Collision energy dependence of product branching in the ionization of HCl molecules by collision with metastable neon atoms

The ionization of HCl by collision with metastable neon atoms has been studied in a crossed beam‐mass spectrometric experiment in the thermal energy range. It was found that the ionization leads to HCl+, NeH+, and NeHCl+ ion products. The HCl+ ion is the main product. Its cross section shows a decreasing trend in the energy range investigated. The NeH+ cross section is about one order of magnitude lower and decreases with collision energy more rapidly than that of HCl+. The NeHCl+ ion is the minor product with a cross section decreasing dramatically with collision energy. The production of these ions can be explained as the result of the postionization dynamics of the two possible ionic complexes formed when the electron is ejected: the ground state [Ne⋅⋅⋅HCl+(X)] ionic complex dissociates into Ne + HCl+(X) or remains as a stable NeHCl+ ion; the excited [Ne⋅⋅⋅HCl+(A)] complex dissociates into Ne+HCl+(A) or reacts to NeH+ + Cl. The theoretical model used to analyze the experimental results gives a satisfactory qualitative account of the cross sections and of their collision energy dependence. The model combines the semiclassical treatment for Penning and associative ionization with the Langevin criterion for the ion–molecule reactions. It assumes a local complex spherical potential for the Ne∗–HCl interaction, the probability for the formation of each of the four possible product ions, HCl+(X), HCl+(A), NeH+, and NeHCl+ being obtained from the distribution of electronic states, kinetic energy, angular momentum, and intermolecular distance of the two nascent [Ne⋅⋅⋅HCl+(X,A)] complexes formed when the electron is emitted by the system.