Nowadays, buffer gas cooling represents an invaluable option to produce cold stable molecules, both in view of secondary cooling/trapping strategies towards the achievement of quantum degeneracy and for fundamental spectroscopic studies. From this follows a demand to establish a pool of specialized, increasingly precise spectroscopic interrogation techniques. By combining saturated cavity ring-down spectroscopy with the buffer-gas-cooling technique, we have demonstrated a general approach to Lamb-dip ro-vibrational measurements on cold molecules. The developed scheme represents the launch pad to high-accuracy molecular tests of fundamental Physics at the electron volt energy scale. Examples include: searching for fifth-force interactions, assessing the space-time stability of the proton-to-electron mass ratio, and probing parity violation in chiral molecules.

The research is published in Optica.