industrial research. By John J. Carty. October, 1916. Pages 16. Price 20 cents. BRAKY Y OF CAL1FOBNU LOS ANGBLBS UCLA-Physics Library QC 715 H87r L 006 583 541 5 AA 001 029 441 1 QC 715 H87r Physics Librarj