Energy Relaxation of N$_2$O in Gaseous, Supercritical and Liquid Xenon and SF$_6$

TL;DR

This study investigates the energy relaxation processes of N$_2$O molecules in xenon and SF$_6$ environments across different phases, combining high-level calculations and molecular dynamics to understand relaxation rates and their dependence on solvent density.

Contribution

The paper provides a detailed molecular-level analysis of rotational and vibrational energy relaxation in various phases, validating simulation results against experimental data and highlighting limitations of binary collision models at high densities.

Findings

RER rates in low-density solvents match experimental data

Binary collision model works up to ~4 M concentration

VER trends are accurately captured by simulations

Abstract

Rotational and vibrational energy relaxation (RER and VER) of N$_2$O embedded in xenon and SF$_6$ environments ranging from the gas phase to the liquid, including the supercritical regime, is studied at a molecular level. Calibrated intermolecular interactions from high-level electronic structure calculations, validated against experiments for the pure solvents were used to carry out classical molecular dynamics simulations corresponding to experimental state points for near-critical isotherms. Computed RER rates in low-density solvent of $k_{\rm rot}^{\rm Xe} = (3.67\pm0.25)\cdot10^{10}$ s$^{-1}$M$^{-1}$ and $k_{\rm rot}^{\rm SF_6} = (1.25\pm0.12)\cdot10^{11}$ s$^{-1}$M$^{-1}$ compare well with rates determined by analysis of 2-dimensional infrared experiments. Simulations find that an isolated binary collision (IBC) description is successful up to solvent concentrations of $\sim 4$ M. For higher densities, including the supercritical regime, the simulations do not correctly describe RER, probably due to neglect of solvent-solute coupling in the analysis of the rotational motion. For VER, the near-quantitative agreement between simulations and pump-probe experiments captures the solvent density-dependent trends.