Microscopic dynamics in simple liquids by subpicosecond birefringences

The transient birefringent responses of three simple liquids (Xe, Ar, and ${\mathrm{O}}_2$) have been measured using subpicosecond optical pulses. In addition to molecular reorientational dynamic contributions to the birefringence (in ${\mathrm{O}}_2$), we have directly observed the subpicosecond dynamics arising from electronic and intermolecular interactions (in Xe and Ar). Our results in Xe and Ar constitute the first unambiguous observations of such intermolecular processes in real time. The theoretical connections between such direct observations and the previous indirect, light-scattering studies in the frequency domain are explicated. The importance of intermolecular processes in the subpicosecond dynamics of all dense fluids is noted.