Single-electron capture processes in slow collisions of helium ions with molecular targets.

摘要

Translational energy-gain spectroscopy (TES) has been used to investigate state-selective dissociative and non-dissociative single-electron capture processes in low-energy collisions of He2+ ions with O 2, H2O, CO2, N2, and NH3 at impact energies between 100 eV and 1600 eV. The measured energy spectra for the He2+-O2 and H2O collision systems show that the dominant exit channel is due to dissociative transfer ionization (DTI), single-electron capture accompanied by ionization of the molecular target-ion, at the lowest collision energies. As the impact energy is increased non-dissociative single-electron capture (SEC) into the He+ (n = 2) states is found to populate at collision energies in excess of 1000 eV. For N2 and CO2 targets, the dominant reaction is due to DTI. In the case of the He2+-NH3 collisions, it is the SEC that is predominantly populated. The dominant reaction channels observed for collisions of He2+ with N2, CO 2, and NH3 remain dominant over the entire impact energy region studied and at laboratory scattering angles between 0° and 8°. The energy-gain spectra are interpreted qualitatively in terms of the reaction windows, which are calculated using the Landau-Zener model and the extended version of the classical-over-barrier model. The energy dependence of total cross sections for single-electron capture was also measured and found to be weakly dependent on the projectile energy. The present cross sections are also compared with the available data and are interpreted in terms of theoretical formulations based on Landau-Zener and Demkov models. These calculations show that the He+ (n = 2) formation (i.e., SEC) proceeds through a single-electron process governed by nucleus-electron interactions. In contrast, the He+ (n = 1) formation (i.e., DTI) mechanisms involves an exothermic two-electron process driven by electron-electron interactions.