NO3 radical studied by laser-induced fluorescence

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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 C2v 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 C2v 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 C2v as minimum‐energy structure, but a model with less fine resolution overlooks the three shallow minima and reports the larger‐scale D3h structure.

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