Ultrafast spectroscopy of defects

The intrinsic luminescence quenching of the F center in NaI and NaBr has been the subject of discussion for many years. The key question was whether the nonradiative electronic transition to the ground state after optical excitation occurs during or after lattice relaxation in the excited electronic state. Ultrafast time resolved techniques showed that in the case of NaBr the electronic transition predominantly occurs from the relaxed excited state (RES), whereas in NaI the electronic transitions during and after lattice relaxation have a comparable efficiency. A similar question raises when the luminescence is quenched by aggregation of the F-center with a molecular impurity. In addition one wants to characterize the electronic-vibrational (e-v) energy transfer associated with the nonradiative F-center relaxation by measuring the population of the vibrational levels after the transfer. Ultrafast spectroscopy can contribute to a better understanding of the e-v transfer process in particular in the case of the OH − perturbed F center: Because of the stronger quenching of electronic luminescence and the much faster (nonradiative) vibrational decay for OH − , much fewer information is available yet than in the case of CN − .