PHOTOGRAPHIC STUDIES OF LUMINESCENCE. 131 



THE PHOSPHORESCENCE SPECTRUM DURING DECAY AND THE QUESTION OF 



TWO OVERLAPPING BANDS. 



In Chapters IV, V, VI, and VII of this memoir, the characteristic form 

 of the curve of decay of phosphorescence for various substances and under 

 a variety of conditions has been given and it has been shown to consist of 

 two straight lines of different slopes which gradually merge into one another. 

 In Chapter VI it was shown, moreover, that Becquerel's 1 expression for 

 the decay could be made to fit the actual curves by assuming the existence 

 of two overlapping bands having very different rates of decay. 2 



In Chapter IV measurements on the phosphorescence of Sidot blende 

 were described and it was shown that in the case of Sidot blende the spec- 

 trum did not change its form during the first few seconds of decay. 3 A 

 similar result was obtained by Waggoner 4 (see Chapter VII) for several 

 other substances exhibiting phosphorescence of short duration. 



While the presence of overlapping bands of differing duration would 

 necessarily cause a gradual change in the distribution of intensities in the 

 phosphorescence spectrum as decay progressed, these measurements were 

 not decisive, as against the hypothesis of the existence of such bands. It 

 was found in the case of the Sidot blende that the "first process" of decay 

 lasted about 10 seconds, whereas the spectrophotometric measurements 

 covered only 3 or 4 seconds. Dr. Waggoner's determinations likewise apply 

 chiefly to the earlier stages of decay. 



The experiments now to be described were undertaken to provide further 

 data bearing upon the two-band theory of phosphorescence decay. The 

 two rectilinear parts of the typical decay curve differ widely in slope. 

 Hence, on the assumption of two bands, most of the light before the bend 

 in the curve is due to the band corresponding to the steeper straight line 

 (see Fig. 79, Chapter VI). At the bend the light is due more or less equally 

 to both bands; and after the bend the light is due mostly to the second 

 band. With a method available for studying the light distribution before 

 and after the bend, the two-band explanation would be proved or disproved, 

 depending on whether a change was or was not found in the distribution of 

 the light. 



As was shown in the first part of this chapter, the fluorescence spectrum 

 of the substance used consisted of one prominent, symmetrical, smooth- 

 sided band with a maximum at about ^ = 0.55 (see Fig. 133). The band 

 extends, approximately, from /jl == 0.46 to ^ = 0.60. Furthermore, the energy 

 distribution immediately after excitation (Fig. 137) is the same as in the 

 fluorescence spectrum. 



The present work is concerned with the light distribution before and 

 after the bend in the decay curve. 



H. Becquerel. Comptes Rendus, 113, p. 618, 1891. 

 2 See Chapter VI; also C. A. Pierce, Physical Review, xxvi, p. 312. 

 3 See also Nichols and Merritt, Physical Review, xxi, p. 247. 

 C. W. Waggoner, Physical Review, xxvn, p. 220. 



