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 d escribes the superoxide (SOO) and ring isomers of SO2 that occur as se condary minima on the ground-state potential surface at high energies (about 75% of the dissociation energy) above the C-2v minimum. Rate co nstants for the S + O-2 and Of SO reactions are calculated to test thi s surface, and to examine the importance of electronically excited sta tes in the O + SO recombination. The Ar + SO2 collisions are described by summing the ab initio potential with empirical intermolecular pote ntials. The resulting average vibrational energy transfer, [Delta E], 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 b etter than was obtained in a previous theoretical study, and our resul ts indicate that the use of improved intramolecular and intermolecular potentials is crucial to obtaining better results. The energy depende nce of [Delta E] is found to be much stronger at energies where second ary minima on the potential surface are accessible; however, much of t his effect is reproduced using a potential that has the same dissociat ion energy but not the secondary minima.