Radiative and nonradiative processes ofFandF′centers in NaBr and NaI

The Dexter et al. criterion for the occurrence of luminescence in deep centers predicts for $F$ centers in NaBr and NaI a strong quenching of the luminescence due to a crossover process between the excited-state- and ground-state-configuration coordinate curves, implying that most optical absorptions do not lead to the relaxed excited state. We have studied the optical absorption, conversion, and emission properties of pure (dilute and concentrated) $F$-center systems in zone-refined NaBr and NaI with the following results: An infrared emission of double spectral structure is present in both systems, but has very low quantum efficiency (<0.01). A detailed study of the electronic kinetics shows that in the optical excitation cycle all electrons reach the relaxed excited state, from which they can be ionized (thermally or by tunneling) to form $F^{\prime }$ centers with full efficiency. Therefore, while the above criterion predicts the existence of nonradiative processes properly, the mechanism of crossover reactions is not confirmed, and a nonradiative rate process from the relaxed excited state is strongly suggested. The role of the luminescence due to electron recapture by ionized $F$ centers has been examined, and it is proposed that the second observed luminescence is due to a radiative recapture process from a higher state.