Raman study of rotational motion and vibrational dephasing dynamics of NO₃^- in molten nitrates

Abstract

The Raman spectra of the symmetric stretching v ₁(A′₁) mode of NO₃ ^- ions have been measured in a series of molten nitrates, LiNO₃, NaNO₃, KNO₃, RbNO₃, CsNO₃, AgNO₃, TlNO₃, at a temperature 10–20 K above each melting point. Temperature dependence of the Raman spectrum was also measured in molten NaNO₃. Reorientation of the symmetry axis of an nitrate ion is represented by the inertial motion of an associated ion cluster for short times and by rotational diffusion for longer times. One observes a point of inflection between two types of rotational motion at 0·2–0·5 ps. The speeds of inertial rotation and rotational diffusion decrease in the order, CsNO₃, RbNO₃, KNO₃, NaNO₃, TlNO₃, LiNO₃, AgNO₃. The ratio of the effective moment of inertia obtained from moment analysis to the theoretical moment of inertia for a free NO₃ ^- ion, I/I ₀, increases in the same order, from 2·0 for CsNO₃ to 20 for AgNO₃.

The vibrational correlation functions are analysed on the basis of a model of simultaneous homogeneous and inhomogeneous vibrational dephasing. The vibrational frequency modulation is rather fast in molten Na, K, Rb, Cs, Ag, Tl nitrates, while homogeneous and inhomogeneous broadening give comparable contributions in strongly interacting molten LiNO₃. As the cation size decreases in molten alkali metal nitrates, the rotational motion seems to be more restricted and the vibrational dephasing seems to be more accelerated. As the temperature increases from 591 to 725 K in molten NaNO₃, the speed of rotational diffusion increases considerably, and the vibrational dephasing rate increases slightly.

We have observed that, in all molten nitrates investigated here, the peak frequency is on the high frequency side of an isotropic Raman band. Two processes for the observed asymmetry are presented: (a) asymmetric distribution of local environmental states of NO₃ ^- ions which have different vibrational frequencies, and (b) the contribution of hot bands to the low frequency side of the main band of ions in the ground state, in the case that the vibrational mode is anharmonic. The observed asymmetric isotropic Raman spectra in molten nitrates could be reproduced by either or both of the two models for some systems. However we could not separate the contribution from each process.