ROLE OF INTERSYSTEM CROSSING IN THE DYNAMICS OF THE O(P-3)+ICL REACTION

Reactive scattering of O(P-3) atoms with ICl molecules has been studie d at an initial translational energy E similar to 98 kJ mol(-1) using a supersonic beam of O atoms seeded in He buffer gas generated from a high temperature radiofrequency discharge source. A mildly peaked angu lar distribution of IO reactive scattering slightly favouring the forw ard direction is observed with a product translational energy distribu tion which peaks at a very low fraction f' similar to 0.04 of the tota l available energy with a long tail extending out to higher energy. Th is compares with a more sharply peaked angular distribution at lower i nitial translational energy E similar to 45 kJ mol(-1). In both cases, the product translational energy distributions are shifted to higher energy for the forward and backward scattering compared with sideways scattering. Extended phase space calculations indicate that reaction l eading to IO reaction product is confined to very small impact paramet ers b(m) similar to 1.2 Angstrom at high initial translational energy increasing to b(m) similar to 2.5 Angstrom at lower energy. This is at tributed to efficient intersystem crossing from the lowest triplet (3) A'' potential energy surface to the singlet (1)A' potential energy sur face in strongly bent OICl configurations. Hence the observed IO produ ct scattering arises from the stable singlet OICl intermediate which h as a complex lifetime similar to 2 rotational periods at E similar to 45 kJ mol(-1). Failure to observe ClO product scattering suggests that formation of the alternative singlet IOCl intermediate is inhibited b y a potential energy barrier to rearrangement of the OICl complex.