Relaxation and optical excitation of the FH(OH-) center was studied by using a ps pump-probe technique for induced transparency. It was found that 3 different relaxation components could be distinguished. The first was an almost temperature-independent component which decayed within a few ps. The second was a component which decayed over more than 10ns at all temperatures. The third was a strongly temperature-dependent component with a time constant that was of the order of 100ps at 50K, and was equal to at least 10ns at temperatures below 20K. Essentially no effect upon the relaxation time of the components was observed as a result of OH-  OD- substitution. Because of its ps time scale, its temperature independence, and the associated Raman data, the first component was attributed to a radiation-less electronic transition during lattice relaxation, which mainly occurred near to the first crossing point that was reached. This corresponded to the excitation of one quantum of the stretching vibration. Because the other components changed from induced transparency to induced absorption under probe-wavelength variation, they were suggested to be unrelated to electronic relaxation processes. The ns component was attributed to vibrational relaxation. It appeared in relaxation scans as a result of the effect of the stretching vibration upon the electronic absorption. The effects of probe power upon relaxation measurements, at temperatures below 30K, also showed that optical conversion between the two FH(OH-) configurations was involved in the relaxation process. It was concluded that these configurations had differing electronic absorption bands, and were essentially different orientations of OH- with respect to the F center. The third, strongly temperature-dependent component was associated with the recovery of thermal equilibrium between these configurations.

E.Gustin, M.Leblans, A.Bouwen, D.Schoemaker: Physical Review B, 1996, 54[10], 6977-87