Identification of a new low energy 1u 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 mixingspectroscopy and high level ab initiocalculations. 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′ 1u. Ab initiocalculations 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 configurationsand resolves a decades-long mystery in the initial assignment of the A state.