Theoretical studies of collisional relaxation of highly excited SO₂ in an Ar bath

Molecular dynamics studies of collisional relaxation of highly excited SO₂ in an Ar bath are described. Most of the calculations use a newly developed global ab initio potential surface for SO₂ that correctly describes the superoxide (SOO) and ring isomers of SO₂ that occur as secondary minima on the ground-state potential surface at high energies (about 75% of the dissociation energy) above the C_2v minimum. Rate constants for the S + O₂ and O + SO reactions are calculated to test this surface, and to examine the importance of electronically excited states in the O + SO recombination. The Ar + SO₂ collisions are described by summing the ab initio potential with empirical intermolecular potentials. The resulting average vibrational energy transfer, (ΔE), per collision is in good agreement with direct measurements (performed at energies where the secondary minima are not populated) at 1000 K, but the agreement is poorer at 300 K. The agreement is significantly better than was obtained in a previous theoretical study, and our results indicate that the use of improved intramolecular and intermolecular potentials is crucial to obtaining better results. The energy dependence of (ΔE) is found to be much stronger at energies where secondary minima on the potential surface are accessible; however, much of this effect is reproduced using a potential that has the same dissociation energy but not the secondary minima.