FTIR study of two types of OH-/OH- and OD-/OD- defect pairs in KBr

In a systematic FTIR study of the stretching mode of OH- (and OD-) defects in KBr, we have identified four sharp absorption lines, all scaling in strength with the square of the absorption strength of the isolated defects. Their equal or opposite changes in strength observed after thermal quenching clearly divide them into two groups: two transitions with a large (similar to 50 cm(-1)) and two with a small (similar to 0.8 cm(-1)) spectral splitting for the case of OH-. We interpret these two groups as the oscillations of two different pair-configurations: the first one from a pair of inequivalent molecular oscillators with crossed [100] orientations on next nearest [200] lattice sites with strong elastic dipole interaction; the second one from a pair of equivalent [100] oriented oscillators on nearest neighbor [110] sites, coupled by static electric dipole interaction and slightly split by dynamic coupling into ''in-phase'' and ''out-of-phase'' modes.