It was again demonstrated that the presence of sufficient amounts of statistically distributed OH- defects drastically quenched F-center luminescence, photoconductivity, and excited state (F*) lifetimes. The present study tested whether the OH- reorientation rate played a decisive role in this strong OH- dipole F*-center interaction effect. In the case of KCl, at temperatures above 10K, it had been found that the OH- reorientation was extremely rapid. The present materials, at 10K, exhibited dipolar reorientation times which were long when compared with the radiative F-center lifetime. However, with increasing temperature the OH- dipolar reorientation times markedly decreased and became much shorter than the radiative F-center lifetime. Thus, in these materials, the F luminescence was nearly entirely efficient and had a long lifetime at temperatures below 15K. But it was largely quenched, and had a short lifetime, when the OH- reorientation rate increased with temperature. A quantitative analysis showed that the OH- reorientation time had to be some 4 orders of magnitude shorter than the radiative F* lifetime in order to quench the F luminescence fully. Unlike these statistically distributed F-center and OH- defect systems, aggregated pairs of both defects [FH(OH-)] in all of the materials exhibited a very strong luminescence quenching which was almost independent of temperature.

L.Gomes, F.Luty: Physical Review B, 1995, 52[10], 7094-101