Ab initio study of the acetylene and vinylidene dications fragmentation

The C2H++2 fragmentation processes have been studied using the complete active space self-consistent field method followed by a multireference perturbative configuration interaction, in order to interpret recent charge separation spectroscopy experiments. For two-body processes, the calculated appearance thresholds of the C2H+/H+ and CH+/CH+ fragment pairs are in good agreement with the experimental data. It is shown that the C2H++2→CH++CH+ dissociation occurs with an important rotation of the CH+ ions. The presence of the CH+2 ion is explained by a preliminary isomerization of acetylene to vinylidene dication. This reaction has been studied for the lowest lying states of C2H++2 (3Σ−g and 1Δg) and compared with other acetylenic ions isomerizations (C2H2, C2H+2, C2H−2). For three-body processes, the calculations are consistent with the mechanisms proposed by the experimentalists.