A Laser Magnetic Resonance Study of Excited Vibronic States of $NCO(\bar{X}^{2}\Pi$)

Abstract

The Renner-Teller effect in NCO splits each bending vibrational level($\nu_{2}$) into several closely spaced vibronic states. This complicates the assignment of ro-vibrational spectra since each vibrational transition has several vibronic bands whose origins are similar and not accurately $known^{1}$. A large number of complex resonances have been observed in a search of the region to the red of the $\nu_{3}$ fundamental$(1921.3cm^{-1}$) using a CO-lmr spectrometer. NCO transitions in this region originate from the excitation of $\nu_{3}$ between vibronic states arising from $\nu_{2}=1$ and $\nu_{2}=2$. So far we have assigned transitions to the `unique' $2^{1}_{1}3^{1}_{0} {^{2}}\Delta_{3/2}$ and $2^{2}_{2}3^{1}_{0} {^{2}}\Phi_{7/2}$ bands which show Hund's case(a) behaviour. Because of the small rotational constant in $NCO(\approx 0.39cm^{-1})$ the resonances show highly non-linear Zeeman effects. This makes the recognition of P, Q and R line $M_{J}$ patterns very difficult, since they are greatly distorted from the familiar intensity and splitting patterns characteristic of simpler systems. Other spectra not fully analysed as yet, show the characteristics of Hund's case(b) vibronic $^{2}\Sigma$ and $^{2}\Pi$ states. Resonances thought to be due to the latter show a large K-type doubling. We have also recorded many other spectra possibly involving states with $\nu_{2} > 2$, but our failure to observe the $\nu_{3}$ hot $bands^{2}$ $3^{2}_{1}$ and $3^{3}_{2}$ suggests that very hot levels are not well populated by our method of NCO generation$(HNCO + F^{\ast}$). The experimental details, the results of our continuing analysis and their interpretation in terms of the Renner-Teller effect will be discussed.