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 stu died 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 c onfigurations B and R, characterized by partially overlapping electron ic absorptions (blue and red shifted from the F band), and by spectral ly well separated sharp VA lines. Irradiation into the B and R electro nic 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) q uantum 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 stre ngth ratio). Guided by recent electron nuclear double resonance result s in KBr: OH-, we attribute bistability and the observed EA and VA beh avior to large linear off-center displacements of the cation between F and OH- along the pair axis, which is strongly coupled to translation al/rotational motion of the OH- or OD-. The resulting anharmonic total -energy potential of these coupled motions decides by shape and relati ve depth of its single or double wells about the possibility of bistab ility, its thermal behavior, and the preference for B and R configurat ions in various hosts.