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 Deltanu=0 band of the CO(2)(+*) B (2)Sigma(u)(+) --> X (2)Pi(g) electronic transition and the Deltanu= +3, +2, +1, 0, -1, -2, -3 vibrational transition progression in the CO(2)(+*) A (2)Pi(u) --> X (2)Pi(g) 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 (2)Pi(u) --> X (2)Pi(g) 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 (2)Pi(u) 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.