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Freezing-Out of Tunneling Motion of OH^- in NaCl Crystal Due to Dipole-Dipole Interaction

It is noticed that the dipole-dipole interactions between pairs of off-center impurity ions in alkali halide crystals effectively freeze out the tunneling motions of the individual ions and this can be optically detected. The concentration dependence of the infrared absorption line shape of NaCl: OH observed by Wedding-Klein and Nanba et al. (the O–H stretching band) is explained from this point of view. The so-called B band is assigned as due to the stretching mode excitation of the frozen pairs of dipoles. A fairly good fit to the observed spectra is obtained by the virial expansion theory for the line shape function applied to the six-state hopping model with the tunneling parameter of 1 cm -1 h c and with the magnitude of the dipole moment of 3.3 Debye.