EXCHANGE A STUDY OF THE EXCITING POWER FOR FLUORESCENCE OF THE DIFFERENT PARTS OF THE ULTRAVIOLET SPECTRUM A THESIS PRESENTED TO THE FACULTY OF THE GRADUATE SCHOOL OF CORNELL UNIVERSITY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY BY LELAND JAYNES BOARDMAN Reprinted from PHYSIC A i . S. S., Vol. XX, No. 6, December, 1922. A STUDY OF THE EXCITING POWER FOR FLUORESCENCE OF THE DIFFERENT PARTS OF THE ULTRAVIOLET SPECTRUM A THESIS PRESENTED TO THE FACULTY OF THE GRADUATE SCHOOL OF CORNELL UNIVERSITY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY BY LELAND JAYNES BOARDMAN Reprinted from PHYSICAL REVIEW, S. S., Vol. XX, No. 6, December, 1922, A STUDY OF THE EXCITING POWER FOR FLUORESCENCE OF THE DIFFERENT PARTS OF THE ULTRAVIOLET SPECTRUM. 507754 [Reprinted from THE PHYSICAL REVIEW, S.S., Vol. XX., No. 6, December, 1922. A STUDY OF THE EXCITING POWER FOR FLUORESCENCE OF THE DIFFERENT PARTS OF THE ULTRAVIOLET SPECTRUM. BY LELAND JAYNES BOARDMAN. SYNOPSIS. Intensity of Fluorescence as a Function of Wave-length of Exciting Light, 0.55 to 0.2 fj.. The purpose of the experiment was to determine what wave-lengths are effec- tive in excitation and what relations exist between these wave-lengths and the corresponding absorption and fluorescence spectra. Light from a source giving a continuous spectrum was dispersed by means of a quartz spectrograph and allowed to fall on the substance to be studied which was spread on a flat surface. Then the parts of the spectrum which excited fluorescence were observed or photographed by means of the fluorescent light. A preliminary study of seventy substances showed that all the oxides (20) and simple chlorides (8) tested were not excited, a few substances (7) including zinc silicate, zinc sulphate and cadmium phosphate fluoresced faintly, a few responded well (anthracene, eosin, fluorescein, phenol- phthalein, calcium tungstate, and phosphorescent willemite), while the uranyl compounds (20) fluoresced strongly. For the last group the effective spectrum extended from 0.55 to 0.35 M only, while for the others it extended continuously to 0.2 n except in the case of four substances for which light from 0.35 to 0.325 was ineffective. Excitation band spectrum for twelve uranyl compounds was determined by measuring the density of the plates as a function of the wave-length by means of a sensitive photoelectric spectrophotometer. Some curves are reproduced and the wave-numbers corresponding to from 35 to 105 maxima for each compound are given. Comparison with absorption spectra shows close agreement, an absorption band corresponding to an excitation band in every case. This relation had pre- viously been found by Howe to hold for phosphorescent sulphides. Absorption Spectrum of Twelve Uranyl Compounds, from 0.55 to 0.32 /JL. Because of the correspondence noted above the excitation bands may be taken to be absorp- tion bands and thus the known absorption spectrum be considerably extended toward both the red and ultraviolet. Comparison of these bands with the fluorescence spectrum indicates clearly many new reversing regions where the fluorescent light obscures the absorbing effect. These are listed. INTRODUCTION. THE purpose of this investigation is to study the behavior of different portions of the ultraviolet spectrum as regards the ability of exciting fluorescence. The major part of previous work in fluorescence has been confined to a study of the fluorescence and absorption spectra of various materials and the relations between the two. This enables one to describe the phenomena, or state what happens as the result of the mechanism producing fluorescence. It also throws some light on the nature of the mechanism itself. VOL. XX. J ULTRAVIOLET SPECTRUM. 553 It seemed probable that something could also be learned about this mechanism by studying the means by which it is set in operation: in other words by studying the conditions and means by which fluorescence is excited. As a part of this problem it is of interest to determine what wave-lengths are effective in excitation and what relations exist between the excitation, absorption, and the fluorescence spectra. Almost the only work that has been done along this line is that of Stokes, 1 and, for a certain group of materials, the work of Lenard. 2 The method of the present investigation is similar to that used by Stokes. Quartz was however used in place of glass, and better sources of ultraviolet light were employed. Fig. i shows the arrangement of the s yy fr/1 apparatus. Light from the source 5 passed through a spectrograph, and then fell upon the fluorescent substance mounted in the plane of the plate-holder of the spectro- graph at A . With the room darkened the fluorescence was studied for color and relative intensity by the eye at E. For more accurate study a photo- graph was taken by means of a camera lens L and a plate at P. In this way the exciting power of any region of the ultraviolet light was easily determined, since the dispersed light fell immediately upon the specimen spread out to intercept the entire beam of light. It was necessary to use a light source which gives a continuous ultra- violet spectrum as free as possible from lines or bands. An electric spark under water was quite satisfactory for a large range of the ultra- violet spectrum. Other sources of light could be used to better advantage however in the visible and near ultraviolet region, since it was very difficult to maintain a spark in water for a sufficiently long time to give a proper exposure for the regions of weak fluorescence. Some photo- graphs were however obtained by long excitation by the spark. By the method of producing the under-water spark used in this work, which is described in another paper soon to be published, it was possible to main- tain a vigorous spark discharge 8 mm. in length for half an hour in dis- tilled water, by means of a Tesla coil operated by a transformer of one kilowatt capacity. It was found that a 400 c.p. nitrogen-filled glass-bulb Mazda lamp gave sufficient intensity in the near ultraviolet, and this was used when 1 Stokes, Phil. Trans., p. 463, 1852. 2 Lenard, P. Ueber Lichtemission und deren Erregung. Annalen der Physik, 31, p. 641- 1910. 554 LELAND JAYNES BO A RDM AN. [?S! possible because of its greater convenience. For somewhat shorter wave- lengths a similar lamp with a quartz bulb was used. Two spectrographs were used : the Fuess type, which gives a spectrum about 5 cm. long, range 580 my. to 200 m^, and a Hilger instrument which gives five regions (5 settings) of the spectrum with a total length of about 30 cm., range 800 m\i to 205 ra/z. This spectrograph, which proved to be excellent for the purpose, is constructed so that five adjustments of its parts can be made for each of the five ranges or parts of the spectrum. The ranges are: 800 to 400 WM, 400 to 305, 305 to 255, 255 to 225 and 225 to 205. The five adjustments are: position of collimating lens, angle at which the prism is set, the angular position of the arm carrying the plate-holder, the position of the objective lens, and the angle which the plane of the plate-holder makes with the axis of the plate-holder arm. The data furnished by the makers for the various adjustments were corrected for this particular instrument and plotted in such a way as to show the relation between each variable and the corresponding range. It was found that practically linear relations existed, so that it was possible to set for any intermediate range desired by merely inter- polating between the given settings. A split quartz prism was used. PRELIMINARY STUDY. A visual study of a great number of substances was made in order to find out what parts of the ultraviolet spectrum were most capable of exciting fluorescence, and also what substances respond best to such excitation, and were therefore suitable for further study. The Fuess spectrograph was mounted in such a way as to make the plane of the plate-holder horizontal. The substance was spread out on a piece of glass or stiff paper and held in the plane of the plate-holder. The follow- ing substances were examined in this way, a record being made in each case of the amount and position of the fluorescence excited by the ultra- violet only (since the dispersion in this region was good whereas the visible part was very narrow). The uranyl compounds exhibited the strongest fluorescence. They are given in the order of relative intensity, the first being the brightest. 1 Rubidium uranyl nitrate, Lead uranyl acetate, Rubidium uranyl sulphate, Ammonium uranyl nitrate, Potassium uranyl sulphate, Mercury uranyl acetate, Potassium uranyl nitrate, Calcium uranyl acetate, Potassium uranyl chloride, Strontium uranyl acetate, 1 The following uranyl compounds were found to fluoresce most strongly under x-ray excitation: Rubidium uranyl nitrate, rubidium uranyl sulphate, cesium uranyl chloride and lithium uranyl acetate. VOL. XX. No. 6. ULTRAVIOLET SPECTRUM. 555 Uranyl acetate, Sodium copper uranyl acetate, Silver uranyl acetate, Uranyl tellurate, Thanous uranyl sulphate. Ammonium potassium uranyl chloride, Cesium uranyl chloride, Lithium uranyl acetate, Cadmium uranyl acetate, Barium uranyl acetate, Other substances responding well to ultraviolet excitation were: Anthracene, If variable but continuous excitation between 550 and Phenolphthalein, j } 200 m/*. Calcium tungstate, j ["excitation between 550 and 350 mju and Phosphorescent willemite, I between 325 and 225 mju approximately. Fluorescein, Last two substances dissolved in water Eosin, J I or alcohol. The following substances fluoresced faintly by ultraviolet excitation: CaC 2 O 4 , Cadmium phosphate, Phosphorescent calcite. Zinc silicate, Sodium uranyl cobalt acetate. Zinc sulphate, Sodium molydate. The following substances were practically unaffected by ultraviolet excitation : Cesium chloride, Didymium chloride, Lead chloride, Naphthol yellow, Potassium iodide, Rubidium chloride, Barium chloride, Barium sulphate, Berylium chloride, Calcium fluospar, Calcium sulphate, Cadmium iodide, and the oxides : A1 2 O 3 , CaO, BaO, CeO 2; Bi 2 O 3 CeO, Sodium chloride, Sodium silicate, Telluric acid, Thallun sulphate, Tungstic acid, Vanadium chloride, Cr 2 O 3 , PbO, NiO, SiO, UO 3 , CuO, MgO, Ni 2 O 2 , SnO 2 , ZnO. FeO, MnO, Sb 2 O 3 , TeO 2 , Three general types of excitation were observed. First, a broad con- tinuous region of excitation, somewhat variable in intensity and becoming gradually weaker further out in the ultraviolet. Second, strong in the violet and near ultraviolet to about 350 mju where the fluorescence seems to dissappear over about 25 m/* then to reappear over a region about 100 m/x long with a maximum at about 275 m/z. Third, strong in the violet and near ultraviolet to about 350 m/x where the intensity drops very rapidly to practically zero in some cases or to a relatively small value beyond which point the intensity gradually fades away. No fluorescence was observed beyond 200 m/j,. Anthracene is a good example of the first type, calcium tungstate and fluorescein are good examples of the second type, and the uranyl compounds illustrate the third type. 556 LELAND JAYNES BOARDMAN. Photographs were obtained of the fluorescent light emitted from nearly all of the substances listed above which show any appreciable effect. The means of mounting the material is described in the following para- graph. It was very difficult to secure good photographs in some cases, particularly with the liquid solutions of fluorescein and eosin. The Fuess spectrograph was used and a camera lens of 7 inches focal length. Exposures ranging from one minute to thirty minutes were necessary. METHOD OF MOUNTING THE FLUORESCENT SUBSTANCE. It was necessary to have the fluorescent material offer a smooth surface to the exciting light in order to obtain a good record on the photographic plate, to which end the following simple method was used. A strip of varnish or glue about two centimeters wide was made across the whole width of the plate, and the powdered substance was sprinkled over this with sufficient depth to cover it completely. Another glass plate was then used to press upon on rub this surface till it was made as smooth as possible. Care was taken not to leave any part of the surface in a matte white condition due to rubbing, as such a part may appear on the photograph to be different from its surroundings. In case of some of the uranyl salts the natural crystals are very hard to reduce to fine enough granulations to make a smooth surface possible in this way. Some of the photographs show this. In case of liquids or solutions a strip of quartz was used, and it was mounted in such a way as to cover the portion of liquid to be exposed. FURTHER STUDY OF THE URANYL COMPOUNDS. It was mentioned above that the uranyl compounds were excited to fluorescence by wave-lengths of light lying between 550 m/* and 350 m/x. (Excitation by shorter wave-lengths than this was very feeble, too faint indeed to be photographed.) In the most intense part of the excitation, as photographed by the Fuess spectrograph, there appears to be a short region in which the excitation is variable, giving what might be called bands, and this region is the same as that in which absorption bands for these same materials are found. Now fluorescence is undoubtedly due primarily to absorption, though it cannot be assumed that greater absorption will produce proportionately greater fluorescence. It seems reasonable nevertheless to expect that there may be some variation in the excitation where there is variable absorption. If it is true that the variation in the intensity of the fluorescence is due to the changing absorption of the dispersed exciting light, this method might well be used in detecting and studying the absorption of materials, for if light of a No L 6 XX> ] ULTRAVIOLET SPECTRUM. 557 particular wave-length is absorbed and this excites fluorescence which emanates from the same, or neighboring, spot, it would be very easy to observe this photographically in case there is good dispersion of the exciting light. The work that follows is an attempt first to test the validity of the assumption by seeing if fluorescence bands do occur where absorption bands are known to exist, and, if the results confirm the assumption, to locate the position of as many absorption bands as this method is capable of giving, and thereby to test the laws which have been found to govern the arrangement of the bands previously observed. The uranyl salts are particularly good for this work because they give many narrow bands, both in the fluorescence region of the spectrum and in the absorption region. THE FOLLOWING ARE THE COMPOUNDS TESTED: 1. Barium uranyl acetate, 7. Cesium uranyl chloride, 2. Lithium uranyl acetate, 8. Potassium uranyl chloride, 3. Mercury uranyl acetate, 9. Potassium uranyl nitrate, 4. Strontium uranyl acetate, 10. Rubidium uranyl nitrate, 5. Uranyl acetate, 1 1. Cesium uranyl sulphate, 6. Sodium copper uranyl acetate, 12. Rubidium uranyl sulphate. Nichols and Howes 1 in their recent treatise entitled "Fluorescence of the Uranyl Salts" give a summary of results showing the distribution and character of the fluorescence spectrum and the absorption spectrum of many uranyl compounds. It is shown that the spectrum of the fluorescent light consists of bands which naturally form eight groups of five members each, approximately, ranging from 640 m/z to 490 m/*, or thereabouts. The absorption spectrum consists also of bands having about the same arrangement, but ranging from 490 my to 380 mju, appar- ently a continuation of the fluorescence spectrum. The frequency inter- vals (reciprocal wave-length intervals) between homologous bands of each group are practically the same throughout the spectrum of either, the interval being about 86 in the fluorescence spectrum and about 70 in the absorption spectrum. The last member of the fluorescence series is usually coincident with or at a distance from the first member of the absorption series of the homologous band of 86 or 70 frequency units. Considering all of the series of the various homologous bands there is an overlapping of the fluorescence and absorption spectra of about three groups, called the "reversing region" because here there are coincidences of fluorescence and absorption bands. In this work a glass-bulb nitrogen-filled tungsten lamp, running at 6 amperes, was used as a source of ultraviolet light. A quartz mercury 1 Nichols and Howes, Carnegie Inst. Wash. Pub., No. 298, 1919. 558 LELAND JAYNES BOARDMAN. [SECOND [SERIES. lamp was used 1 for calibration purposes, the calibration spectrum being photographed alongside of the other spectrum by exposing an adjacent portion of the slit of the spectrograph, both exposures taking place at the same time. The intensity of blackening of the plates was measured by means of a device set up by J. O. Perrine in connection with his work 2 on " A Spectro- graphic Study of Ultraviolet Fluorescence Excited by X- rays." The apparatus makes use of a photo-electric cell, C in Fig. 2, and a sensitive galvanometer G. The cell, which was selected after trial of several types, was made by Kunz. 3 A Leeds and Northrup type C galvanometer was used about 6 meters from Fig. 2. the scale near the comparator and cell. The comparator carried the photographic plate P just under a slit 2 mm. by .25 mm., through which a strong beam of light from an incandescent lamp L was passed. The lamp had one filament carrying a current of about 6 amperes from a storage battery. The current could be adjusted to meet the needs of the plate. is a constant potential dry battery of about 80 volts. Further description of the apparatus is given in the paper just cited. Much of the success of the present work is due to this apparatus. The instrument, which is highly sensitive to any variation in the density of the photographic image, has many advantages over other methods such as those depending on the eye, but faulty places in the photograph must be carefully avoided or eliminated by comparison with other photo- graphs. METHOD OF PLOTTING. Curves were first made from the galvanometer deflections and the positions on the comparator. The intensities of the light transmitted by the negative were plotted as ordinates and the comparator readings as abscissae, these plots being made while the measurements were being made, i.e., plotting rather than recording the numbers. The intensities are merely relative, so that a convenient arbitrary scale was chosen to represent them. The positions of the mercury lines were located on the 1 The well-known, yellowish green fluorescence of these substances is of a color to which most photographic plates do not respond well. The most satisfactory of the numerous plates tested was found to be the polychrome plate made by the Eastman Kodak Company. 2 J. O. Perrine; Thesis in M.S. in Cornell University Library. 3 Kunz and Stebbins, PHYSICAL REVIEW, 7, p. 282. VOL. XX.1 No. 6. ULTRAVIOLET SPECTRUM. 559 plot by observing the comparator reading for the smallest deflection of the galvanometer; thus the densest part of the line was taken as the proper position of the line. A setting could be made to the nearest tenth of a millimeter. Five to seven mercury lines were thus located on each plot. Knowing their wave-lengths to four significant figures their reciprocals were plotted (on another sheet) as ordinates against their recorded positions on the plot as abscissae, and thus a calibration plot was obtained, giving the reciprocal wave-lengths for any comparator reading on the first plot. Using this calibration plot the final plot was obtained where the intensities of the light were plotted against the reciprocal wave-lengths. A new calibration plot was used whenever the mercury lines were found to be spaced differently due to different magnifications caused either by a change in the settings of the spectro- graph or by a change in the camera adjustment. Final curves were made in this way for each of the twelve uranyl compounds. In these plots a millimeter represents 2.5 frequency units, 1 and one tenth of a millimeter on the photographic plate. The accuracy of the data recorded in Tables I. to V. is such that the results are prob- ably correct to within three frequency units. An estimated "probable error" is near one frequency unit. Typical curves are presented, greatly reduced, in Figs. 3, 4, 5 and 6. LITHIUM URAf/YL ACETATE TH Fig. 3. J A frequency unit is such that 500 mju corresponds to 2000 frequency units, i.e. i .000000500 in meters 2000 X io 3 . 56o LELAND JAYNES BOARD MAN. [SECOND [SERIES. Peaks in the curves represent regions of greater fluorescence. They are undoubtedly regions of greater absorption, and many of them coincide with absorption bands of these substances that have been located by LITHIUH URANYL ACETATE 2 ,J I (I, , 1 1, .IT, Jl 11(1,1 II, ,1, ,1111 ,1,1.1 1 M '/#?