It was noted that a defect pair which consisted of an F center and an OH- ion exhibited a dual optical behavior at temperatures below 5K. That is, there were 2 unpolarized optical absorption bands that were red-shifted and blue-shifted away from the F-center absorption band. The defect pair was bistable because weak red-shifted irradiation reduced the red band and enhanced the blue-shifted absorption band. The process was reversible. Heating to 10K progressively decreased the red-shift until this band merged with the blue band. At temperatures of between 10 and 150K, the blue band moved downwards through the F-center peak so as to become the red band. An attempt was made to establish static configurations, of the K+ and OH- ions, which could account for both the red shift and the blue shift, and the observed half-widths below 5K; all within the context of a point-ion model. The results were compared with configurations that were deuced from electron nuclear double resonance measurements of the isotropic and anisotropic super-hyperfine constants. A perturbation calculation was carried out by using basis states that gave good agreement with the F band. The model parameters included the displacements of the K+ and OH- ions, and the intrinsic dipole orientation of the OH- ions. The parameters were varied until agreement was obtained with the peak shifts and half-widths of the FH(OH-) red and blue center spectra. The K+ ion displacements were found to be about one third of those obtained previously. A comparison with the electron nuclear double resonance so-called super-hyperfine a-constant indicated that the ground state was some 25% too large at the K+ ion site. A second configuration was identified for both the red and blue center.

P.Gash: Materials Science Forum, 1997, 239-241, 373-6