Raman scattering by the hydroxyl ion in KCl and KBr

摘要

Raman studies have been performed on KCl:OHminus;and KBr:OHminus;crystals between 2 K and room temperature. They provide additional information about previously measured infrared bands near 30 cmminus;1, 300 cmminus;1, and 3600ndash;4000 cmminus;1. The 300 cmminus;1band, its weak satellites, and its combination bands with the Osngbnd;H stretching mode can be seen in Raman spectra only below 100 K. All these bands haveT2gsymmetry, consistent with libration of the Osngbnd;H bond about 100 equilibrium directions. From the intensity of the combination bands relative to one component of the main stretching band a numerical value of the matrix element thgr; of the librational coordinate was determined and found to be consistent with a picture of angular harmonic oscillation of the hydroxyl ion with its center of mass fixed. The 30 cmminus;1bands share the intensity and symmetry properties of the 300 cmminus;1bands. A librational picture was applied to them also. The measured thgr; yielded a large moment of inertia (18 amuhyphen;Aring;2) that implies that this low frequency librational motion involves displacement of the center of mass as well as rotation of the Osngbnd;H bond direction. The same conclusion resulted from a fit to the Devonshire theory for the combined motion. The fitted moments of inertia were in the range 14ndash;18 amuhyphen;Aring;2, and the dimensionless barrier parameterkwas about 45 for KCl:OHminus;and 85 for KBr:OHminus;. At higher temperatures the depolarized components in the Osngbnd;H stretching region suggested strongly damped freehyphen;rotorhyphen;like behavior. Its onset coincided approximately with thermal population of levels that are essentially unbound in the Devonshire potential. The cause of the damping was assigned to greatly increased phonon emission and absorption that accompanies largehyphen;angle rotations of the offhyphen;center hydroxyl ion.