Vibrational and electronic absorption study of bistable and stable F-H(OH-) centers in alkali halides

The closest possible F center-OH- defect pairs, located on (200) next-nearest-neighbor anion sites separated by a host cation, have been studied in terms of their electronic (EA) and stretching-mode vibrational absorption (VA) in various hosts. These pairs can exist in KBr, RbBr, and RbI at T less than or similar to 10 K in two different bistable configurations B and R, characterized by partially overlapping electronic absorptions (blue and red shifted from the F band), and by spectrally well separated sharp VA lines. Irradiation into the B and R electronic bands achieves at 4 K reversible B reversible arrow R conversions, which we determined to occur in both directions with high (0.2-0.6) quantum efficiency. Fourier-transform infrared measurements parallel to these EA conversions established the proper assignment of the two VA lines to the B and R configurations (and yielded their oscillator strength ratio). Guided by recent electron nuclear double resonance results in KBr: OH-, we attribute bistability and the observed EA and VA behavior to large linear off-center displacements of the cation between F and OH- along the pair axis, which is strongly coupled to translational/rotational motion of the OH- or OD-. The resulting anharmonic total-energy potential of these coupled motions decides by shape and relative depth of its single or double wells about the possibility of bistability, its thermal behavior, and the preference for B and R configurations in various hosts.