Ethane Molecular Energy Relaxation in High-Pressure Rare Gases

We use two different sets of molecular dynamics simulations to calculate the rotational and vibrational energy relaxation of C₂H₆ in Ar bath gas over a pressure range of 10-400 atm and at temperatures of 300 and 800 K. C₂H₆ is instantaneously excited by 80 kcal/mol randomly distributed into both vibrational and rotational modes. The first set of three simulations modifies the potential energy surface by eliminating attractive potentials between Ar-Ar or between Ar-C₂H₆ or between both Ar-Ar and Ar-C₂H₆. In all three simulations, the vibrational and rotational relaxation rates decrease, implying the presence of an <i>n</i>-mer or multiple collision partners in the high-pressure dynamics. The second set of simulations on the unmodified potential energy surface involves 208 trajectories at 300 K and at 5 pressures (from 75 to 400 atm), where all atomic and molecular positions were saved every 0.05 ps or 20,001 "snapshots" for the full 1,000 ps simulation time. To this archive, we applied ChemNetwork software to tag each snapshot with the nature and number of Ar species in close interactions with C₂H₆ that are primarily responsible for vibrational relaxation. We make the approximation that each snapshot samples a "collision event" whose vibrational energy change is that energy's average over the current and subsequent snapshot minus that energy's average over the current and previous snapshot. We show how this approximation leads to the decomposition of the vibrational relaxation rate into per-time or per-collision rates tagged for specific collision types. This analysis shows the pressure-driven importance of both multiple independent Ar species and larger Ar species in C₂H₆ vibrational relaxation. Surprisingly, this analysis also shows the role of weak collisions, i.e., snapshots with no close C₂H₆ interactions, which decline in frequency with pressure but increase in effectiveness. The modified potential and unmodified potential sets of simulations indicate that the vibrational relaxation rate's high-pressure curvature away from linear pressure dependence is due to the growth of larger and multiple collision partners whose effectiveness does not linearly grow with pressure.