Dynamics in the BMIM PF6/acetonitrile mixtures observed by femtosecond optical Kerr effect and molecular dynamics simulations

We have performed the measurements of the optical Kerr effect signal time evolution up to 4 ns for amixture of 1-alkyl-3-methyl-imidazolium hexafluorophosphate (BMIM PF6) ionic liquid and acetonitrile inthe whole mole fractions range. The long delay line in our experimental setup allowed us to capturethe complete reorientational dynamics of the ionic liquid. We have analysed the optical Kerr effect signalin the time and frequency domains with help of molecular dynamics simulations. In our approximationof the slow picosecond dynamics with a multi-exponential decay, we distinguish three relaxation times.The highest two are assigned to the reorientation of the cation and acetonitrile molecules that are in thevicinity of the imidazolium ring. The third one is recognized as originating from cation rotations andreorientation of acetonitrile molecules in the bulk or in the vicinity of the aliphatic chains of the cation.With help of the simulation we interpret the intermolecular band in the reduced spectral density,obtained from Kerr signal, as follows: its low-frequency side results from oscillations of one of thecomponents in the cage formed by its neighbors, while the high-frequency side is attributed to thelibrations of the cation and acetonitrile molecule as well as the intermolecular oscillations of systemcomponents involved in specific interactions. We use this assignment and concentration dependence ofthe spectra obtained from velocity and angular velocity correlations to explain the mole fraction dependenceof Kerr reduced spectral density.