Linear probing of molecules at micrometric distances from a surface with sub-Doppler frequency resolution

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.