ENDOR analysis of the microscopic structures of the thermally driven bistable configurations of F_H(OH^-) centres in KBr

Abstract

he aggregation of F centres and OH-molecules to FH(OH-) centres in KBr leads Io a very interesting optical and thermal bstabibty. The htruc~~ms of the bistable FH(OH-) centres were investigated by using optical and mnventional detection of electron nuclear double w n a n c e (ENWR). The anaiyis of the "WR spectra of both FH(OH-) mnfigurations b prsented. It b shown that in both tistable configurations the OH-molecule resides on a nat-nearest anion site (fourth shell). ?he bistable mnfigurations differ in the orienfation of the OH-dipole with respect to the F centre. The thermal bstabilily ir tentatively explained as being due to enmpy changes with temperature.

Electronic, vibrational, and configurational relaxation of theFH(OH−Gustin1, Leblans2, Bouwen3 et al. 1996Phys. Rev. B10010View full textAdd to dashboardCite

“…Little is known about this vibration, but it is assumed to be soft, so that it would be easily excited at higher temperatures. 27 The relaxation time of the intermediate component, which we associated with the reorientation rate, is very similar for OH Ϫ and OD Ϫ . One may expect that the different librational frequencies and masses of the OH Ϫ and OD Ϫ result in different reorientation rates.…”

“…In particular the motion of the K ϩ cation between the F center and the OH Ϫ might stimulate the reorientation of the OH Ϫ , since the position of this ion is closely related to the orientation of the OH Ϫ . 27 It is also possible that the reorientation is related to the electronic transition itself: the libration of the OH Ϫ might accept some of the energy that is dissipated by this process, or even act as a promoting mode for the transition. 14 Below 30 K, the OH Ϫ remains in its new orientation for at least several nanoseconds, whereas at 50 K thermal reorientation occurs on a time scale of the order of 100 ps.…”

“…At low temperatures there is optical bistability between a redshifted and a blue-shifted absorption band, with peak positions of 587 and 615 nm at 4 K, respectively. 26,27 The different absorption bands have been associated with different orientations of the OH Ϫ ion, which is in a ͗200͘ position with respect to the F center, in units of the interatomic distance. 1,27 This adds a complication to the system, since electronic ground and excited states should be associated with each configuration, as well as vibrational states.…”

“…26,27 The different absorption bands have been associated with different orientations of the OH Ϫ ion, which is in a ͗200͘ position with respect to the F center, in units of the interatomic distance. 1,27 This adds a complication to the system, since electronic ground and excited states should be associated with each configuration, as well as vibrational states. At all temperatures, the two configurations are efficiently converted into each other by resonant optical excitation ͑see paper I and II͒.…”

Thermally induced coloration of KBr at high pressures

“…2), consistent with the color centers observed in lower pressure studies of KBr. [38,39] For this reason, we attribute this defect state to high-pressure F-centers in KBr. This is in contrast with early work that observed an increase in absorption energy of the F-center in KBr with pressures to ~15 GPa and predicted a continual increase in the absence of a phase transition.…”