Role of intersystem crossing in the dynamics of the O(³P)+ICl reaction

Abstract

Reactive scattering of O(³P) atoms with ICl molecules has been studied at an initial translational energy E ∼ 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 angular distribution of IO reactive scattering slightly favouring the forward direction is observed with a product translational energy distribution which peaks at a very low fraction f′ ∼ 0·04 of the total available energy with a long tail extending out to higher energy. This compares with a more sharply peaked angular distribution at lower initial translational energy E ∼ 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 leading to IO reaction product is confined to very small impact parameters b _m ∼ 1·2 Å at high initial translational energy increasing to b _m ∼ 2·5 Å at lower energy. This is attributed to efficient intersystem crossing from the lowest triplet ³A″ potential energy surface to the singlet ¹A′ potential energy surface in strongly bent OICl configurations. Hence the observed IO product scattering arises from the stable singlet OICl intermediate which has a complex lifetime ∼ 2 rotational periods at E ∼ 45 kJ mol^-1. Failure to observe ClO product scattering suggests that formation of the alternative singlet IOCl intermediate is inhibited by a potential energy barrier to rearrangement of the OICl complex.