ULTRAFAST SPECTROSCOPY OF DEFECTS

The intrinsic luminescence quenching of the F center in NaI and NaBr h as been the subject of discussion for many years. The key question was whether the nonradiative electronic transition to the ground state af ter optical excitation occurs during or after lattice relaxation in th e excited electronic state. Ultrafast time resolved techniques showed that in the case of NaBr the electronic transition predominantly occur s from the relaxed excited state (RES), whereas in NaI the electronic transitions during and after lattice relaxation have a comparable effi ciency. A similar question raises when the luminescence is quenched by aggregation of the F-center with a molecular impurity. In addition on e wants to characterize the electronic-vibrational (e-v) energy transf er associated with the nonradiative F-center relaxation by measuring t he population of the vibrational levels after the transfer. Ultrafast spectroscopy can contribute to a better understanding of the e-v trans fer process in particular in the case of the OH- perturbed F center: B ecause of the stronger quenching of electronic luminescence and the mu ch faster (nonradiative) vibrational decay for OH-, much fewer informa tion is available yet than in the case of CN-.