Aggregates of F centers and OH- or OD- impurities in face-centered cubic alkali halides were studied by using resonant Raman scattering techniques. At low temperatures, 2 different configurations of the FH(OH-) aggregate were observed under optical pumping conditions in all of the hosts. At higher temperatures, the modes which were associated with the FH center disappeared. From the relative intensities of anti-Stokes Raman scattering data, it was deduced that the E-V energy transfer populated mainly the first level of the OH- or OD- stretching mode. With regard to the effect of Na+ impurities, it was suggested that 2 new modes which appeared were associated with a single aggregate center, which was described as: FAH(Na+OD-). This center was characterized by a slower reorientation of the OD- ion. One of the associated modes persisted up to 60K. Because it was believed that the ion between the F center and the OH- impurity played an important role in the reorientation of the FH center, it was tentatively assumed that the Na+ ion took up this position. It was also queried whether there existed a second, low-temperature configuration of the FAH center, which might not have been observed here because of the use of an excitation wavelength which was at the red end of the absorption band. It was recalled that absorption measurements, performed after prolonged optical conversion, had revealed an optical bistability in KCl that was doped with Na+ and OD-. However, it was concluded that this was probably not related to the simple FAH center because it persisted to above 130K. The E-V transfer efficiencies of the FAH center were difficult to determine precisely, because of the presence of nearby FH modes, but they were thought to be similar to that of the FH center.

E.Gustin, M.Leblans, A.Bouwen, W.Van Renterghem, D.Schoemaker, F.Luty: Materials Science Forum, 1997, 239-241, 477-80