Collision spectroscopy of ion-diatom systems: ${\mathrm{He}}^{+}$ -${\mathrm{N}}_2$ and -${\mathrm{O}}_2$ at medium energies

Energy-loss measurements of direct-excitation and electron-capture processes are performed for the ${\mathrm{He}}^{+}$ -${\mathrm{N}}_2$, -${\mathrm{O}}_2$ collisional systems in the 0.2-4-keV energy range and in the 0°-3° angular range. Pure vibrational excitation and vibrational excitation in excited electronic states are studied using a high-resolution spectrometer (100 meV). Pure vibrational excitation is always found to be a weak process. The analysis of vibrational distributions in excited states strongly supports an interpretation in terms of the Franck-Condon principle. Relative probabilities as functions of energy and angle are reported for the identified excitation and electron-capture processes. The results show the importance of electron-capture processes among which quasiresonant and endothermic processes dominate exothermic processes. The striking resemblance of the present data with those available on the ${\mathrm{He}}^{+}$ -rare-gas systems conveys a similar quasimolecular interpretation of the collision mechanisms.