Computer simulation of vibrational dephasing in carbon disulfide

Abstract

Vibrational dephasing of the totally symmetric stretching mode of liquid carbon disulfide at different pressures at room temperature is investigated by molecular dynamics simulation. The thermodynamic states simulated correspond to the liquid at atmospheric pressure, 1 kbar, 4 kbar and 8 kbar, all at room temperature (294 K). An - effective vibrational-rototranslational coupling potential, which contains an exponential repulsive term and an attractive long range term, is assumed. The fundamental role of anisotropy in this potential is emphasized. The experimental trend of average gas-to-liquid vibrational frequency shifts and vibrational correlation functions are well reproduced by molecular dynamics. The simulations show that correlation functions of vibrational frequency fluctuation of CS2 are not a single exponential, what invalidates the well-known Kubo's formula for vibrational correlation function. The long time behaviour seen in the frequency fluctuation correlation functions, present even if one assumes a purely repulsive, short range coupling potential, is due to the characteristic hindrance of the rototranslational dynamics of CS2. The simulations also show a small correlation between vibrational and reorientational relaxations, even when an isotropic coupling potential is considered, what is an indication of coupling between vibrational and reorientational degrees of freedom by translational motions.