NEW ACCURATE MEASUREMENT OF ³⁶ ArH ⁺ AND ³⁸ ArH ⁺ RO-VIBRATIONAL TRANSITIONS BY HIGH RESOLUTION IR ABSORPTION SPECTROSCOPY

Abstract

The protonated Argon ion, $^{36}$ArH$^{+}$, has been identified recently in\nthe Crab Nebula (Barlow et al. 2013) from Herschel spectra. Given the\natmospheric opacity at the frequency of its $J$=1-0 and $J$=2-1 rotational\ntransitions (617.5 and 1234.6 GHz, respectively), and the current lack of\nappropriate space observatories after the recent end of the Herschel mission,\nfuture studies on this molecule will rely on mid-infrared observations. We\nreport on accurate wavenumber measurements of $^{36}$ArH$^{+}$ and\n$^{38}$ArH$^{+}$ rotation-vibration transitions in the $v$=1-0 band in the\nrange 4.1-3.7 $\\mu$m (2450-2715 cm$^{-1}$). The wavenumbers of the $R$(0)\ntransitions of the $v$=1-0 band are 2612.50135$\\pm$0.00033 and\n2610.70177$\\pm$0.00042 cm$^{-1}$ ($\\pm3\\sigma$) for $^{36}$ArH$^{+}$ and\n$^{38}$ArH$^{+}$, respectively. The calculated opacity for a gas thermalized at\na temperature of 100 K and a linewidth of 1 km.s$^{-1}$ of the $R$(0) line is\n$1.6\\times10^{-15}\\times N$($^{36}$ArH$^+$). For column densities of\n$^{36}$ArH$^+$ larger than $1\\times 10^{13}$ cm$^{-2}$, significant absorption\nby the $R$(0) line can be expected against bright mid-IR sources.\n