Experimental investigations of exciton spectra in ionic crystals

Abstract An account is given of the experimental results which have been obtained since 1953 in the author's laboratory for the spectroscopy of crystals. It has been shown that, with a number of crystals, including Cu2O, CuCl, CuBr, CuI, AgI, HgI2, PbI2, TlCI, TlBr, and TlI, line spectra can be observed at low temperatures in absorption and in emission. The reflection spectra have also been studied. The occurrence of a line spectrum appears to be a characteristic feature of pure ionic crystals and although the subject is still under investigation it is now possible to outline the general properties of these spectra. Three classes of line spectra have been observed. The spectre of two of the classes are in reasonable agreement with the predictions of recent theories of exciton spectra while those of the third class are most probably also exciton spectra of a type which has not yet been treated theoretically. Other interpretations are, however, possible. When very thin samples (0·1 μ) are used for the observation of spectra of the first class, there is only a weak continuum and the spectra consist mainly of a few strong characteristic lines (‘raies ultimes’); these might possibly be used for analytical purposes. The intensities of lines in the spectra of the third class are extremely sensitive to the quality of the crystal. The results of our experiments are in qualitative agreement with recent theories of exciton spectra. It will, however, be necessary to extend both the experimental and the theoretical work before an entirely satisfactory interpretation of the spectra of solids can be given.