The relaxation after optical excitation of the ${\mathit{F}}_{\mathit{H}}$(${\mathrm{OH}}^{\mathrm{-}}$) center in KBr is studied with a picosecond pump-probe technique for induced transparency. 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 (iii) 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 ${\mathrm{OH}}^{\mathrm{-}}$→${\mathrm{OD}}^{\mathrm{-}}$ substitution. Because of its picosecond time scale, its temperature independence, and the Raman measurements presented in an earlier paper, we identify the first component as a radiationless electronic transition during lattice relaxation, which occurs mainly near the first 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-wavelength variation, they are very probably not related to electronic relaxation processes. The nanosecond component is interpreted as vibrational relaxation. It appears in the relaxation scans as a result of the influence 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:${\mathit{F}}_{\mathit{H}}$(${\mathrm{OH}}^{\mathrm{-}}$) configurations is involved in the relaxation process. These configurations possess different electronic absorption bands and are essentially different orientations of ${\mathrm{OH}}^{\mathrm{-}}$ with respect to the F center. The third, strongly temperature-dependent component is associated with the recovery of the thermal equilibrium between these configurations. © 1996 The American Physical Society.

• Received 2 February 1996

DOI:https://doi.org/10.1103/PhysRevB.54.6977