Raman spectroscopic study on the vibrational and rotational relaxation of OH− ion in molten LiOH

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Abstract
Raman scattering spectra have been measured for molten LiOH at 773 K. The highly corrosive melt was held as a drop under a silver wire ring. While the Rayleigh wing spread over more than 1000 cm−1, only one distinct peak for the OH− stretching mode appeared at 3607 cm−1, the band shape being asymmetric owing to inhomogeneous broadening. The vibrational and rotational autocorrelation functions for the OH− ion, Gv(t) and Gr(t), respectively, have been evaluated from the Fourier transformation of the observed polarized and depolarized band contours. The Gv(t) is virtually of an exponential form and its decay is faster than that of the NO−3 ion in molten LiNO3. The Gr(t) can be obtained with good accuracy, whereas inhomogeneous broadening is obviously observed. The relaxation rate of Gr (t) is extremely high as compared with that for the NO−3 ion in molten LiNO3 . The difference is explained in terms of the difference mainly in the moments of inertia and partly in the temperatures. The Gr (t) has oscillatory character with a period of ∼0.04 ps at small t. This must be attributed to the precession and/or the pendulum motion of the ion, which supports the M‐ or J‐extended rotational diffusion model rather than Debye’s rotational diffusion model.