A DOUBLE-EXPONENTIAL MODEL FOR THE MEMORY FUNCTION OF VIBRATIONAL DEPHASING AS SUGGESTED BY MOLECULAR-DYNAMICS OF CARBON-DISULFIDE

Computer simulation of vibrational dephasing of liquid CS2 indicates t hat the correlation function of frequency fluctuation is not a single exponential, which invalidates the use of Kubo's formula for the vibra tional correlation function. The two different time scales for the dec ay of the frequency fluctuation correlation function allow one to prop ose a double-exponential model for the memory function of the vibratio nal correlation function. The model is used to fit vibrational correla tion functions obtained from molecular dynamics of CS2 at 1 bar, 1 kba r and 4 kbar at room temperature. The hindrance of the rotational and translational dynamics under high pressures results in a larger contri bution from the slow decay term of the frequency fluctuation correlati on functions and memory functions.