VIBRONIC COUPLING OF THE $\tilde{X}^{2}A_{1}$ AND $\tilde{A}^{2}B_{2}$ STATES OF NITROGEN DIOXIDE

Abstract

The physical geometry of the $\tilde{X}$ and $\tilde{A}$ states of nitrogen dioxide gives rise to favorable Franck-Condon factors for coupling between low-lying vibrational levels of the $\tilde{A}$ state and levels of the $\tilde{X}$ state in which the bending vibration $\nu_{2}$ is excited by 8--12 quanta. Vibronic coupling between the electronic states leads to large perturbations in the vibration-rotation manifold of the $\tilde{A}$ state, and hence to non-regular vibrational and rotational structure in the $\tilde{A}$$\leftarrow$$\tilde{X}$ band system, seemingly-erratic isotope shifts, and many ``extra” features in absorption. An attempt to de-perturb the rotational structure of the 0-0 band of the $\tilde{X}$ $\leftarrow$ $\tilde{A}$ system will be described. This calculation is necessarily highly parameterized but the “best” values determined for the A and B constants, are A = 2.77, B = 0.495 cm$^{-1}$, corresponding to the structure r$_{0}$ = 1.26 {\AA} $\theta_{0} - 102^{\circ}$ for the $\tilde{A}$ state. Evidence for rotational perturbation in this system will also be discussed.