V. THE MORE INTIMATE STRUCTURE OF URANYL SPECTRA 



AS REVEALED BY COOLING. 



It was first shown by J. and H. Becquerel and Onnes, 1 who studied 

 the spectra of several of the uranyl salts when excited to fluorescence 

 at the temperature of liquid air and ultimately at that of liquid hydro- 

 gen, that each band of the spectrum as we know it at +20 is resolved 

 into a group of much narrower bands. It was further shown by these 

 investigators that all of the various groups of bands in a given spectrum 

 were resolved in precisely the same manner, the homologous com- 

 ponents forming series. 



This more intimate structure, which is revealed by cooling, may be 

 studied to great advantage in the case of the double chlorides, which 

 salts, as has been noted in Chapter III, have spectra sufficiently 

 resolved at +20 so that the origin of the components observed at 

 185 can be traced and the relation of the two spectra to one another 

 much more definitely determined than is the case where the spectrum 

 at +20 consists of unresolved bands. 



Four of these chlorides have the following composition : 



U0 2 C1 2 . 2KC1+2H 2 O. UO 2 Cl 2 .2RbCl+2H 2 O. 



U0 2 C1 2 . 2NH 4 C1+2H 2 0. UOCl 2 .2CsCl. 



They crystallize in triclinic plates which are strongly fluorescent 

 and their spectra, which are almost identical in structure, are resolved 

 at room temperature into 8 groups of narrow bands. Each group, 

 which corresponds to a single band of the ordinary uranyl fluorescence 

 spectrum, consists of 5 nearly equidistant bands. The symmetry of 

 these spectra, as they appear to the eye when viewed with a spectro- 

 scope of moderate dispersion, is most striking. The bands are well 

 separated from their neighbors and are about one-tenth as wide as the 

 bands of the ordinary type of uranyl spectra. 



The distribution of intensities within the group has been determined 

 for the visually brightest group in the spectrum of the ammonium 

 uranyl chloride by means of the spectrophotometer. The results of 

 such a determination are given in table 18, and are shown graphically 

 in figure 52. 



The curve (fig. 52) which forms an envelope of the group of bands is 

 of the same type as that for the distribution of intensities in a single 

 band of the ordinary uranyl fluorescence spectrum and of the envelope 

 of the set of bands in such a spectrum and is also similar to curves of 

 distribution of the fluorescent spectra having a single broad band. 2 



The effect of cooling is likewise analogous, the envelope for 185 

 being narrower on account of the great relative reduction in brightness 

 of the outlying members of the group. All the bands are shifted in 



1 Becquerel and Onnes, Leiden Communications, 110. 1909. 



2 See Chapters II and III. 



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