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 D_3h symmetry with three identical shallow minima, each representative of a local C_2v structure and located with threefold symmetry around the central axis. Such a potential‐energy function can split degenerate D_3h 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 C_2v structures to give D_3h structure and rotational spectra. This model qualitatively explains both the five fundamental frequencies observed by fluorescence and the definite D_3h properties of high‐resolution infrared spectra. We suggest that a molecular theoretical model with fine spatial resolution sees the miniwells and reports C_2v as minimum‐energy structure, but a model with less fine resolution overlooks the three shallow minima and reports the larger‐scale D_3h structure.