The ν5 and 2 ν9 bands of the 15N isotopic species of nitric acid (H 15NO 3): Line positions and intensities

Abstract

Nitric acid (HNO 3) plays an important role in the Earth’s atmosphere as a reservoir molecule of NO x species. It has a strong infrared signature at 11 μm which is one of the most commonly used for the infrared retrieval of this species in the atmosphere since this spectral region coincides with an atmospheric window. It is therefore essential to have high quality spectral parameters in this spectral region. For the H 14NO 3 (main) isotopic species, the 11 μm bands were already the subject of numerous extensive studies which involve not only the ν 5 and 2 ν 9 cold bands but also the first hot bands. The present work is the first high resolution Fourier transform analysis of the 11 μm bands for H 15NO 3, which is the second most abundant isotopomer of nitric acid [ a ≅ 0.00365(7)]. In this way, the analysis of the ν 5 and 2 ν 9 cold bands centered at 871.0955 and 893.4518 cm −1 was performed, and as for H 14NO 3, these bands are significantly perturbed since rather strong resonances couple the 5 1 and 9 2 rotational levels. The theoretical model that we used to compute the line positions and line intensities is directly issued from the one which we used recently for H 14NO 3 [A. Perrin, J. Orphal, J.-M. Flaud, S. Klee, G. Mellau, H. Mäder, D. Walbrodt, M. Winnewisser, J. Mol. Spectrosc. 228 (2004) 375–391]. Actually, for the H 15NO 3 line positions, the Hamiltonian matrix accounts for the rather strong Fermi and the weaker Coriolis interactions linking the 5 1 ⇔ 9 2 rotational levels. Using this model which accounts correctly for the strong mixing of the 5 1 and 9 2 upper state energy levels, the ν 5 and 2 ν 9 line intensities for H 15NO 3 were satisfactorily computed using the ν 5 and 2 ν 9 transition moment operators achieved previously for the 14N (main) isotopic species. In this way, the transfer of intensities from the ν 5 fundamental (and presumably strong) band to the 2 ν 9 overtone (and presumably weak) band could be explained for H 15NO 3 as it was done previously for the 14N (main) isotopic species. Finally, the position of the H 15NO 3 ν 5 + ν 9 − ν 9 hot band was identified at 875.245 cm −1.