Infrared and Raman studies of hydroxide ions in NaCl extended to H -> D and O-16 -> O-18 isotope exchange - art. no. 134301

Substitutional OH- and OD- defects in NaCl have been studied with Fourier-t ransform infrared (FTIR) and polarized Raman-scattering (RS) spectroscopy. For the first time, the motional levels of the OH- were clearly resolved in their stretch-mode (SM) absorption spectra, showing no change in splitting under (OH-)-O-16 --> (OH-)-O-18 isotope exchange. Due to extremely small o scillator strengths, the first harmonic SM absorptions of (OD-)-O-16 and (O D-)-O-18 defects were not detectable. In contrast to this, the almost equal RS strengths of OH- and OD- defects permitted their detailed comparative i nvestigation with polarized Raman scattering. The resulting low-frequency m otional levels of OD- in both ground and excited SM states are found to be quite different from those of OH- and are hard to reconcile with some of th e existing models. A reasonably good fit of the OH- and OD- data can be ach ieved with a simple V-4 Devonshire model; this suggests hindered rotation o f a strongly off-center shifted center of mass in a potential with smaller energy barriers for OD- compared to OH-. Despite the unexplained weakness o f the first harmonic SM absorption of OD-, phonon and librational sidebands are observed with almost similar strength as those of OH-. Their symmetry was determined by polarized RS and/or FTIR absorption measurements under un iaxial stress alignment. The "normal" OH-/OD- isotope shift of the libratio nal sideband and the observation of its second harmonic show that this angu lar motion occurs around the center of mass in a strong and nearly harmonic potential.