Coherent Raman Metrology of Molecular Hydrogen: application to the Q(1) 1-0 line

International audienceWe introduce a comb-calibrated spectrometer for frequency metrology of Raman-active transitions and thus of quadrupole transitions. It may operate in a two-octave spanning range, from 50 to 5000 cm–1, covering all fundamental rovibrational bands as well as purely rotational bands. The spectrometer exploits a stimulated-Raman-scattering interaction of the gas target with two comb-calibrated near-infrared cw lasers in a multi-pass cell. We demonstrate its metrological quality by measuring the transition frequency of the most famous line of molecular hydrogen, namely the Q(1) 1-0 line at 4155 cm–1, with a few-parts-per-billion accuracy of 1×10–5 cm–1. This improves by an order of magnitude the experimental state of the art obtained by Resonantly-Enhanced Multiphoton-Ionization1 and overcomes by a factor of about 2 the current theoretical benchmark obtained by initio-calculations2.The Raman spectra are acquired over a 2-decades-spanning pressure range, from 40 mbar to 4 bar, and globally fitted with a β-corrected Hartmann-Tran profile to retrieve the zero-pressure line center ν0. The fitting uses a properly chosen set of collisional parameters fixed against ab-initio values inferred from quantum-scattering calculations on H2-H2 collisions3. To mitigate the impact on ν0 of an inadequate modeling over such a large range of relatively high pressures we investigate how the inferred ν0 modifies upon restricting the fit to experimental data at lower and lower pressures (i.e., progressively discarding high-pressure spectra). Interestingly, the analysis of ν0 as a function of the maximum pressure used in the global fit for a variety of conditions - namely linear pressure shifts and ab-initio parameters varied in their confidence intervals - returns a family of converging trajectories that define ν0 with a systematic uncertainty as low as 235 kHz. The overall uncertainty, summing up in quadrature a statistical error of 205 kHz, is 310 kHz. The experimental line center agrees withtheory within 1-σ. Prospectively, further reducing both experimental and theoretical uncertainties, highly stringent tests of molecular quantum electrodynamics can be performed in a robust way with this highly versatile spectrometer on a large number of transition