NO3 radical studied by laser-induced fluorescence

Abstract

The fluorescence emission spectra of NO, excited at 14 742, 15 109, 15 882, 16 053, and 16 555 cm ’ are reported. On the basis of fundamentals, overtones, and combination of five vibrational frequencies (368, 753, 1053, 1500, and 2010 cm- ‘) 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,, symmetry with three identical shallow minima, each representative of a local C,, structure and located with threefold symmetry around the central axis. Such a potential-energy function can split degenerate D,, 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,,, structures to give D,, structure and rotational spectra. This model qualitatively explains both the five fundamental frequencies observed by fluorescence and the definite D,,, properties of high-resolution infrared spectra. We suggest that a molecular theoretical model with fine spatial resolution sees the miniwells and reports Czu as minimumenergy structure, but a model with less fine resolution overlooks the three shallow minima and reports the larger-scale DJh structure.