Linear Probing of Molecules at Micrometric Distances from a Surface with Sub-Doppler Frequency Resolution

We report on precision spectroscopy of subwavelength confined molecular gases. This was obtained by rovibrational selective reflection of NH3 and SF6 gases using a quantum cascade laser at λ ≈ 10.6 μ m . Our technique probes molecules at micrometric distances (≈ λ/2π) from the window of a macroscopic cell with submegahertz resolution, allowing molecule-surface interaction spectroscopy. We exploit the linearity and high resolution of our technique to gain novel spectroscopic information on the SF6 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.