Energy transfer, stabilization, and dissociation in collisions of helium with highly excited HO₂

This paper considers application of the classical trajectory method to the determination of cross sections and rate constants for energy transfer, stabilization, and dissociation in collisions of He with highly excited HO₂. The resulting rate constants are used to evaluate several models of the termolecular recombination kinetics in He + H + O₂, such as the weak and strong collision models of stabilization. An ab initio potential energy surface is used for the HO₂ molecule, and the He + HO₂ intermolecular potential is approximated as a sum of pairs. Cross sections and rate constants are evaluated for two types of initial HO₂* ensembles, one which is microcanonical for the HO₂ vibrations but cold for rotation, and one which is microcanonical for both vibration and rotation. For both the rotationally cold and hot ensembles, the stabilization cross sections were found to decrease rapidly with internal energy E_int and increase rapidly with translational energy E₀ above the dissociation threshold, with the cross sections always smaller for cold HO₂* than for hot. This difference between cold and hot is interpreted in terms of the greater efficiency of R → T energy transfer for rotationally hot HO₂*. For both H + O₂ and OH + O dissociation, the cross sections increase rapidly with increasing E_int, with the rotationally cold cross sections generally much larger than rotationally hot. The stabilizing influence of centrifugal barriers was found to be important in determining the smaller rotationally hot cross sections. An analysis of stabilization and H + O₂ dissociation rate constants indicates that dissociation is faster than stabilization for cold HO₂*, but neither rate constant is more than a few percent of the gas kinetic rate constant. For hot HO₂*, stabilization is much faster than dissociation, and the stabilization rate constant is 14-18% of gas kinetic. Roughly speaking, hot HO₂* is pretty well approximated by the strong collision model (with an appropriate efficiency factor), while cold HO₂* is better approximated by using the weak collision approximation. These results emphasize the importance of using accurate stabilization rate constant information in the kinetic modeling of termolecular recombination rates, and the potential importance of collision-induced dissociation in the recombination mechanism.