ELECTRONIC, VIBRATIONAL, AND CONFIGURATIONAL RELAXATION OF THE F-H(OH-) CENTER IN KBR

The relaxation after optical excitation of the F-H(OH-) center in KBr is studied with a picosecond pump-probe technique for induced transpar ency. Three different relaxation components can be distinguished: (i) a nearly temperature-independent component decaying in a few ps; (ii) a component which decays slower than 10 ns at all temperatures; and (i ii) a strongly temperature-dependent component with a time constant of the order of 100 ps at 50 K and at least 10 ns below 20 K. We observe essentially no effect on the relaxation time of the components under OH--->OD- substitution. Because of its picosecond time scale, its temp erature independence, and the Raman measurements presented in an earli er paper, we identify the first component as a radiationless electroni c transition during lattice relaxation, which occurs mainly near the f irst crossing point reached. This corresponds to the excitation of one quantum of the stretch vibration. Because the other components change from induced transparency to induced absorption under probe-wavelengt h variation, they are very probably not related to electronic relaxati on processes. The nanosecond component is interpreted as vibrational r elaxation. It appears in the relaxation scans as a result of the influ ence of the stretch vibration on the electronic absorption. Effects of the probe power on the relaxation measurements below 30 K, show that also optical conversion between the two KBr:F-H(OH-) configurations is involved in the relaxation process. These configurations possess diff erent electronic absorption bands and are essentially different orient ations of OH- with respect to the F center. The third, strongly temper ature-dependent component is associated with the recovery of the therm al equilibrium between these configurations.