Isotope and temperature effects of the vibrational and rotational relaxation in molten alkali hydroxides

Abstract

Polarized and depolarized Raman spectra have been measured for the symmetric stretching modes of hydroxide ions in molten NaOH, KOH, NaOD, and KOD over the temperature range 593–873 K. The vibrational and rotational autocorrelation functions, C v(t) and C r(t), respectively, have been evaluated from Fourier transformation of the spectra. The vibrational and rotational relaxation rates of OD- ions obtained are compared with those of OH- ions. For all the systems, the vibrational relaxation time τv became larger with increasing temperature and, after reaching a maximum, it decreased. This observation can be explained on the assumption that, at lower temperatures, the motional narrowing effect is predominant for the band width and, at higher temperatures, inhomogeneous broadening by the activated motions of the oscillator becomes crucial. All the rotational autocorrelation functions show rapid and oscillatory decay for short times t and slow and exponential decay for longer times. The former is caused by the...