A theoretical study of the low-lying states of Ti2 and Zr2

Abstract

The 1Σ+g, 3Δg, 3Σ+u, and 7Σ+u states of Ti2 and Zr2 have been studied using a multireference configuration-interaction (MRCI) approach. Although our best calculation produces a 1Σ+g ground state for Zr2, the 3Σ+u and 3Δg states are found to be very low lying. Additional support for a 1Σ+g ground state assignment comes from the fact that the resulting vertical excitation spectrum is consistent with the optical spectrum of Zr2 observed in noble gas matrices. For Ti2, it proved more difficult to make a definitive assignment of the ground state, because of the many low-lying states and the large effect of inner-shell (3s and 3p) correlation. With only valence correlation included, the ground state is predicted to be 7Σ+u at both the MRCI and averaged coupled-pair functional (ACPF) levels of correlation treatment. However, inner-shell correlation effects, estimated based on modified coupled-pair functional (MCPF) and contracted configuration-interaction (CCI) calculations, preferentially lower the 1Σ+g and 3Δg states, resulting in a 3Δg ground state and a very low-lying 1Σ+g state. Since the 3Δg state lies below the 1Σ+g state at all levels of correlation treatment and both states are derived from the same atomic asymptote, we prefer a 3Δg assignment for the ground state of Ti2. This is consistent with the failure to observe an electron-spin resonance (ESR) signal, but such an assignment requires an explanation for the absence of subcomponents on the lines observed in the resonance Raman spectrum of Ti2.