Rovibrational investigation of a new high-lying u+ state of Cu2 by using two-color resonant four-wave-mixing spectroscopy

A highly excited electronic state of dicopper is observed and characterized for the first time. The [39.6] 0 u + -X 1 Σ g + (0 g + ) system is measured at rotational resolution by using degenerate and two-color resonant four-wave-mixing, as well as laser induced fluorescence spectroscopy. Double-resonance experiments are performed by labeling selected rotational levels of the ground state by tuning the probe laser wavelength to transitions in the well known (1-0) band of the B 0 u + -X 1 Σ g + (0 g + ) electronic system. Spectra obtained by scans of the pump laser in the UV wavelength range were then assigned unambiguously by the stringent double-resonance selection rules. The absence of a Q-band suggests a parallel transition (∆Ω = 0) and determines the term symbol of the state as 0 u + in Hund's case (c) notation. The equilibrium constants for 63 Cu 2 are T e = 39559.921(92) cm −1 , ω e = 277.70(14) cm −1 , B e = 0.104942(66) cm −1 and r e = 2.2595(11) Å. These findings are supported by high level ab initio calculations at the MRCI+Q level, which clearly identifies this state as resulting from a 4p ← 3d transition. Additionally, three dark perturber states are found in the v = 1 and v = 2 vibrational levels of the new state. A deperturbation analysis characterizes the interaction and rationalizes the anomalous dips in the excitation spectrum of the [39.6] 0 u + -X 1 Σ g + (0 g + ) system.