82 



FLUORESCENCE OF THE URANYL SALTS. 



FL. 



So far as the spectra at +20 are concerned, we find that: 



(1) All absorption bands toward the violet from the reversing region 

 occur in series with constant-frequency intervals. 



(2) For every fluorescence series there is a corresponding absorption 

 series. 



Whether the relation between absorption and fluorescence outlined 

 above is significant can best be determined by the study of the spectra 

 for --185. 



If, for example, the explanation of the numerous instances of inexact 

 coincidence is valid, we should expect exact reversals of the components; 

 also that the components of the resolved absorption spectra form series 

 definitely related to the components of the fluorescence spectra in a 

 manner consistent with the system indicated for the spectra at +20. 

 From a study of the exactness of the reversals in the resolved spectra at 

 low temperatures it appears that 25 out of 38 fluorescence series are 

 certainly reversed and that 36 fluorescence series join absorption series 

 in the seventh group. The experimental error in this group does not 

 exceed 1.5 units. The difference in position between fluorescence and 

 absorption is sometimes greater than 1.5, but this may 

 be ascribed to the dissymmetry in the form of the bands. 



Fluorescence bands have their crest toward the violet, 

 absorption bands toward the red. In the case of reversals, 

 these regions tend to annul each other, leaving a rem- 

 nant of fluorescence on the red side and a remnant of 

 absorption on the violet. The result is that in regions 

 where fluorescence and absorption exist together, fluo- 

 rescence bands are apt to be given too great a wave- 

 length, and vice versa. In the C 2 series of the rubidium 

 chloride, for example, there is a displacement of 2.6 units 

 between the observed positions of fluorescence and 

 absorption. 



If, however, we compute the proper positions of these bands, using 

 the average intervals for the C 2 and c 2 series respectively, thus elimi- 

 nating the displacements in the reversal region, the fluorescence band 

 and absorption thus established agree in position within 0.3 unit. 

 The impossibility of excluding all absorption when fluorescence is 

 present, and the impossibility of preventing a tendency toward 

 fluorescence when absorption alone is sought for may well account 

 for the resulting displacement. The case of the C 2 series is not an 

 isolated one probably every reversal is affected somewhat and the 

 stronger bands the most; there being always an apparent shift of the 

 absorption band toward the violet and of the fluorescence band 

 toward the red. This phenomenon has long been recognized by the 

 authors in connection with broad fluorescence bands, and it must now 

 be recognized in the reversing of the narrow, line-like bands at the 

 temperature of liquid air. 



ABS. 



FIG. 68. 



