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

In contrast to the well-documented case of F-CN- pairs, little is know n 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-multich annel detection, we were able to obtain more detailed and reliable dat a than was the case in previous measurements on this center in KCl. St epwise F-->F-H conversion experiments revealed the existence of severa l F-OH- aggregate centers with stretching-mode frequencies within simi lar to 10 cm(-1) from that of the undisturbed impurity ion. In particu lar, 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 configurati ons. The primary E-V transfer process for both isotopes was characteri zed by accurately measuring the excitation-power dependence of the pop ulation of the vibrational levels. The influence of V-V transfer is ne gligible, 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 Lifet ime of OD- and OH- in the F-H Center is estimated to be of the order o f 100 and 10 ns, respectively. Its isotope effect and its temperature independence suggest that decay into hindered rotations is the relaxat ion mechanism of the stretch vibration.