On the collision energy dependence of the reaction cross section

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Abstract
A microcanonical transition state model calculation is presented to account for the kinetic energy dependence of the reaction cross section σR(E T). An atom-diatom reaction scheme was used to obtain different expressions for the reaction cross section as a function of the internal density of states of the transition state and, on the other hand, of the adiabatic character of the rotational and vibrational motion of the diatomic. The model was applied to the M + RX → MX + R, M + X 20 → M0 + X 2, etc., where M = alkali, X = halogen or nitrogen and R = alkyl group. The model seems to explain satisfactorily the shape of these excitation functions and shows that the shape evolution of σR with E T is a direct consequence of the collision energy dependence of the transition state location. Indeed, it is shown that for several exoergic reactions the maximum in σR(E T) can be caused by the shift of the transition state from an early location (at the entrance channel) towards the product valley as E T increases.