On the ground electronic states of copper silicide and its ions

The low-lying electronic states of SiCu, SiCu⁺, and SiCu^- have been studied using a variety of high-level ab initio techniques. As expected on the basis of simple orbital occupancy and bond forming for Si(S²P²)+ Cu(S¹) species, ²Π_r, ¹Σ⁺, and ³Σ^- states were found to be the ground electronic states for SiCu, SiCu⁺, and SiCu^-, respectively; the ²Π state is not that suggested in most recent experimental studies. All of these molecules were found to be quite strongly bound although the bond lengths, bond energies, and harmonic frequencies vary slightly among them, as a result of the nonbonding character of the 2π-MO (molecular orbital) [composed almost entirely of the Si 3p-AO (atomic orbital)], the occupation of which varies from 0 to 2 within the ¹Σ⁺, ²Π_r, and ³Σ^- series. The neutral SiCu is found to have bound excited electronic states of ⁴Σ^-, ²Δ, ²Σ⁺, and ²Π_i symmetry lying 0.5, 1.2, 1.8, and 3.2 eV above the ²Π_r ground state. It is possible but not yet certain that the ²Π_i state is, in fact, the "B state" observed in the recent experimental studies by Scherer, Paul, Collier, and Saykally,