Stimulated Raman scattering metrology of molecular hydrogen

Frequency combs have revolutionized optical frequency metrology, allowing one to determine highly accurate transition frequencies of a wealth of molecular species. These progresses have only marginally benefited infrared-inactive transitions, due to their inherently weak cross-sections. Here we overcome this limitation by introducing stimulated-Raman-scattering metrology, where a frequency comb is exploited to calibrate the frequency detuning between the pump and Stokes excitation lasers. We apply this approach to the investigation of molecular hydrogen, which is a recognized benchmark for tests of quantum electrodynamics and of theories that describe physics beyond the standard model. Specifically, we measure the transition frequency of the Q(1) fundamental line of H 2 around 4155 cm −1 with few parts-per-billion uncertainty, which is comparable to the theoretical benchmark of ab initio calculations and more than a decade better than the experimental state of the art. Our comb-calibrated stimulated Raman scattering spectrometer extends the toolkit of optical frequency metrology as it can be applied, with simple technical changes, to many other infrared-inactive transitions, over a 50-5000 cm −1 range that covers also purely rotational bands. Molecular hydrogen has a simple structure that makes it a unique benchmark for molecular quantum physics. The authors determined the transition energy of its fundamental Q(1) vibrational line with an unprecedented parts-per-billion accuracy by a novel spectrometer that combines Stimulated-Raman-Scattering with comb calibration of optical frequencies.