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.