Competition between dissociation and exchange processes: contrasting dynamical behaviors in collinear H+H₂ and He+H⁺₂ collisions

Abstract

Dissociative, exchange, and nonreactive collisions of the H+H₂ and He+H⁺₂ systems in collinear geometry are examined. The behavior of the two systems is found to differ qualitatively and quantitatively. For H+H₂ (v=0), quasiclassical trajectory (QCT) calculations on the Siegbahn-Liu-Truhlar-Horowitz surface show that the dynamic threshold energy (E^dyth) for dissociation is twice the energetic threshold (E^e_th). For v=1, the elevation of E^dy_th is slightly less. There is vibrational enhancement of collision induced dissociation (CID) near threshold, but slight vibrational inhibition at higher energies. At energies above that required for dissociation, a second threshold to exchange is observed and the exchange process eventually takes over from dissociation. For He+H⁺₂ (v=0,1), QCT calculations on the McLaughlin-Thompson surface yield E^dy_th∼E^e_th for dissociation, but also show an antithreshold, with the exchange process becoming dominant at a higher energy. There is only vibrational enhancement of the dissociation process in the energy range investigated. The differences in the dynamical behavior of the two systems are analyzed in terms of reactivity band plots and individual trajectories. Examination of the dynamics for different mass combinations on the HeH⁺₂ potential energy surface gives an insight into the kinematic factors governing CID in collinear geometries. We also consider the qualitative and quantitative differences between this collinear study and our earlier three dimensional work on the same systems.