Laser excitation spectroscopy of the A and B states of jet-cooled copper dimer: evidence for large electronic isotope shifts

Fluorescence excitation spectra recorded for the A-X system of jet-cooled Cu2 show conclusive evidence of a ?? = 0 transition, and the A state is thereby definitively assigned as ??u+. A previous assignment of the B state as ??u+ is confirmed, but the vibrational levels of this state are complicated by the presence of a perturbation at v' = 0. The perturbing state does not, however, appear to be either of the two optically accessible electronic states in this spectral region. Anomalously large electronic isotope shifts are observed for the A and B states, and this behavior is discussed in terms of the correspondingly large "specific mass shifts" observed in the optical spectra of atomic copper for transitions that couple states differing in the number of d electrons. Due to the large spin-orbit coupling constants in the "cMiole" configurations, it is proposed that the low-energy-excited molecular states of Cu2 derived from these configurations should be described by Hund's case (c) coupling. Dynamical effects observed in the gas phase and in solid matrices are briefly discussed in terms of this bonding scheme.