Rotationally resolved dicopper (Cu2) laser-induced fluorescence spectra

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Abstract
We have used an RF sputter source to produce gas‐phase copper dimer molecules for study with laser‐induced fluorescence spectroscopy. Rotationally resolved spectra of transitions from the electronic ground (X) state to the A, B, C, G, and J states have been obtained. Previous rotational analyses of the X, A, and B states are confirmed. Analysis of the X–C spectrum shows the C state to have Π(Ω=1) symmetry, and a bond length of 2.26 Å. Its vibrational constants are refined slightly. The J state has a bond length of 2.15 Å, and shows evidence of extensive perturbation. Emission lifetimes at the bandheads of its lowest two vibrational states are 80 ns. Franck–Condon factors for the X–C and X–J transitions have also been obtained. The new J state bond length supports conclusions drawn from a recently obtained photoelectron spectrum of Cu2: the cation ground state indeed corresponds to loss of a 4sσ electron, and the excited state is due to removal of a 3d electron. Bond strengths and distances in the various states of the neutral are rationalized in terms of 4sσ bonding orbitals and nonbonding (slightly repelling) 3d cores; creation of a 3d vacancy allows 3d shell bonding to occur, increasing bond strengths and vibrational frequencies slightly.