Abstract
Reactivity of the dication CO 2 2+ in dependence on its internal energy was studied in collision with CO 2 and CO. The dication was obtained by photoionization of the CO 2 molecule by synchrotron radiation and the guided beam apparatus CERISES was used in these studies. Spontaneous dissociation of CO 2 2+ to CO + and O + was observed without any target gas in the reaction cell. With CO 2 or CO as a target gas, CO + and O + fragment ions were formed as products of dissociative charge transfer, while the contribution of collision-induced dissociation of the reactant CO 2 2+ to their formation was found negligible. The CO + yield showed a considerable dependence on the internal energy of the reactant dication CO 2 2+: the branching ratio CO +/O + increased gradually with photon energy between 38.7 and 39.4 eV. We interpret this observation as being due to the increase in the instantaneous distances at the extrema of vibrational motion of the dication with increasing internal energy. This modifies the Franck-Condon factors in the charge transfer process and leads to a preferential population of the vibrationally excited states of the product cation CO 2 +(C) known to lead exclusively to the dissociation products CO ++O. An indirect mechanism that requires the coupling of the first excited (a) state of the dication with the higher vibrational states of the CO 2 2+ (X) state is proposed to explain the threshold behaviour observed in our results.
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