Cation dependence of the vibrational and rotational relaxation of OH− ion in molten MOH(M=Li, Na, K, Rb, and Cs) by Raman scattering measurements

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Abstract
Polarized and depolarized Raman scattering measurements have been performed for the stretching vibrational mode of OH− ion in molten MOH (M= Li, Na, K, Rb, and Cs). Vibrational and rotational autocorrelation functions, Cv(t) and Cr(t), have been evaluated from Fourier transformation of the spectra. The relaxation rate of Cv(t) becomes more rapid and the asymmetry of the isotropic band becomes larger as the cation size becomes smaller. These results can be interpreted in terms of the coulombic interaction. Applying the two-parameters model based on the Kubo line shape theory to the observed spectra, it is suggested that the OH− ion apparently follows the fast modulation regime due to the libration of OH− ion. For all melts, the Cr(t) has oscillatory character caused by libration of the OH− ion at small t <0.1 ps. The function shows ordinary exponential decay at large t≳0.1 ps. The cation dependence of the libration at t< 0.1 ps and the rotational diffusion process at t≳0.1 ps were investigated separately. The larger the cation size becomes, the longer and larger the period and amplitude of the libration become, respectively. This indicates that in the libration process the OH− ion seems to have more extended space in the melt as the cation size becomes larger. With respect to the rotational diffusion process, the rate shows rather small cation dependence.