J. F. STANTON, Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, TX 78712.
Spectral lines associated with the NO3 molecule were first detected and reported by Chappuis in the period 1880-1882. Although the carrier of these lines was not determined without ambiguity, it still remains a fact that this may well have represented the first spectroscopic detection of a polyatomic radical. In the intervening century and a quarter, NO3 has emerged as both an important species in the nighttime atmosphere as well as a major challenge for molecular spectroscopy. The difficulties in the spectroscopy lay in the exceedingly complex quantum mechanics that governs the molecule; there are five low-lying electronic states, and vibronic interactions mixing these states are ubiquitous. This talk focuses on the ground electronic state of NO3, which is strongly coupled ( via a pseudo-Jahn-Teller interaction) with an excited state having 2E' symmetry that is positioned roughly 2 eV higher in energy. Through use of a model Hamitonian approach, the identity of most vibronic levels within 3000 cm-1 of the zero-point level is revealed, and a number of spectroscopic misassignments are corrected and other unassigned lines are properly accounted for. The question the molecular structure of NO3 is discussed towards the end of the talk, where it will be argued that viewing the molecule as a symmetric D3h species is appropriate to understand its spectroscopy, while viewing it as a C2v species effectively rationalizes its remarkably high reactivity. The question of the symmetry associated with the minimum on the adiabatic potential energy surface is largely irrelevant in this regard.