Modeling disequilibrium in gas ensembles: how quantum state populations evolve under multicollision conditions;...

The method of Marsh and McCaffery [J. Chem. Phys. 117, 503 (2002)] is used to quantify how rovibrational populations and mode temperatures change as an ensemble of CO molecules, initially excited to (v;j)=(8;12), evolves to thermal equilibrium in a bath gas. The bath gases considered are Ar, N(2), O(2), and CO all at 300 K with the diatomics in their (0;8) rovibrational states. Ensembles generally contain 1000 molecules, 10% of which are excited CO (CO( *)) molecules. State (v;j) populations and mode temperatures of CO* and bath molecules are calculated for successive collisions to 1000 or more. We find that relaxation to local thermodynamic equilibrium occurs in distinct phases that vary widely in rate of cooling. There is especially fast vibration-vibration (VV) exchange in CO*-CO mixtures that is largely decoupled from rotation and translation. Several aspects of ensemble behavior may be rationalized using concepts established in quantum state resolved single collision studies. We demonstrate the existence of a simultaneous energy quasiresonant, angular momentum conserving, low Deltaj VV process that can cause either ultrafast relaxation or up pumping of the kind seen in a number of experiments.