A Study of the Exciting Power for Fluorescence of the Different Parts of the Ultraviolet Spectrum

Intensity of Fluorescence as a Function of Wave-length of Exciting Light, 0.55 to 0.2 {μ}.---The purpose of the experiment was to determine what wave-lengths are effective 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, phenolphthalein, 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 {μ} only, while for the others it extended continuously to 0.2 {μ} 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 previously been found by Howe to hold for phosphorescent sulphides.Absorption Spectrum of Twelve Uranyl Compounds, from 0.55 to 0.32 {μ}.---Because of the correspondence noted above the excitation bands may be taken to be absorption 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.