Rovibrational investigation of a new high-lying 0 state of Cu 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 -XΣ (0 ) 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 B0 -XΣ (0 ) 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 (ΔΩ = 0) and determines the term symbol of the state as 0 in Hund's case (c) notation. The equilibrium constants for Cu are T = 39 559.921(92) cm, ω = 277.70(14) cm, B = 0.104 942(66) cm, and r = 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. In addition, 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 -XΣ (0 ) system.