  We report on precision spectroscopy of sub-wavelength confined molecular
gases. This was obtained by rovibrational selective reflection of
$\mathrm{NH_3}$ and $\mathrm{SF_6}$ gases using a quantum cascade laser at
$\lambda\approx 10.6 \mu m$. Our technique probes molecules at micrometric
distances ($\approx \lambda/2\pi$) from the window of a macroscopic cell with
sub-MHz resolution, allowing molecule-surface interaction spectroscopy. We
exploit the linearity and high-resolution of our technique to gain novel
spectroscopic information on the $\mathrm{SF_6}$ greenhouse gas, useful for
enriching molecular databases. The natural extension of our work to thin-cells
will allow compact frequency references and improved measurements of the
Casimir-Polder interaction with molecules.
