PICOSECOND RADIATIONLESS RELAXATION OF THE F-CENTER IN NABR INDUCED BY HYDROGEN IMPURITIES

With a pump-probe laser-induced transparency technique the ground-stat e recovery of the F center in hydrogenated NaBr is measured. For a ran dom distribution of F and substitutional hydrogen (H-s(-)) centers a n onexponential F-center relaxation is observed, faster than the one in undoped crystals. The aggregation of F and H-s(-) centers by subsequen t optical bleachings is monitored by means of resonant Raman scatterin g. The corresponding relaxation measurements reveal three decay compon ents, which we associate with the formation of F-H(H-s(-)) and FH2(H-s (-)) centers and to the remaining F centers. The relaxation time of th e F-H(H,) center is established to be similar to 290 ps, which is 20 t imes faster than the one of the pure F center. An estimate of the deca y rate assuming dipole-dipole coupling between the F-center electron a nd the H-s(-) vibration shows that this interaction may play an import ant role in reducing the lifetime of the excited F center. The frequen cy shift of the hydrogen vibration of the F-H(H-s(-)) center with resp ect to the one of the. H-s(-) centers can not be explained by dipole-d ipole coupling only. This is possibly related to a displacement of the H-s(-) equilibrium position of the F-H(H-s(-)) center, away from the lattice site.