New accurate measurement of 36ArH+ and 38ArH+ ro-vibrational transitions
  by high resolution IR absorption spectroscopy
  The protonated Argon ion, $^{36}$ArH$^{+}$, has been identified recently in
the Crab Nebula (Barlow et al. 2013) 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 $^{36}$ArH$^{+}$ and
$^{38}$ArH$^{+}$ rotation-vibration transitions in the $v$=1-0 band in the
range 4.1-3.7 $\mu$m (2450-2715 cm$^{-1}$). The wavenumbers of the $R$(0)
transitions of the $v$=1-0 band are 2612.50135$\pm$0.00033 and
2610.70177$\pm$0.00042 cm$^{-1}$ ($\pm3\sigma$) for $^{36}$ArH$^{+}$ and
$^{38}$ArH$^{+}$, respectively. The calculated opacity for a gas thermalized at
a temperature of 100 K and a linewidth of 1 km.s$^{-1}$ of the $R$(0) line is
$1.6\times10^{-15}\times N$($^{36}$ArH$^+$). For column densities of
$^{36}$ArH$^+$ larger than $1\times 10^{13}$ cm$^{-2}$, significant absorption
by the $R$(0) line can be expected against bright mid-IR sources.
