LASER-INDUCED FLUORESCENCE MEASUREMENT OF SPIN-ORBIT SPLITTINGS IN EXCITED VIBRATIONAL LEVELS OF $NCO (X^{2}\Pi_{i})^{\ast}$

$^{\ast}$Supported by the U.S. Army Research Office, Contract No. DAAG29-80-K-0049. $^{1}$R.N. Dixon, D. Field and M. Noble, Chem. Phys. Lett. 50, 1 (1977); K.G. Weyer, R.A. Beaudet, R. Straubinger, and H. Walther, Chem. Phys. 47, 171 (1980). $^{2}$R.N. Dixon and D.A. Ramsay, Can. J. Phys. 46, 2619 (1968).Author Institution: Molecular Physics Department, SRI InternationalIn this study on NCO we use the combination of laser excitation and a $CH_{4}/N_{2}O$ atmospheric pressure flame to furnish new high resolution spectroscopic information on a ground state transient molecule in excited vibrational levels. The linear NCO molecule is of interest both spectroscopically, since the $X^{2}\Pi_{i}$ state has a Renner-Teller split bending vibration with possible Fermi resonances, and chemically, as an intermediate in several combustion mechanisms. We have undertaken a detailed investigation of these NCO ground state levels $0 \nu_{2}0, 1\nu_{2}0, 2\nu_{2}0, 0\nu_{2}1$ and $1\nu_{2}1$ where $\nu_{2}$ varies from 0 to 2 in some cases. Spin-orbit splitting constants A, rotational constants and vibrational band origins are extracted for several previously unobserved vibrational levels. A preliminary fit to band head positions shows that the magnitude of A (in $cm^{-1}$, with typical $0.3 cm^{-1}$ error) decreases with $\nu_{1}$ and increases with $\nu_{3}: A_{000}=-95.5; A_{100}=-90.0; A_{200}=-71.8; A_{001}=-97.3; A_{101}=-92.6$. The Renner-Teller splitting also changes with stretching vibrations. Similar variations in the spin-orbit coupling constants are seen in $BO_{2}(X^{2}\Pi_{g})^{1}$ and $NCS(A^{2}\Pi_{i})^{2}$. These molecules possess a strong Fermi interaction which may also contribute to the large spin-orbit splitting variations in the $\nu_{1}$ levels of the $X^{2}\Pi_{i}$ state of NCO