New Results on the Decay Properties of Perturbed and Unperturbed M‐Centers in NaCl:CaCl 2

Lifetime measurements concerning the relaxed excited state of the perturbed and the unperturbed M‐center in X‐ray colored NaCl:CaCl 2 samples, give the following results: a) the decay spectra are characterized by two components, due to the perturbed ((18.2 ± 0.5) ns at 9 K)) and to the unperturbed ((26.5 ± 0.5) ns at 9 K) M‐center respectively, in the temperature range 9 to 320 K: the component due to the perturbed M‐center disappears at higher temperatures; b) the lifetime of the M‐perturbed center is a decreasing function of the temperature contrary to what happens in KCl samples doped with divalent impurities, where it is constant up to room temperature; c) no difference from the data obtained in undoped NaCl samples is noted for the unperturbed M‐center and it is shown, for the first time, that its lifetime decreases above 310 K; d) the simple scheme of a two‐channel decay process is not valid for the perturbed M‐center whereas it enables to fit with accuracy the data concerning the unperturbed M‐center (by assuming a de‐excitation energy value equal to (0.46 ± 0.02) eV). An analysis of the optical properties of the M‐perturbed center in NaCl:CaCl 2 samples leads to the assumption that the perturbative effect is low compared to that observed in KCl doped with divalent impurities: in particular, it seems to be inefficient as regards the absorption properties; this hypothesis also enables to explain the absence of formation of Z 4 ‐centers in the same samples.