Infra-red luminescence of zinc and cadmium sulphide phosphors and the possible role of cation vacancies

Some of the outstanding difficulties of the interpretation of the banded emission spectra (1·3-2·2 μm range) in zinc and cadmium sulphide in terms of internal transitions within the Cu 2+ ion are discussed. Determinations of the decay times for the infra-red emission in cadmium sulphide for various exciting conditions at different temperatures confirmed that these difficulties exist. It is proposed that the cation vacancy may constitute the luminescence centre and an LCAO-MO treatment is developed to provide quantitative evidence for this argument. By combining experimental and theoretical results it was possible to derive two models for the vacancy which adequately explain all the infra-red emission and absorption properties of this centre. A description of energy level models in terms of the many-electron wave function approach provides a much more realistic account of luminescence centres than the one-electron functions normally adopted. The vacancy models are self-consistent and are flexible enough to explain many of the observations of infra-red stimulation and quenching effects of visible luminescence in zinc sulphide.