Formation of HCO⁺by the associative ionization of CH+O

Abstract

The cross section and rate constant of the chemi-ionization reaction CH+O to HCO++e are calculated from a theoretical model. The treatment assumes that the collision partners follow classical adiabatic trajectories up to the region where the potential surfaces cross, and that either the Landau-Zener model or a unit probability gives the transition probability for curve-crossing along the single dimension defined by the trajectory. Potential surfaces were obtained from experimental data and INDO calculations. Cross sections and rates were computed for a variety of parameters, in order to test their sensitivity to the model. The process can proceed from CH+O in their ground states; of the available potential surfaces of neutral HCO, the most important is that of the 2 Sigma - state, followed by the two surfaces associated with the 2 Delta state. The cross section for the 2 Sigma - surface may be as high as 8 AA2 for collisions of very low velocity. The rate coefficient curve deemed most probable gives values of ca. 3.5*10-11 cm3 s-1 at 1000 K, and about 7*10-12 cm3 s-1 at 25 K.