The molecular origins of the two-dimensional Raman spectrum of an atomic liquid. I. Molecular dynamics simulation
As complex as it may seem, a two-dimensional (fifth-order) nonresonant Raman spectrum may provide one of the simplest ways to get at the character of intermolecular dynamics in liquids. Its status as an echo spectroscopy means that it should not only permit us to survey the intermolecular vibrations, it should allow us to ascertain the extent of their coherence. Arriving at a microscopic interpretation of those spectra, however, poses some genuine theoretical challenges. We describe here the first complete molecular dynamics simulation of such a spectrum. By using classical dynamics and focusing on liquid Xe, we find that we are able to produce a spectrum strikingly similar to the experimentally reported (nonmagic-angle) spectra of liquid CS2. We observe, in particular, that there is no discernable echo, suggesting that the dynamics is strongly homogeneously broadened. We turn, in a companion paper, to the implications of these results for instantaneousnormal-mode models of liquids.