First analysis of the ν₂+ν₇ and ν₂+ν₉ and ν₂+ν₆ combination bands of HNO₃: evidence of perturbations due to large amplitude OH torsion in the 2¹9¹ excited state

This paper is the first of two back-to-back works, whose goal is to update the existing line positions and intensities spectroscopic parameters for the 5.8 µm region of nitric acid in the HITRAN or GEISA spectroscopic databases. This first study is devoted to the generation of first linelists for the ν₂+ν₉-ν₉, ν₂+ν₇-ν₇ and ν₂+ν₆-ν₆ hot bands in the 5.8 µm region, whereas the second study will concern the positions and intensities for the ν₂ band centred at 1709.567 cm⁻¹. High-resolution Fourier transform spectra were recorded in the 5.8 µm and in the 4.2–4.6 µm spectral ranges. The identification of the ν₂+ν₉, ν₂+ν₇ and ν₂+ν₆ bands of HNO₃ at 2165.0036, 2285.9657 and 2351.216 cm⁻¹, respectively, was performed, and several assignments were confirmed by searching for ν₂+ν₉-ν₉ and ν₂+ν₇-ν₇ transitions within those of ν₂ at 5.8 µm. Both ν₂+ν₆ and ν₂+ν₉ bands are highly perturbed. Surprisingly, both ν₂+ν₉ and ν₂+ν₉-ν₉ bands exhibit large amplitude torsional splittings of ∼0.043 cm⁻¹. This is presumed to be due to the existence of an anharmonic resonance that couples together the 2¹9¹ energy levels with those of a dark state involving high excitation in the ν₉ large amplitude OH torsional mode.