New Analysis of the ν3 and ν4 Bands of HNO3 in the 7.6 μm Region

Abstract

A new line position analysis of the ν3 and ν4 bands of nitric acid (HNO3) at 1326.186 and 1303.072 cm(-1) together with its associated interacting bands is presented. The 3(1) and 4(1) energy levels were obtained from an extended analysis of high-resolution Fourier transform spectra recorded at Giessen in the 7.6 μm region. The energy levels of 3(1) and 4(1) upper states of nitric acid are strongly interacting with those of the 9(3), 6(2), 5(1)9(1), and 7(1)8(1) dark states centered at 1288.899, 1289.46, 1341.05, and 1343.78 cm(-1), respectively. Informations on these perturbing dark states were achieved through previous partial investigations of hot bands in high-resolution Fourier transform spectra recorded at 22 μm in Giessen (for 3ν9-2ν9 and 3ν9-ν5), at 12 μm in Denver (for 3ν9-ν9), and at 11 μm in Orsay (for ν5+ν9-ν9). The energy levels calculation accounts for the various Fermi, anharmonic, A-type, B-type, and C-type Coriolis resonances, which couple together the {6(2),9(3),4(1),3(1),5(1)9(1),7(1)8(1)} interacting energy levels. For nitric acid, the ν9 mode (OH torsion relative to the -NO2 moiety) is a large amplitude motion. The theoretical model used in this work accounts also for large amplitude effects in the 9(3) dark state, which lead to a splitting of the 9(3) energy levels of about 0.060 cm(-1). In this way, the existence of torsional splittings for several ν4 perturbed lines was explained by the occurrence of local A-type and B-type Coriolis resonances coupling the 4(1) energy levels with those of 9(3). Because four dark bands had to be accounted for in the model, the results of the energy level calculations are reasonable, although not perfect. However, a very significant improvement was achieved in terms of understanding the 7.6 μm absorbing bands of nitric acid as compared to the analysis of the ν3 and ν4 bands performed several years ago [Perrin, A.; Lado-Bordowski, O.; Valentin, A. Mol. Phys. 1989, 67, 249-267]. Finally, the present analysis also features, for the first time, the ν3+ν9-ν9 hot band located at 1331.09 cm(-1). This study will help to improve HNO3 measurements by satellites. This will be indeed the case for the "Infrared Atmospheric Sounding Interferometer" (IASI) experiment.