Electron-impact ionization of hydrogen chloride

Abstract

Relative partial ionization cross sections for the formation of Cl⁺, Cl²⁺ and H⁺ from the hydrogen chloride molecule have been determined, from simultaneously recorded mass and two-dimensional ion–ion coincidence spectra, as a function of the ionizing electron energy from 30 to 200 eV. Precursor specific cross sections have been derived which quantify the contribution to the yield of each fragment ion from each level of ionization and the kinetic energy released in the fragmentation reactions of multiply charged HCl has been used to obtain thermodynamic information on the doubly and triply charged states. Analysis shows that at 100 eV electron energy double ionization contributes 12% of the total ion yield and 3% at 50 eV. At 100 eV the decay of HCl²⁺ by the loss of an H atom to form Cl²⁺ is a result of approximately 41% of the double ionization events, comparable to the frequency of charge separating dissociation (52%). The formation of Cl²⁺ has a threshold of 46.8 ± 1.5 eV and is assigned to population of the c⁵Σ⁻ dication state. The observed threshold for forming long-lived dications (HCl²⁺) is consistent with the population of the ground state of the dication, and higher vibrational levels of this state appear to contribute to the yield of H⁺ + Cl⁺ ion pairs. The formation of metastable HCl²⁺ contributes a maximum of 0.6% to the ion yield. Long-lived HCl³⁺ ions are not detected but dissociative triple ionization makes up approximately 0.7% of the ion yield at 200 eV. We estimate the lowest lying dissociative state of HCl³⁺ lies at 72 ± 2 eV above the ground state of the neutral molecule.