Pressure Effects on the Relaxation of an Excited Ethane Molecule in High-Pressure Bath Gases

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2021-09, Vol.125 (39)
• Main Authors: Hren, Zackary R., Lazarock, Chad R., Vincent, Tasha A., Rivera-Rivera, Luis A., Wagner, Albert F.
• Format: Article
• Language: English
• Subjects: Cluster chemistry; Collisions; Energy; Hydrocarbons; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Molecules
• ISSN: 1089-5639; 1520-5215

Here, we use molecular dynamics to calculate the rotational and vibrational energy relaxation of C2H6 in Ar, Kr, and Xe bath gases over a pressure range of 10 to 400 atm and at temperatures of 300 K and 800 K. The C2H6 is instantaneously excited by 80 kcal/mol randomly distributed into both vibrational and rotational modes. The computed relaxation rates show little sensitivity to the identity of the noble gas in the bath. Vibrational relaxation rates show a non-linear pressure dependence at 300 K. At 800 K the reduced range of bath gas densities covered by the range of pressures do not yet show any non-linearity in the pressure dependence. Rotational relaxation is characterized with two relaxation rates. The slower rate is comparable to the vibrational relaxation rate. The faster rate has a linear pressure dependence at 300 K but an irregular, nonlinear pressure dependence at 800 K. To understand this, a model was developed based on approximating the periodic box used in the molecular dynamics simulations by an equal-volume collection of cubes where each cube is sized to allow only single occupancy by the noble gas or the molecule. Combinatorial statistics then leads to a pressure and temperature dependent analytic distribution of the bath gas species the molecule encounters in a collision. This distribution, the dissociation energy of molecule/bath gas complexes and bath gas clusters, and the computed energy release per collision combine to show that only at 300 K is the energy release sufficient to dissociate likely complexes and clusters. This suggests that persistent and pressure-dependent clusters and complexes at 800 K may be responsible for the non-linear pressure dependence of rotational relaxation.