Abstract
It is shown that the vibrational structure appearing in the luminescent emission close to the fundamental absorption bands of ZnS, CdS and ZnO corresponds to the frequency of the longitudinal lattice vibrations of the optical branch, either alone or in combination with the transverse vibrations of this branch. This proves that the edge emission is a property of the pure, unperturbed lattice. In connection with observations by Klick as well as from fundamental theoretical reasoning it is concluded that the fluorescence is due to excitons. Weak bands, appearing at shorter wavelengths are attributed to band-band transitions or to higher exciton states. The appearance of double peaks in the infra red absorption spectrum of ZnO and BeO is explained by a process which involves phonons of the optical and acoustical branch according to a theory given by Born and Blackman.
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