Temperature dependence of fast neutral-neutral reactions: a triatomic model study

Some neutral-neutral reactions are known to proceed rapidly at low temperat ures as a consequence of strong inverse temperature power-law dependences o f the rate constants. Previous calculations, based on capture approximation s, failed to account for these experimental data. In this article, short-ra nge effects (subsequent to capture) are investigated using a simple planar atom-diatom toy model based on pairwise atomic interactions. Reaction rate constants have been estimated in the temperature range of 25-300 K using a quasi-classical trajectory Monte Carlo approach. It is shown that a small s hort-range barrier in the entrance valley may significantly influence the r eactivity. In particular, our crude triatomic model can reproduce a strong inverse temperature dependence of the rate constant in good agreement with experimental evidence for more complex systems. These predictions are inter preted using a 3-D representation of the effective potential surfaces, illu strating the crucial importance of vector correlations between partial angu lar momenta. Thus, the strong inverse power-law temperature dependence of t he rate constant may be attributed to the population of higher rotational s tates of the reactants with increasing temperature. (C) 2000 Elsevier Scien ce B.V. All rights reserved.