The (A)over-tilde2 Δ5/2 ← (X)over-tilde2 Δ5/2 electronic band system of nickel acetylide, NiCCH

Optical spectra of the supersonically cooled NiCCH radical have been recorded in the 530-650 nm region using the resonant two-photon ionisation method, and five of the observed bands have been rotationally resolved. The rotationally resolved studies demonstrate that the ground state of NiCCH is of [GRAPHICS] symmetry, corresponding to a 3d(9) configuration on the nickel atom, with the hole located in a 3d delta orbital. Most of the observed bands are assigned to the [GRAPHICS] electronic transition, which shows a progression in the Ni-C stretching mode, nu(3). In addition, single excitations of the modes nu(2) (C equivalent to C stretch), nu(4) (C equivalent to C-H bend), and nu(5) (Ni-C equivalent to C bend) are observed, allowing these vibrational intervals to be determined for the [GRAPHICS] state. Hot bands also allow the determination of nu(5) in the ground [GRAPHICS] state. Quantum chemical computations have also been performed, confirming that the ground state is of (2)Delta symmetry. The results are placed in context via comparison to other monovalent, monoligated Ni complexes, particularly NiCu, NiH, and NiCN.