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 performed with time resolutions of both 7 ps and 200 fs. The measurements confirm the expectation of an electronic relaxation with a decay time of the order of tens of nanoseconds, which can be associated with a nonradiative transition from the relaxed excited state (RES) at low temperatures. Different from the case of NaBr, the decay channel related to the 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 related to the RES lifetime to the retrapping component. Measurements of 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 closely the vibrational lifetime derived from linewidth measurements of the resonant Raman spectrum. This feature is expected for the contribution to the induced transparency arising from an electronic transition during vibrational relaxation, the so-called crossover process. © 1994 The American Physical Society.