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.

Automated summary

This paper aims to update the spectroscopic parameters of nitric acid in the 5.8 μm region and generate linelists for specific bands. High-resolution Fourier transform spectra were utilized to identify different bands of nitric acid and study their perturbations. Surprising findings include large amplitude torsional splittings and the existence of anharmonic resonance between energy levels.