Theoretical study of the spectroscopy of Al2

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Abstract
The singlet and triplet states of Al2 below about 30 000 cm−1 have been studied at the multireference configuration‐interaction level in a [8s 7p 5d 2f] Gaussian basis. We attempt to identify and characterize the band systems in both the singlet and triplet manifolds that should be most amenable to experimental study. The spectroscopy of Al2 can be understood in terms of an X 3Πu ground state, and except for the well known (1)3Σ−u–A 3Σ−g emission system, all other transitions that we can unambiguously assign involve the X 3Πu ground state. Above about 27 000 cm−1 the spectrum of Al2 is complicated by the presence of several overlapping transitions. The calculations suggest that the assignments of the E and F systems observed recently in a jet‐cooled beam are correct, although the considerable remaining differences between the experimental and theoretical spectroscopic constants and radiative lifetimes preclude a definitive assignment. The very intense E’ system observed by Morse is assigned to the (3)3Πg–X 3Πu band system. Several singlet–singlet transitions are studied to help determine whether metastable singlet states exist in jet‐cooled beams of Al2.