Collisional excitation of CO by slow molecular ions: wavefunction overlap effects

Abstract

High-resolution translational-energy spectrometry has been applied to study excitation of CO molecules in 1.8 keV collisions with H₂⁺, N₂⁺, CO⁺, CO₂⁺, O₂⁺ and CS₂⁺ projectiles. Transitions to the optically forbidden triplet states a³ Pi and a'³ Sigma ⁺, optically allowed states A ¹ Pi , B ¹ Sigma ⁺ and G ¹ Pi , and ionizing transitions to the low-lying states of CO⁺, X² Sigma ⁺, A ² Pi and B ² Sigma ⁺, are observed. Oscillator strengths for electronic transitions to individual states are obtained by simulating the measured energy-loss spectra with calculations using either spectroscopically determined or calculated Franck-Condon factors. Wavefunction overlap effects manifest themselves in the observation that the cross section for collisional excitation of CO is found to be sensitively dependent on whether the molecular projectile ion possesses Sigma or Pi symmetry. Excitation cross sections are at least a factor of 10 larger in the case of Sigma projectiles.