Should a franck-condon or a curve-crossing picture be applied to ion-target collisional activation? A study of keV CO2+·/He collisions by emission spectroscopy

Collision-induced photon emissions (CIE) were observed for keV CO 2 +· /He collisions from 190 to 1020 nm. The emissions were assigned to the Δν=0 band of the CO 2 +· B 2Σ u + → X 2Πg electronic transition and the Δν = +3, +2, +1, 0, −1, −2, −3 vibrational transition progression in the CO 2 +· A 2Πu → X 2Πg electronic transition. The other peaks arise from the emissions of excited O· fragment atoms and the target gas. The relative intensities of the CO 2 +· and O· emissions are independent of the ion translational energy above 3 keV, supporting the curve-crossing mechanism for collisional excitation. Investigation of the relative intensities within the A 2Πu → X 2Πg emission of CO 2 +· indicates that the vibrational distribution is well described by the Franck-Condon principle at high collision energy, a consequence of short collision time but not necessarily an indication of vertical transitions. Below 3 keV ion translational energy, vibrational excitation in the A 2Πu electronic state was observed. The observation is consistent with the explanation that the reaction occurs at small impact parameters, in which short-range, repulsive interactions between the projectile and the target result in direct translational-vibrational excitation.