EVIDENCE FOR THE CROSSOVER PROCESS AFTER OPTICAL-EXCITATION OF THE F-CENTER IN NAI

Ground-state recovery measurements on the F center in NaI were perform ed with time resolutions of both 7 ps and 200 fs. The measurements con firm the expectation of an electronic relaxation with a decay time of the order of tens of nanoseconds, which can be associated with a nonra diative transition from the relaxed excited state (RES) at low tempera tures. Different from the case of NaBr, the decay channel related to t he recapture of conduction electrons cannot clearly be distinguished. This is possibly related to the fact that the available time delay is considerably shorter than the ground-state recovery in the temperature range in which the dominant contribution changes from the component r elated to the RES Lifetime to the retrapping component. Measurements o f the ground-state recovery with a 200-fs time resolution show that a very fast relaxation component is present for the F center in NaI. It possesses a time constant of 9 ps at low temperature and its relative contribution with respect to the total induced transparency signal is roughly 33%. The temperature dependence of its time constant follows c losely the vibrational lifetime derived from linewidth measurements of the resonant Raman spectrum. This feature is expected for the contrib ution to the induced transparency arising from an electronic transitio n during vibrational relaxation, the so-called ''crossover process.''