Homogeneous and inhomogeneous vibrational dephasing processes in aqueous thiocyanate solutions

Abstract

The vibrational correlation functions are calculated from the isotropic Raman spectra for the CN stretching mode of SCN- anions in aqueous LiSCN, NaSCN and KSCN solutions, at concentrations 1–10 M and at temperatures 30, 55 and 80°C. These correlation functions are analysed on the basis of a model of simultaneous homogeneous and inhomogeneous vibrational dephasing, and both the homogeneous and inhomogeneous broadening contributions are extracted. The CN mode in aqueous thiocyanate solutions is found to be inhomogeneously broadened to various degrees depending on cation species, concentration and temperature. The homogeneous contribution increases and the inhomogeneous contribution decreases with increasing temperature because of increased molecular motion. Increase of salt concentration is found to develop inhomogeneity, probably because the strong interaction in concentrated solutions results in long lived local structures. An increase of inhomogeneous and a decrease of homogeneous broadening contributions with increasing salt concentration compensate in KSCN solutions. However, in LiSCN and NaSCN solutions, the increase of inhomogeneous contribution dominates the homogeneous contribution, which leads to concentration broadening. The band in 10 M LiSCN solution at 30°C, which is the broadest of all the CN bands investigated here, is found to be broadened only by the inhomogeneous process.