Abstract

The spectroscopy and photochemistry of nitrogen dioxide has been the subject of numerous experimental and theoretical studies; yet, these efforts have been little rewarding so far. Regarding the spectroscopy, the ground electronic state X²A₁ is the only state which has been reasonably well characterized through EPR, ESR, LMR, MW and IR studies; equilibrium geometry, the three vibrational fundamentals, dipole moment, rotational constants, force and anharmonicity constants, and various others fine and hyperfine constants, are now known. Conversely, most of these data are unknown or controversial for the four lowest excited electronic states of NO₂, namely, ${\tilde{A}}^2{B}_2,{\tilde{B}}^2{B}_1,{\tilde{C}}^2{A}_2$ and $1^2{\Sigma }_g^{+}$, lying in the far-IR, visible and near-UV regions; it is particularly striking that even the equilibrium geometries and vibrational frequencies of these states are still being sought for. Perhaps the best known of the excited states is 2²B₂, which lies in the near-UV, for which equilibrium geometry, vibrational frequencies, rotational constants and To are known. At higher energies, in the near-UV and VUV, there are various valence states: three quartets, 1⁴A₂, 1⁴B₂ and 1⁴A₁, and a number of doublets; these states have been predicted theoretically, yet none of them has been observed so far. In addition, a number of fragments of Rydberg series have been observed in the VUV and EUV regions, but only few features have been assigned unambiguosly.

© 1993 Optical Society of America