Low-temperature inelastic collisions between hydrogen molecules and helium atoms

Area of Science:

• Physical Chemistry
• Chemical Physics
• Molecular Dynamics

Background:

• Understanding inelastic collisions is crucial for modeling chemical reactions and energy transfer in various environments.
• Accurate potential energy surfaces (PES) are essential for reliable theoretical calculations of collision dynamics.

Purpose of the Study:

• To experimentally and theoretically investigate inelastic hydrogen-molecule:helium (H2:He) collisions.
• To calculate state-to-state cross sections and rates using recent potential energy surfaces (PES).
• To compare experimental measurements with theoretical predictions for fundamental rates k(2-->0) and k(3-->1).

Main Methods:

• Theoretical calculations performed at the converged close-coupling level using MR-PES, MMR-PES, and BMP-PESs.
• Experimental assessment of rates using a master equation approach to describe rotational population evolution.
• High-sensitivity Raman spectroscopy employed to measure H2 rotational populations in supersonic H2+He jets.

Main Results:

• Good agreement between experimental and theoretical k(2-->0) rates was found for the MR-PES, but not for the BMP-PES.
• Experimental k(3-->1) rates were found to be compatible with calculations from both MR-PES and MMR-PESs within experimental uncertainty.
• The study supports the MR-PES and MMR-PESs, along with the employed gas-dynamic equations.

Conclusions:

• The MR-PES provides a reliable description of inelastic H2:He collisions, particularly for the k(2-->0) rate.
• The consistency between experimental and theoretical results supports the validity of the employed molecular dynamics models.
• Further investigation may be needed to definitively resolve discrepancies for the k(3-->1) rate.