NONRADIATIVE RELAXATION AND IONIZATION OF THE F-CENTER IN NABR STUDIED WITH PICOSECOND OPTICAL PULSES

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 efficiency 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 efficiencies 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 influence 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.