Electron-impact ionization of hydrogen chloride

Relative partial ionization cross sections for the formation of Cl+, Cl2+ a nd H+ from the hydrogen chloride molecule have been determined, from simult aneously recorded mass and two-dimensional ion-ion coincidence spectra, as a function of the ionizing electron energy from 30 to 200 eV. Precursor spe cific cross sections have been derived which quantify the contribution to t he 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 cha rged states. Analysis shows that at 100 eV electron energy double ionizatio n contributes 12% of the total ion yield and 3% at 50 eV. At 100 eV the dec ay of HCl2+ by the loss of an H atom to form Cl2+ is a result of approximat ely 41% of the double ionization events, comparable to the frequency of cha rge separating dissociation (52%). The formation of Cl2+ has a threshold of 46.8 +/- 1.5 eV and is assigned to population of the c (5)Sigma (-) dicati on state. The observed threshold for forming long-lived dications (HCl2+) i s consistent with the population of the ground state of the dication, and h igher vibrational levels of this state appear to contribute to the yield of H+ + Cl+ ion pairs. The formation of metastable HCl2+ contributes a maximu m of 0.6% to the ion yield. Long-lived HCl3+ ions are not detected but diss ociative 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.