Millimeter Wave Absorption of OH⁻ in NaCl Crystal

The absorption spectrum and its temperature dependence of the off-center OH - ion in NaCl crystal has been observed in the millimeter wave region from 1.1 to 2.5 cm -1 ( λ=4 to 9 mm). Two absorption bands due to the motion of the center of mass of the OH - ion have been observed at 1.85 and 1.56 cm -1 and are assigned to the transitions between A 1g and T 1u states and between T 1u and E g states, respectively. The transition dipole moments of the former and the latter transitions are estimated as 1.6 and 3.7 debye. The ratio of the two transition energies is 1.19 and this value is different from the ratio of 2 predicted by the tight well tunneling model. The energy levels are explained well by the LCAO-MO calculation based on a three dimentional adiabatic potential proposed by Gomez, Bowen and Krumhansl.