Infrared and Raman studies of hydroxide ions in NaCl extended toH→Dand16<

Abstract

Substitutional OH Ϫ and OD Ϫ defects in NaCl have been studied with Fourier-transform infrared ͑FTIR͒ and polarized Raman-scattering ͑RS͒ spectroscopy. For the first time, the motional levels of the OH Ϫ were clearly resolved in their stretch-mode ͑SM͒ absorption spectra, showing no change in splitting under 16 OH Ϫ → 18 OH Ϫ isotope exchange. Due to extremely small oscillator strengths, the first harmonic SM absorptions of 16 OD Ϫ and 18 OD Ϫ defects were not detectable. In contrast to this, the almost equal RS strengths of OH Ϫ and OD Ϫ defects permitted their detailed comparative investigation with polarized Raman scattering. The resulting low-frequency motional levels of OD Ϫ in both ground and excited SM states are found to be quite different from those of OH Ϫ and are hard to reconcile with some of the existing models. A reasonably good fit of the OH Ϫ and OD Ϫ data can be achieved with a simple V 4 Devonshire model; this suggests hindered rotation of a strongly off-center shifted center of mass in a potential with smaller energy barriers for OD Ϫ compared to OH Ϫ . Despite the unexplained weakness of the first harmonic SM absorption of OD Ϫ , phonon and librational sidebands are observed with almost similar strength as those of OH Ϫ . Their symmetry was determined by polarized RS and/or FTIR absorption measurements under uniaxial stress alignment. The ''normal'' OH Ϫ /OD Ϫ isotope shift of the librational sideband and the observation of its second harmonic show that this angular motion occurs around the center of mass in a strong and nearly harmonic potential.