A study was made of the closest possible pairs, of F centers and OH- defects, that were located on <200> next-nearest neighbor anion sites separated by a host cation. This was done by monitoring their electronic and stretching-mode vibrational absorption in the various hosts. These pairs could exist, at temperatures below 10K, in 2 different bistable configurations that were characterized by partially overlapping electronic absorptions that were blue- or red-shifted from the F-band. They were also characterized by well-separated sharp VA lines. The bistable behavior was attributed to large linear off-center displacements of the cation along the pair axis between F and OH-. This was strongly coupled to a translational cum rotational motion of the OH- or OD-. The resultant anharmonic total-energy potential of these coupled motions governed, via the shape and relative depth of its single or double wells, the possibility of bistability and the preference for B-shifted or R-shifted configurations in various hosts.

V.Dierolf, F.Luty: Physical Review B, 1996, 54[10], 6952-62