ELECTRONIC-VIBRATIONAL ENERGY TRANSFER BETWEEN F CENTERS AND OH- IMPURITIES IN KBR STUDIED BY STOKES AND ANTI-STOKES RAMAN SCATTERING

摘要

In contrast to the well-documented case of F-CN- pairs, little is known about the electronic-vibrational (E-V) energy transfer between an F-center electron and the stretching mode of a neighboring OH- impurity. We studied this phenomenon for F-OH- and F-OD- pairs in KBr by means of Stokes and anti-Stokes resonant Raman scattering. Using CCD-multichannel detection, we were able to obtain more detailed and reliable data than was the case in previous measurements on this center in KCl. Stepwise F-->F-H conversion experiments revealed the existence of several F-OH- aggregate centers with stretching-mode frequencies within similar to 10 cm(-1) from that of the undisturbed impurity ion. In particular, the stretching modes of the two bistable configurations of the F-H(OH-) center in KBr were identified. Polarized measurements were not able to distinguish between parallel and perpendicular orientations of the impurity with respect to the F-H defect axis in these configurations. The primary E-V transfer process for both isotopes was characterized by accurately measuring the excitation-power dependence of the population of the vibrational levels. The influence of V-V transfer is negligible, as was verified by variation of the impurity concentration. With the assumption that each excited F-H electron transfers energy to the impurity vibration in its relaxation cycle, the vibrational Lifetime of OD- and OH- in the F-H Center is estimated to be of the order of 100 and 10 ns, respectively. Its isotope effect and its temperature independence suggest that decay into hindered rotations is the relaxation mechanism of the stretch vibration. [References: 36]