Substitutional OH- and OD- defects were studied by means of Fourier-transform infra-red and polarized Raman-scattering spectroscopy. The motional levels of the OH- were clearly resolved in their stretching-mode absorption spectra, and exhibited no change in splitting under 16OH- → 18OH- isotopic exchange. Due to the extremely small oscillator strengths, the first harmonic stretching-mode absorptions of 16OD- and 18OD- defects were undetectable. However, the almost-equal Raman-scattering strengths of OH- and OD- defects permitted their detailed comparative investigation using polarized Raman scattering. The resultant low-frequency motional levels of OD-, in both the ground and excited stretching-mode states, were found to be quite different to those of OH-. They were difficult to reconcile with some existing models. A reasonably good fit to the OH- and OD- data could be achieved by using a simple V4 Devonshire model. This implied the hindered rotation of a strongly off-center shifted center of mass in a potential having smaller energy barriers for OD- than for OH-. In spite of an unexplained weakness of the first harmonic stretching-mode absorption of OD-, phonon and librational side-bands were observed which had almost similar strengths as those of OH-. Their symmetry was determined, by means of polarized Raman-scattering and/or Fourier-transform infra-red absorption measurements, under uniaxial stress alignment. The so-called normal OH-/OD- isotopic shift of the librational side-band, and observation of its second harmonic, showed that this angular motion occurred around the center of mass in a strong and nearly harmonic potential.

Infrared and Raman Studies of Hydroxide Ions in NaCl Extended to H → D and 16O → 18O Isotope Exchange. E.Gustin, A.Bouwen, D.Schoemaker, C.P.An, F.Luty: Physical Review B, 2001, 63[13], 134301 (13pp)