Dispersed fluorescence studies of the NCO radical

Rotational transitions to excited vibrational levels of the [Xtilde] 2Π state of the NCO free radical have been studied in the gas phase by dispersion of laser induced fluorescence from the (000) level of the first excited electronic state (à 2∑+). The vibrational levels involved are the 2Π vibronic components of the (100) and (020) states which perturb each other through a Fermi resonance. Transitions from the (000) level of the à state to the (100) level of the [Xtilde] state are much stronger than those to the k 2Π(020) levels which in turn are stronger than those to the μ2Π (020) levels. Measurements have been made of the wavenumbers of 199 such transitions and the whole data set has been fitted to a model which takes the perturbing effects of Renner-Teller coupling and Fermi resonance explicitly into account. The model is more than adequate, the quality of the fit being limited by the precision of the experimental data (about ±0·15 cm-1). The positions of the k 2Π and μ 2Π components of the (020) level are reliably established for the first time. The parameters determined in the fit are discussed and the need for further work is identified.