Rotational Transitions of CO + Induced by Atomic Hydrogen

Abstract

The CO+ molecular ion has been observed in photon-dominated regions. Recent modeling has not been able to explain the abundances of CO + in these regions. The most abundant collision partners are believed to be hydrogen atoms, hydrogen molecules, and electrons. The reactions of these species with CO+ have been studied previously and found to be fast. The only inelastic processes studied before were collisions of CO+ with electrons. Here we investigate the inelastic collisions of CO+ with hydrogen atoms. We argue that this can be done on the lowest triplet electronic state. This implies that CO+ ions, in a hydrogen atom dominated surrounding, experiences a few inelastic collisions before reacting. Rotationally inelastic state-to-state cross sections (j ≤ 8) are presented together with state-resolved thermal rate coefficients. The results suggest that the rotationally inelastic collision cross sections can be of comparable magnitude to the reactive ones. Implications for the rotational temperature of CO+ in PDRs are discussed. To perform the calculations, a potential energy surface is fitted using the reproducing-kernel Hubert space method based on multi-reference configuration-interaction calculations employing the aug-cc-pVQZ basis set. © 2008. The American Astronomical Society. All rights reserved.