Ground-state recovery measurements on the F center in NaI were performed with time resolutions of both? ps and 200 fs. The measurements con6rm 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 kom 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. "

The dephasing relaxation times T2 of the Raman-active J =2 rotons in para-Hz crystals, intentionally doped with HD impurities, were measured by means of the time-resolved stimulatedRaman-gain (TRSRG) technique. The relaxation rates T2 corresponding to the spectral linewidth 5v=(~T2) are found to increase linearly with HD concentration with the slopes 0. 148+0.05 cm ', 0. 126+0.05 cm ', and 0. 140+0.05 cm ' per percent for the~M~=0, 1, and 2 rotons, respectively, which, within experimental error, are the same as in the case of ortho-H2 impurities. This result demonstrates the equivalence of HD and ortho-H2 for scattering of J =2 rotons in para-H, .Furthermore the previously suggested possibility that the extrapolated values for the roton relaxation rate in pure para-H2 would originate from HD impurities at the natural isotope concentration (cHD =0.03%) is eliminated.

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With a picosecond pump-probe laser technique the recovery of the ground-state population after optical excitation of the F center in NaBr is studied. Two decay components are observed, the fastest one dominating at low temperatures. Its time constant is interpreted as the lifetime of the relaxed excited state and is established to be 6+1 ns at 10 K. This value is perfectly consistent with the experimental emission efEciency and the expected radiative lifetime, provided that the nonradiative transition to the ground state is assumed to occur after the relaxed excited state is reached. Within the same assumptions a nonradiative lifetime of 27 ns at zero temperature is predicted for the F center in NaI. A theoretical expression for vibronic tunneling to the ground state from a thermalized excited state is applied to the F center in alkali halides and is shown to agree equally well with the experimental emission efBciencies as the Dexter-Klick-Russell criterion. The same expression also accounts for the strong temperature dependence of the nonradiative relaxation process observed in NaBr below 100 K. At higher temperatures an accurate analysis of the radiationless transition rate is encumbered by the infiuence of ionization on the lifetime of the excited state. The relative contribution of the second, much slower decay component increases rapidly above 70 K and is related to retrapping of electrons released by ionization of F and F' centers.

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