Picosecond dynamics of gas-phase dimers in liquid carbon dioxide

A study of the anisotropic interactions in liquid CO2 based on the combination of inelastic neutron scattering data for the dynamic structure factor S(Q,ω) and molecular dynamics simulations using up to six different site-site interaction potentials is reported. The efficiency of dynamical measurements in discriminating among possible potentials clearly emerges. By analysing the salient features of the interaction models used in the simulations and their ability to reproduce neutron spectra at low and intermediate Q values (2 < Q < 17 nm−1), we could detect the short-range pair configurations which mostly contribute to the measured picosecond dynamics. We show that these are the (CO2)2 slipped parallel dimers characteristic of the gas phase. Such a memory of the gas-phase structures was not observed in liquid methane. The role of anisotropy and overall strength of the attractive energy between molecular pairs appears to be crucial to explain why stable dimers of the gas phase are (as in CO2) or are not (as in methane) the picosecond-living two-body structures also detected in the dense liquid.