The electronic spectrum of NiCN in the 500-630 nm region has been observed by laser-induced fluorescence, following the reaction of laser-ablated nickel atoms with cyanogen under free jet expansion conditions. Seven electronic states have been identified. Three of these, X1 2 5/2 , X2 2 3/2 , and W1 2 3/2 , are derived from the electron configuration Ni + (3d 9 ) CN -, and the other four, Γ2 5/2 , B2 3/2 , C2 7/2 , and D2 5/2 , are derived from the configuration Ni + (3d 8 4s) CN -. Rotational analysis of bands of NiC 14 N and NiC 15 N at high resolution has given the bond lengths in the X1
2 5/2 ground state as r 0 (Ni-C) = 1.8292 Β± 0.0028 A β ; r 0 (C-N) = 1.1591 Β± 0.0029 A β . The orbital angular momentum splits the bending fundamental of the X1 2 5/2
state into two vibronic components, which lie at 243.640 cm -1 ( 2 3/2 ) and 244.964 cm -1 ( 2 7/2 ). Exceptionally strong Fermi resonance occurs in the ground state between the Ni-C stretching vibration, Ξ½ 3 , and the overtone of the bending vibration. Sixty vibrational levels of the ground state with |l| = 0 and 1 have been assigned. They could be fitted by least squares to a simple matrix representation of the Fermi resonance that ignores the orbital angular momentum; the interaction matrix element, as a fraction of the bending frequency, turns out to be larger than that in the "prototype" molecule, CO 2 . The two low-lying excited electronic states, X2 2 3/2 at 830 cm -1 and W1 2 3/2 at 2238 cm -1 , have very similar properties to the ground state. The energies of the excited states in the visible region bear a remarkable resemblance to those found in NiH (S. A. Kadavathu et al., 1991, J. Mol. Spectrosc. 147, 448-470). Again, the effects of Fermi resonance in them are large but those of vibronic coupling are surprisingly small. The most significant vibronic coupling occurs between the Γ2 5/2 and B2 3/2 states, which are separated by 79 cm -1 , an interval which is less than half the frequency of the bending vibration that couples them; large numbers of vibronically induced bands appear in the excitation and dispersed emission spectra, but the splitting between the vibronic components of the bending fundamental of the Γ2 5/2 state is only 6.988 cm -1 . Rotational perturbations are widespread in the Γ2 5/2 state, particularly in the levels of the Ξ½ 3 (Ni-C stretching) progression. Vibrational assignments have been made on the basis of the 58 Ni-60 Ni isotope shifts at high resolution and the Franck-Condon patterns in dispersed emission spectra. Various weak unassigned bands, both in the excitation and the dispersed emission spectra, give evidence for the existence of at least three more excited electronic states which it has not been possible to characterize in detail.