Ground-state recovery measurements of the F center were carried out with time resolutions of 7ps and 200fs. The results confirmed the expected electronic relaxation, with a decay time of the order of tens of ns, which would be associated with a non-radiative transition from the relaxed excited state at low temperatures. In contrast with the case of NaBr, the decay channel which was related to the re-capture of conduction electrons could not be clearly distinguished. This was suggested to be related to the fact that the available time delay was considerably shorter than the ground-state recovery in the temperature range where the predominant contribution changed. It changed from a component which was related to the relaxed excited state lifetime, to a re-trapping component. Measurements of the ground-state recovery, at 200fs resolution, showed that a very rapid relaxation component was present for the F center in NaI. It exhibited a time constant of 9ps at low temperatures, and its relative contribution with respect to the total induced transparency signal was about 33%. The temperature dependence of its time constant closely followed the vibrational lifetime which was deduced from line-width measurements of the resonant Raman spectrum. This was to be expected of a contribution to the induced transparency which arose from an electronic transition during vibrational relaxation.

F.De Matteis, M.Leblans, W.Slootmans, D.Schoemaker: Physical Review B, 1994, 50[18], 13186-93