COLLISIONAL EXCITATION OF CO BY SLOW MOLECULAR-IONS - WAVE-FUNCTION OVERLAP EFFECTS

High-resolution translational-energy spectrometry has been applied to study excitation of CO molecules in 1.8 keV collisions with H-2+, N2+, CO+, CO2+, O2+ and CS2+ projectiles. Transitions to the optically for bidden triplet states a 3PI and a' 3SIGMA+, optically allowed states A 1PI, B1SIGMA+ and G1PI, and ionizing transitions to the low-lying stat es of CO+, X2SIGMA+, A2PI and B2SIGMA+, are observed. Oscillator stren gths for electronic transitions to individual states are obtained by s imulating the measured energy-loss spectra with calculations using eit her spectroscopically determined or calculated Franck-Condon factors. Wavefunction overlap effects manifest themselves in the observation th at the cross section for collisional excitation of CO is found to be s ensitively dependent on whether the molecular projectile ion posseses SIGMA of PI symmetry. Excitation cross sections are at least a factor of 10 larger in the case of SIGMA projectiles.