NO3 RADICAL STUDIED BY LASER-INDUCED FLUORESCENCE

Abstract

The fluorescence emission spectra of NO3 excited at 14 742, 15 109, 15 882, 16 053, and 16 555 cm-1 are reported. On the basis of fundamentals, overtones, and combination of five vibrational frequencies (368, 753, 1053, 1500, and 2010 cm-1) we assign 18 out of 20 observed bands. The fluorescence bands exhibit two different shapes, one shows a sharp spike overlapped with a broadband, and the other shows a broadband only. From the literature we obtain a potential-energy surface that has D3h symmetry with three identical shallow minima, each representative of a local C2-upsilon structure and located with threefold symmetry around the central axis. Such a potential-energy function can split degenerate D3h vibrational modes, giving "pseudorotations," as a structure with one long and two short bonds permutes around the three minima. On the time scale of molecular rotations, vibrational motions average over the three local C2-upsilon structures to give D3h structure and rotational spectra. This model qualitatively explains both the five fundamental frequencies observed by fluorescence and the definite D3h properties of high-resolution infrared spectra. We suggest that a molecular theoretical model with fine spatial resolution sees the miniwells and reports C2-upsilon as minimum-energy structure, but a model with less fine resolution overlooks the three shallow minima and reports the larger-scale D3h structure.