State-to-state chemistry with fast hydrogen atoms. Reaction and collisional excitation in H + CO2

Abstract

We present a detailed theoretical study of vibrational–rotational excitation and reaction in collisions of CO₂ with 1.9–2.6 eV hydrogen atoms. Minima and saddle points on the potential surface have been characterized using ab initio configuration interaction calculations, and a global surface has been developed by a combination of many-body expansion and surface-fitting methods. The collision dynamics have been studied using quasiclassical trajectories, with the CO₂ vibrational states characterized by a Fourier-transform action calculation. For non-reactive scattering there is reasonable correlation between the vibrational modes that are excited and the regions of the potential surface sampled during the collisions. Most of the lower states of CO₂ are excited by direct collisions that do not sample potential wells. Collisions which do sample wells lead to short-lived HOCO and HCO₂ complexes, in which either the H dissociates to produce highly excited overtone and combination states of CO₂, or a CO bond breaks to give OH + CO having close to statistical vibrational–rotational distributions. Comparison of cross-sections and final-state distributions with experiment is excellent for the reactive collisions, and is good on a relative but not absolute basis for the non-reactive collisions.