Resolution and interpretation of the luminescent spectra of some solids at low temperatures

Abstract

Luminescent inorganic solids may be divided into a number of classes. Some, like the uranyl salts and rare earth salts, show luminescence in the pure state, and some, like the Lenard and Klatt sulphides, seem to require a Small trace of impurity. The alkali halides, which may be rendered luminescent by prolonged heating in the vapour of the alkali metal, by electrolysis, or by exposure to short waves, may possibly be regarded as belonging to the second class, since it its generally recognized that the luminescence is connected with the presence of neutral atoms which must be considered as impurities in the ionic lattice. The chromium-activated substances studied so extensively by Deutschbein (1932) appear to belong to a third class. The intimate relations which have been found between the line-like emission and absorption spectra of the first class suggests that their luminescence may be quite simply explained as a kind of resonance phenomenon. Indeed, in the case of the rare earths the spectra appear to have been satisfactorily interpreted in terms of electronic transitions of the metallic ion. But, in spite of the very large amount of work on the second class of luminescent substances, a satisfactory explanation of their luminescence is still lacking. This is chiefly due to the fact that their spectra generally consist of broad and featureless bands. Various workers have claimed to have resolved these very broad bands into a number of partial bands, and Schmidt (1932) and Schellenberg (1932) have attempted to establish general series relationships among the partial band maxima. The resolution, however, has been obtained partly by visual observations and partly by reconstruction of the broad bands from a number of assumed partial bands. Howes (1928) obtained by visual spectrophotometry distinct structure in the bands of some of the alkali earth sulphides. His results will be referred to later in this paper. Tanaka (1921) carried out an extensive series of observations on the cathodo-luminescence of a large number of substances activated by various impurities and deduced the existence of a frequency interval characteristic of the activator. But as many of his numbers are the result or rearranging the slight fluctuations which appear on the visually observed spectrophotometric curves, the validity of his deductions is very doubtful.