Identification of a new low energy 1_u state in dicopper with resonant four-wave mixing

The low energy electronic structure of the copper dimer has been re-investigated using non-linear four-wave mixing spectroscopy and high level ab initio calculations. In addition to the measurement of the previously reported A, B, and C electronic states, a new state denoted A' is identified with T₀ = 20 100.4090(16) cm^-1 (⁶³Cu₂). Rotational analysis of the A'-X (0,0) and (1,0) transitions leads to the assignment of A' 1_u. Ab initio calculations present the first theoretical description of the low energy states of the copper dimer in Hund's case (c) and confirm the experimental assignment. The discovery of this new low energy excited state emphasizes that spin-orbit coupling is significant in states with d-hole electronic configurations and resolves a decades-long mystery in the initial assignment of the A state.