Statisticodynamical approach of final state distributions in associative desorptions

In associative desorptions governed by short-range forces, strong energy transfers between rotational, translational, and vibrational degrees of freedom of the nascent molecule take place beyond the transition state (TS). State or energy distributions are thus different at the TS and in the vacuum. Inspired by previous studies on unimolecular reactions, a statisticodynamical approach (SDA) is proposed which combines the transition state theory for the description of state distributions at the TS and a simple dynamical model to account for the effect of the post-TS dynamics on these distributions. The agreement found between the predictions of SDA, the much more demanding classical trajectory calculations, and experimental results in the case of H(2) and D(2) desorbing from Cu(111) is very satisfying. In addition to that, the formulation highlights some key factors of state distributions which would have been difficult to directly extract from a dynamical study. The notions of vibrational heating and rotational cooling are discussed within this context.