Crossed beam studies of the O(P-3,D-1)+CH3I reactions: Direct evidence of intersystem crossing

The angular and velocity distributions of the IO product from the reactions O((PD)-P-3)+CH3I have been obtained in crossed beam experiments with a rot ating mass spectrometer detector at collision energies of 55.2 and 64.0 kJ mol(-1). The center of mass product angular and translational energy distri butions for both the O(P-3) and O(D-1) reactions have been derived, and the effect of electronic excitation and the role of intersystem crossing (ISC) assessed. The O(P-3) reaction proceeds, with comparable cross-section, bot h via a direct mechanism on the triplet potential energy surface with rebou nd dynamics and via a long-lived complex mechanism following ISC from the t riplet to the singlet surface. The O(D-1) reaction proceeds on the singlet surface via formation of a complex that lives about one rotational period a nd also, with comparable cross-section, via direct rebound dynamics followi ng a nearly collinear O-I-CH3 approach geometry. ISC from the triplet to th e singlet surface is attributed to the presence of the heavy halogen atom a nd occurs for bent geometry. These findings are corroborated by recent theo retical calculations on the stationary points of the potential energy surfa ces for the system. Calculations based on phase space theory, which assumes conservation of energy and angular momentum and takes into account the var ious degrees of freedom involved, have been performed; the product angular and translational energy distributions derived for the O(P-3) reaction proc eeding via ISC and long-lived collision complex formation are in very good agreement with the experimental quantities.