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(s(2)p(2))+Cu(S-1) species, (2) Pi(r), (1) Sigma(+), and (3) Sigma(-) states were found to be the ground electronic states for SiCu, SiCu+, and SiCu-, respectively; the (2) Pi, 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 2 pi-MO (molecular orbital) [composed almost entirely of the Si 3p-AO (atomic orbital)], the occupation of which varies from 0 to 2 within the (1) Sigma(+), (2) Pi(r), and (3) Sigma(-) series. The neutral SiCu is found to have bound excited electronic states of 4 Sigma(-), (2) Delta, (2) Sigma(+), and (2) Pi, symmetry lying 0.5, 1.2, 1.8, and 3.2 eV above the (2) Pi(r). ground state. It is possible but not yet certain that the 2 Pi(i) state is, in fact, the "B state" observed in the recent experimental studies by Scherer, Paul, Collier, and Saykally. (C) 1998 American Institute of Physics.