Quantum state-resolved energy redistribution in gas ensembles containing highly excited N2

A computational model is used to quantify the evolution of quantum state populations as highly vibrationally excited N-14(2) (N-14(2)*) equilibrates in various bath gases. Multicollision energy disposal follows general principles established in related single collision processes. Thus when state-to-state routes permit, maximum amounts of energy are deposited into partner species by direct vibration-to-vibration (V-V) exchange. When these pathways are absent, e.g., when Ar is the bath species, relaxation is very slow and multistaged. Conversely, in a bath of v = 0 N-14(2) molecules, 16 vibrational quanta (Delta v = +/- 8) are resonantly exchanged from (v;j) = (8;10) with vibrational equilibration so rapid that rotation and translation still lag far behind after 1000 collisions. Near-resonant V-V exchange dominates the initial phase when N-15(2) forms the bath gas and although some rotational warming occurs, vibrational modes remain decoupled from, and significantly hotter than, the low heat capacity modes. These forms of behavior seem likely to characterize excited and bath species that have closely similar vibration and rotation constants. More generic in nature is N-14(2) in O-2 or in a mixture that closely resembles air. Here, asymmetric V-V exchange is a dominant early feature in ensemble evolution but energy differences in the key vibration and rotation quanta lead to V-V energy defects that are compensated for by the low energy modes. This results in much more rapid ensemble equilibration, generally within 400-500 collisions, when O-2 is present even as a minor constituent. Our results are in good general agreement with those obtained from experimental studies of N-2 plasmas both in terms of modal temperatures and initial (first collision cycle) cross-sections. (C) 2011 American Institute of Physics. [doi:10.1063/1.3532086]