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 μ m region of nitric acid in the HITRAN or GEISA spectroscopic databases. This first study is devoted to the generation of first line lists for the ν₂+ν₉-ν₉, ν₂+ν₇-ν₇ and ν₂+ν₆-ν₆ hot bands in the 5.8 mu m 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 mu m 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 similar to 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. (authors)