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 mu m region of nitric acid in the HITRAN or GEISA spectroscopic databases. This first study is devoted to the generation of first linelists for the nu(2)+nu(9)-nu(9), nu(2)+nu(7)-nu(7) and nu(2)+nu(6)-nu(6) hot bands in the 5.8 mu m region, whereas the second study will concern the positions and intensities for the nu(2) 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 mu m spectral ranges. The identification of the nu(2)+nu(9), nu(2)+nu(7) and nu(2)+nu(6) bands of HNO3 at 2165.0036, 2285.9657 and 2351.216 cm(-1), respectively, was performed, and several assignments were confirmed by searching for nu(2)+nu(9)-nu(9) and nu(2)+nu(7)-nu(7) transitions within those of nu(2) at 5.8 mu m. Both nu(2)+nu(6) and nu(2)+nu(9) bands are highly perturbed. Surprisingly, both nu(2)+nu(9) and nu(2)+nu(9)-nu(9) 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 2(1)9(1) energy levels with those of a dark state involving high excitation in the nu(9) large amplitude OH torsional mode.