Electronic states of Al2

Ab initio multi-configuration self-consistent field and first-order configuration interaction (FOCI) calculations in an extended basis set have been carried out for the lower energy electronic states of Al₂. The ten core electrons of each Al atom were replaced by an accurate compact effective core potential. The FOCI calculated T_o value for the ³Σ_g^?-³Σ_u^? transition agrees with the experimentally observed emission band to within 90 cm^?1. ³Π_u is calculated to be the electronic ground state of Al₂. Based on FOCI energies and qualitative intensity arguments, the reported optical absorption spectrum of matrix isolated Al₂ also agrees best with a ³Π_u ground state. The ³Σ_g^?1 state is calculated (T_e) at only 324 cm^?1 above the ³Π_u state, and the ¹ΣE_g⁺ state is predicted to lie higher.