New line intensities for the far infrared bands of the Trans- and Cis-conformer of nitrous acid (HONO), new determination of the Trans-Cis conformer barrier and its impact on the astrophysical detection of nitrous acid in protostellar clouds

•Determination of the energy difference (ΔECis-Trans) between the Cis- and Trans-HONO conformers of nitrous acid.•Relative line intensities for the B-type transitions of pure rotational bands of Trans-HONO and Cis-HONO measured.•Far infrared FTS spectra on SOLEIL.•Line list for HONO in the far infrared region. The first goal of this work is to improve the determination of the energy difference (ΔECis-Trans) between the ground vibrational state of the Cis- and Trans-HONO conformers of nitrous acid. For this, high resolution spectra were recorded in the 50–200 cm−1 spectral region at three different temperatures, 240, 270 and 296 K. The relative line intensities for the B-type transitions of pure rotational bands of Trans-HONO and Cis-HONO achieved from our measurements were combined in least squares fit computations to those measured previously by Sironneau et al. [Sironneau V, Flaud JM, Orphal J, Kleiner I, Chelin P. Absolute line intensities of HONO and DONO in the far-infrared and re-determination of the Energy Difference between the trans- and cis-species of nitrous acid. J Mol Spectrosc 2010;259:100–104]. In this way, we can significantly improve the accuracy on the HONO conformer energy difference, with a value for ΔECis-Trans = 95.8 ± 9.2 cm−1 compared to SIRΔECis-Trans = 99 ± 25 cm−1 in the previous study of Sironneau et al. The second goal is to generate a line list with “absolute” line intensities for the pure rotational bands in the far infrared region of Trans-HONO and Cis-HONO, with both A- and B-type transitions. This new line list proved to be more robust for an improved detection of HONO in astrophysical objects [Coutens A, Ligterink NFW, Loison JC, Wakelam V, Calcutt H, Drozdovskaya MN, Jørgensen JK, Müller HSP, Van Dishoeck EF, Wampfler SF. The ALMA-PILS survey: First detection of nitrous acid (HONO) in the interstellar medium. Astronomy & Astrophysics 2019;623:L13]. Comparison of the ALMA observations of the HONO transitions towards the protostar IRAS16293 B (black) with the model obtained with the JPL line list for Tex = 300 K and N = 1.4 1015 cm−2 blue dashed line) and the one obtained with our new line list for similar parameters (red solid line). [Display omitted]