COMB · Precision measurements in molecules with frequency combs

Precise frequency measurements enable accurate determinations of physical constants, stringent tests of fundamental theories and searches for possible drifts of the fundamental constants. Simple atoms, hydrogen in particular, have long been the dedicated systems for confronting experimental data and accurate quantum electrodynamics theoretical calculations. With continued progress to ab initio calculations, precision measurements in small molecules, such as molecular hydrogen H2, are gaining much relevance and may be envisioned as an independent way to determine fundamental constants and to test quantum-chemistry theory. In this project, we will develop an instrument of precision molecular spectroscopy, based on frequency combs, broad spectra composed of equidistant narrow lines whose absolute frequency can be known within the accuracy of an atomic clock. Building on our unique know-how, this revolutionary ultraviolet spectrometer will simultaneously combine broad spectral coverage, Doppler-free resolution and extreme accuracy for precise studies of small molecules. Using two-photon excitation and dual-comb spectroscopy with comb lasers of low repetition frequency, we will devise an optical analogue of the Ramsey-fringe method where many molecular transitions will be simultaneously and unambiguously observed and assigned. While such a spectrometer will enable significant progress in our understanding of the structure of many small molecules, it will first be applied to absolute-frequency measurements of rovibronic transitions in the EF – X system of H2 around 3000 THz. The measured frequencies can be used to benchmark molecular theory in the involved ground and excited states. They may contribute to an improved determination of the dissociation energy of H2, set new basis for an independent determination of the proton-charge radius and for searches of variations of the proton-electron mass ratio via comparison to astrophysical measurements.