Dynamical structure of XCl (X = Li, Na, K) aqueous solutions by low-frequency Raman scattering: relation between 50 cm−1 vibration mode and relaxation mode

Depolarized low-frequency Raman spectra of LiCl, NaCl and KCl have been measured as a function of temperature from 266 to 350 K. The concentration dependence of the spectra of LiCl solutions from 0.00 to 0.20 molar ratio has been also measured at room temperature. The spectral profiles have been analyzed with a superposition of one relaxation mode and two damped-harmonic oscillator modes. The multiple random telegraph (MRT) model which takes into account the inertia and the non-white effects has been adopted for a relaxation component. The relaxation time is considered as an average lifetime of the vibration unit of intermolecular vibrations. Two damped oscillator modes are observed around 50 and 180 cm−1 which are considered as a bending-like vibration and a stretching-like vibration, respectively. Above 320 K, the bending-like mode disappears in the fitting results of the 0.08 molar ratio solution. This means that strongly disrupted vibrational mode cannot be distinguished from the relaxation described by the MRT model. At high temperature the relaxation time becomes fast, which means the lifetime of the vibration unit of intermolecular vibration becomes short and the bending-like mode can only vibrate several times during the duration time. On the other hand, at low temperature or at high concentration, the 50 cm−1 component definitely appeared in our analysis with MRT model, because the relaxation time becomes large or the relaxation process becomes slow.