On the vibrational frequency correlation in liquids: Raman study of aqueous thiocyanate solutions

Abstract

The vibrational frequency correlation functions 〈ω(0) ω(t)〉 are calculated from isotropic Raman spectra for the CN stretching mode of the SCN− anion in aqueous LiSCN, NaSCN, and KSCN solutions, at concentrations 1–10 M, and at temperatures of 30, 55, and 80 °C. The experimental 〈ω(0) ω(t)〉 shows rapid relaxation within a particular environment on a subpicosecond time scale, accompanied by constant positive correlation. It is concluded that the homogeneous dephasing is due to the fast relaxation of 〈ω(0) ω(t)〉, and that the inhomogeneous dephasing comes from the static positive correlation, which should be attributed to a distribution of environments persisting over the time scale of Raman probe. The analysis of 〈ω(0) ω(t)〉 confirms that the inhomogeneous broadening increases with decreasing temperature, with increasing salt concentration, and in the order KSCN<NaSCN<LiSCN.