Assignment of the photoelectron spectrum of the nitrate anion NO 3 − and vibronic interactions in the nitrate free radical

The unpaired electron orbital of NO3 is of a2′ symmetry in the ground electronic state, and thus its motion about the symmetry axis of the molecule is free rotation. When a degenerate vibration is excited, however, the free azimuthal rotation of the unpaired electron is perturbed much by nuclear motions of the degenerate mode, as evidenced by high-resolution spectroscopic studies. Thus the ν4 fundamental state, for example, bears some characters of the B̃ excited electronic state through the Herzberg-Teller (H-T) interaction, and Neumark et al. explained anomalous ν4 progression in the photoelectron spectra of the NO3− anion by the H-T mechanism. However, the interaction parameter Neumark required was too large to reproduce the ν4 molecular parameters in the ground electronic state precisely determined by high-resolution IR spectroscopy. This discrepancy was resolved by the fact that the upper ν4 overtone/combination states of Neumark’s photoelectron transitions were primarily of vibrational in nature. The present study thus showed that NO3 bears both vibrational and H-T induced electronic characters in excited states of degenerate modes in the ground electronic state.