Dilution effect on the vibrational frequency and vibrational relaxation of PbCl2–KCl ionic liquids

Abstract

The Raman spectra of molten xPbCl2–(1−x)KCl mixtures have been measured in the liquid state over a wide composition range from dilute to dense solutions, namely x=0.2, 0.33, 0.53, 0.67, 0.77 and 1mol fraction. We studied the vibrational dephasing and vibrational frequency modulation by calculating time correlation functions of vibrational relaxation by fits in the frequency domain. The fitting method enables us to model the real line profiles intermediate between Lorentzian and Gaussian by a function, which has an analytical counterpart in the time domain. The experimental vibrational data are interpreted in terms of the Kubo model stochastic line-shape theory assuming that the environmental modulation arises from a single relaxation process. Successful application of the formalism proposed by Kubo shows that vibrational dephasing is the dominant relaxation mechanism for the isotropic Raman line shapes studied. The calculated vibrational frequency modulation time and vibrational dephasing correlation time exhibit a continuous variation with opposite trends indicating an interplay between fast (τω<τV) and slow (τω>τV) modulation and is associated to the different coordination polyhedra dominating the structure at low and high PbCl2 contents. The “packing” of the anions around central Pb2+ cation seems to be the key factor for the structure and the dynamics of the melts. The study of picosecond dynamics enables us to discern between instantaneous, short-lived and long-lived species in molten halides. Theoretical models of vibrational second moments and experimental results have been tested and compared by diluting PbCl2 in KCl solvent. The changes in second moments of vibrations suggest the strengthening of attraction between coordination polyhedra in a less crowded environment.