Near-infrared band of the nitrate radical NO3 observed by diode laser spectroscopy

Abstract

We have analyzed the near-infrared band of NO3 observed at 7602 cm−1 by using diode laser spectroscopy. Most of the spectral lines were recorded using source-frequency modulation. Zeeman modulation was found useful in selectively detecting some Q branch lines, which provided us with a clue to the assignment of the observed spectra. The band satisfied selection rules for a parallel band and was thus ascribed to a 2A1″–2A2′ vibronic component associated with the 2E′′–X̃ 2A2′ electronic transition, namely, to a transition from the ground vibronic state to the A1″ vibronic state resulting from excitation of the degenerate in-plane bending mode in the 2E′′ electronically excited state manifold. The band was almost free of perturbations, except for some K=6 lines. The least-squares analysis of 581 assigned lines led to molecular parameters of the upper state, where ground-state parameters were fixed to those obtained from the infrared study previously reported [K. Kawaguchi, E. Hirota, T. Ishiwata, and I. Tanaka, J. Chem. Phys. 93, 951 (1990)]. The upper-state B rotational constant gave the effective N–O distance of 1.271 Å, which is to be compared with 1.240 Å in the ground vibronic state. The εbb spin–rotation interaction constant of the upper state was close in magnitude to that in the ground vibronic state, but of opposite sign. This observation indicates that the spin–rotation interaction is primarily caused by that between the 2E′′ excited and the ground electronic states.