Fragmentation of Na₂⁺ dimer ions in kilo-electron-volt collisions with He: A coupled wave-packet study

The dynamics of the fragmentation of Na₂⁺ dimer ions in keV collisions with He is investigated theoretically in the framework of the semiclassical coupled wave-packet method. The fast collisional motion is treated classically, whereas both the electronic motion and the vibrational or dissociative motion of the dimer are treated quantally. The frozen vibrational and rotational approximations are used to lighten the computational effort. The method is able to describe both the impulsive and electronic fragmentation mechanisms. The calculations are undertaken in a basis of 14 electronic states of the Na₂⁺-He system. Aside from the entrance Σ_g3s channel, the main contributions to the Na₂⁺ fragmentation are those of the Σ_u3s, Σ_g3p, and Π_u3p channels. For each electronic state dissociation probabilities as functions of the impact parameter are presented for various dimer orientations to extract physical insight in the fragmentation mechanism. This analysis shows in particular that the contribution of the electronic mechanism is due to collisions where the He atom passes between the two Na nuclei. Doubly differential cross sections for dissociation are calculated for the v=0 initial state of the dimer and for the distribution of vibrational states. Good agreement with experiment is obtained assuming a small amount of initial vibrational excitation of the dimer.