Knowledge Bank

The physical geometry of the $X~$ and $A~$ states of nitrogen dioxide gives rise to favorable Franck-Condon factors for coupling between low-lying vibrational levels of the $A~$ state and levels of the $X~$ 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 $A~$ state, and hence to non-regular vibrational and rotational structure in the $A~$$\leftarrow$$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 $X~$ $\leftarrow$ $A~$ 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$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, corresponding to the structure r$\mathrm{<mrow\; data-mjx-texclass="ORD">0</mrow>}$ = 1.26 {\AA} $\theta 0-102\circ$ for the $A~$ state. Evidence for rotational perturbation in this system will also be discussed.