Knowledge Bank

Potential surfaces in $C2v$ space have been constructed for the low-lying electronic states of $NO2$ from ab initio multi-configuration self-consistent-field (MC-SCF) wave functions. Franck-Condon factors calculated from these potential surfaces yield a theoretical $A~2B2\leftarrow X~2A1$ absorption spectrum. This spectrum, based solely on the ab initio data, is successful in accounting for many of the low resolution absorption features of $NO2$ between 8500 and 6000{\AA}. A proposal is made chat the $A~2B2$ origin is possibly lower than the present experimental assignment of 1.48 eV on the basis of hot and cold band intensity ratios. Also discussed is the large difference in the experimentally determined $A~2B2$ rotational constants and those inferred from the ab initio equilibrium geometry.