Internal photoelectric absorption in halide crystals

Abstract

The possession of absorption bands in the ultra-violet by otherwise transparent crystals has been recognized for a long time as determining the dispersion of the medium. This absorption of radiation is one which produces no progressive change in the crystal. On the other hand, the illumination of crystals which gives rise to photochemical and photo-conduction phenomena causes a progressive change in the crystal and consequently in its absorption spectrum. Measurements have recently been made of the original and induced absorption spectra of many photo-conducting crystals, in the hope of elucidating the formation of the latent photographic image. 1. Quantum mechanics gives us entirely new ideas as to how to correlate the absorption spectra of crystals with their insulating properties. Both phenomena are alike concerned with the quantized electron levels of the crystal. These allowed levels belong not to individual atoms or ions but to the crystal as a whole. The wave functions representing the valence electrons are not localized at individual lattice points, but are oscillatory throughout the crystal. Even in an insulator they are the wave functions of an electron gas moving in the periodic field of the lattice. The opacity of a metal for all wavelengths from the infra-red far into the ultra-violet is due to the fact that electron transitions are possible to vacant levels of all higher energies. For other pure crystals, however, the allowed bands of levels are broken up by wide zones of disallowed energies; and it is these zones which at the same time give to a crystal the properties both of transparency to light and non-conduction to electric current. More exactly, these properties are due to the fact that a, zone of forbidden energies divides a band of levels, which at low temperatures is completely filled with electrons, from a similar higher band which is completely empty. For crystals which are transparent to all visible light we can at once say that the width of this particular zone must be more than 3 ε-volts; for the alkali halides which are transparent into the Schumann region it must be at least more than 5 or 6 ε-volts. When illuminated with ultra-violet light of the right wave-lengths, however, such a crystal is almost on a par with a metal; it contains as many electrons, and the levels are similar; over narrow regions in the ultra-violet, therefore, it absorbs as if it were a conductor and shows selective metallic reflection. The theory of the insulating properties of these crystals has been discussed by A. H. Wilson. The same scheme of levels must be used for correlating conductivity with opacity, and insulation with transparency in the case of liquids, such as mercury and water.