Theoretical prediction of the potential curves for the lowest-lying states of the CSi+ and Si2+ molecular ions

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Abstract
Large‐scale ab initio CI calculations have been carried out for the prediction of the potential curves of the electronic states of the isovalent CSi+ and Si2+ molecular ions which dissociate into C (3Pg)+Si+(2Pu) and Si (3Pg)+Si+(2Pu), respectively. The present results have been compared with previous findings for C2+ and the various parent neutral molecules, and the ionization potentials of the neutral species and the dissociation energies of all these diatomic species have been calculated. The ground and the first excited states of the ions are invariably π2(u)σ(g) X 4Σ(g)−, π3(u)–π(u)σ2(g) 1 2Πu, and π2(u)σ(g) 1 2Δ(g). The relative stabilities and potential curves of the various electronic states are markedly influenced by silicon substitution, as is the relative stability of the key σ(g) and π(u) MO’s. An increase in the number of Si atoms progressively favors double σ(u) occupation and leads to a lowering of the energy spacings between corresponding electronic and vibrational states, while π(u) ionization gradually becomes preferred over that of σ(g). The calculated ionization potentials and dissociation energies confirm, in general, the indirect experimental estimates of these quantities and allow for a more realistic assignment for the appearance potential of Si2+.