On the estimation of cross sections for the formation of triatomic collision complexes

Abstract

The traditional procedure of associating the formation of a collision complex with the passage over a centrifugal potential barrier is reinvestigated. It is argued that the complex is better defined in terms of a spatially localized critical surface (or separation), such that the complex is strongly coupled, while the system is weakly elsewhere. Weak coupling implies that the rotational and vibrational energies of the diatomic fragment are, to a good approximation, conserved. Strong coupling implies that only total energy and angular momentum is conserved. Graphical methods are used to obtain upper bounds for the cross section within weak or strong coupling dynamics by optimizing the attraction within the given constraints. Results for a K + NaCl system and a O( 1D)+H 2 system are presented and compared with results of molecular dynamic simulations available in the literature. The weak coupling constraints are found to give a useful representation of the simulation data. Discontinuities in the optimal orientation of approach indicate the presence of orientational barriers.