Ethane Molecular Energy Relaxation in High-Pressure Rare Gases

Area of Science:

• Physical Chemistry
• Chemical Physics
• Computational Chemistry

Background:

• Understanding energy relaxation dynamics is crucial for chemical kinetics and thermodynamics.
• High-pressure effects on molecular energy transfer are complex and not fully understood.
• Ethane (C2H6) in an argon (Ar) bath gas provides a model system for studying these phenomena.

Purpose of the Study:

• To calculate the rotational and vibrational energy relaxation rates of C2H6 in Ar gas.
• To investigate the influence of pressure (10-400 atm) and temperature (300, 800 K) on relaxation dynamics.
• To elucidate the role of multiple and larger collision partners in high-pressure energy transfer.

Main Methods:

• Employed two sets of molecular dynamics simulations with modified and unmodified potential energy surfaces.
• Utilized ChemNetwork software to analyze trajectory snapshots and identify Ar-C2H6 interaction types.
• Developed an approximation to decompose vibrational relaxation rates into contributions from specific collision events.

Main Results:

• Vibrational and rotational relaxation rates decreased when attractive potentials were modified, suggesting n-mer or multiple collision partner importance.
• Analysis revealed pressure-driven contributions from multiple independent Ar species and larger Ar clusters to C2H6 vibrational relaxation.
• Weak collisions, though less frequent at high pressure, increased in effectiveness, contributing to non-linear pressure dependence of relaxation rates.

Conclusions:

• The high-pressure curvature in vibrational relaxation rate is attributed to the emergence of larger and multiple collision partners.
• These complex collision configurations enhance energy transfer efficiency beyond simple pairwise interactions.
• The findings highlight the necessity of considering multi-body interactions for accurate modeling of gas-phase energy relaxation at high pressures.