Observation of a gerade symmetry state of Cu 2 using two‐color resonant four‐wave mixing

The first observation of an excited 0g +state of dicopper is reported. “g” inversion symmetry states are not observable by direct one‐photon absorption from the X1Σ g +ground state. Measurements are performed by applying nonlinear two‐color resonant four‐wave mixing (TC‐RFWM) spectroscopy. Cold Cu 2 molecules in a molecular beam environment are generated by applying a pulsed laser vaporization source and subsequent near supersonic expansion of the vaporized metal plume entrained in a helium pulse. Spectra in the wavenumber region of the [37.5( v )]0g +‐ B0u +electronic transition are recorded by using the UNFOLDED double‐resonance scheme. Specific rotational levels of the intermediate state B0u + , v ′ =1 are accessed from the X1Σ g +ground state. Dipole selection rules from the labeled levels define unambiguously the rotational quantum number and symmetry of the final state (Δ J =±0,1 and g ↔ u). The UNFOLDED scheme is verified by additional measurements via the origin level, v ′ =0, of the intermediate B0u +state. Spectroscopic constants of the [37.5( v )]0g +state have been determined for both 63 Cu 2 and 63 Cu 65 Cu isotopologues by analysis of the rotationally resolved spectra. The potential of TC‐RFWM to study the unexplored “g” symmetry manifold of Cu 2 is discussed.