The computed spectrum of AlC

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Abstract
The low-lying (up to about 40 000 cm−1) doublet and quartet states of AlC have been studied at the CASSCF and MRCI levels of theory. The bonding in the X 4Σ− state is shown to involve three one-electron bonds. The first excited state, a 2Π, lies about 8700 cm−1 higher and like the X 4Σ− state is derived from ground state Al and C, but has a two-electron π bond leading to a much shorter bond length. Those states derived from Al 3s13p2 are much higher in energy, since the promotion energy is not recovered by the formation of two-electron bonds. Transition moments are presented for all the dipole-allowed transitions. The very strong B 4Σ−–X 4Σ− transition near 22 600 cm−1 is probably the best way to observe AlC in either emission or absorption.