The ν5 and 2ν9 bands of the N isotopic species of nitric acid (H15NO3): Line positions and intensities

Nitric acid (HNO₃) 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 H¹⁴NO₃ (main) isotopic species, the 11 μm bands were already the subject of numerous extensive studies which involve not only the ν₅ and 2ν₉ 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 H¹⁵NO₃, which is the second most abundant isotopomer of nitric acid a ≅ 0.00365(7)]. In this way, the analysis of the ν₅ and 2ν₉ cold bands centered at 871.0955 and 893.4518 cm⁻¹ was performed, and as for H¹⁴NO₃, these bands are significantly perturbed since rather strong resonances couple the 5¹ and 9² 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 H¹⁴NO₃ 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 H¹⁵NO₃ line positions, the Hamiltonian matrix accounts for the rather strong Fermi and the weaker Coriolis interactions linking the 5¹⇔9² rotational levels. Using this model which accounts correctly for the strong mixing of the 5¹ and 9² upper state energy levels, the ν₅ and 2ν₉ line intensities for H¹⁵NO₃ were satisfactorily computed using the ν₅ and 2ν₉ transition moment operators achieved previously for the ¹⁴N (main) isotopic species. In this way, the transfer of intensities from the ν₅ fundamental (and presumably strong) band to the 2ν₉ overtone (and presumably weak) band could be explained for H¹⁵NO₃ as it was done previously for the ¹⁴N (main) isotopic species. Finally, the position of the H¹⁵NO₃ν₅ + ν₉ − ν₉ hot band was identified at 875.245 cm⁻¹.