Electronic excitation ofH2in slow collisions with molecular ions

Excitation of molecular hydrogen has been studied in 1.8-keV collisions with molecular projectile ions H[sub 2][sup +], N[sub 2][sup +], CO[sup +], O[sub 2][sup +], and CO[sub 2][sup +] by means of energy-loss spectrometry using a high-resolution ion translational-energy spectrometer. Excitation of the target H[sub 2] is observed only in the case of collisions with [ital H][sub 2][sup +]. Oscillator strengths for electronic transitions to various electronic states are determined by simulating the measured energy-loss spectra with calculations using calculated Franck-Condon overlap integrals. Excitation of the projectile ions is observed in the case of collisions with N[sub 2][sup +] and CO[sup +] projectiles. Neither projectile excitation nor target excitation is observed when experiments are carried out with O[sub 2][sup +] and CO[sub 2][sup +] projectiles. The results obtained in the present study are interpreted as a manifestation of wave-function-overlap effects involving the highest occupied molecular orbitals of the target and projectile ions.