New accurate measurement of ³⁶ArH+ and ³⁸ArH+ ro-vibrational transitions by high resolution IR absorption spectroscopy

The protonated argon ion, 36ArH+, was recently identified in the Crab Nebula from Herschel spectra. Given the atmospheric opacity at the frequency of its J = 1-0 and J = 2-1 rotational transitions (617.5 and 1234.6 GHz, respectively), and the current lack of appropriate space observatories after the recent end of the Herschel mission, future studies on this molecule will rely on mid-infrared observations. We report on accurate wavenumber measurements of 36ArH+ and 38ArH+ rotation-vibration transitions in the v = 1-0 band in the range 4.1-3.7 μm (2450-2715 cm–1). The wavenumbers of the R(0) transitions of the v = 1-0 band are 2612.50135 ± 0.00033 and 2610.70177 ± 0.00042 cm–1 (±3σ) for 36ArH+ and 38ArH+, respectively. The calculated opacity for a gas thermalized at a temperature of 100 K and with a linewidth of 1 km s–1 of the R(0) line is 1.6 × 10–15 × N(36ArH+). For column densities of 36ArH+ larger than 1 × 1013 cm–2, significant absorption by the R(0) line can be expected against bright mid-IR sources