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Highlights• Close-coupling calculations of pressure broadening and shifting coefficients • Close-coupling calculations of Dicke parameters • Data for anisotropic Raman S (j=0-5) and O (j=2-5) lines for purely rotational transitions, fundamental band and first four overtones

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We report the most accurate measurement of the position of the weak quadrupole S(2) 2-0 line in D2. The spectra were collected with a frequency-stabilized cavity ring-down spectrometer (FS-CRDS) with an ultra-high finesse optical cavity (F = 637 000) and operating in the frequencyagile, rapid scanning spectroscopy (FARS) mode. Despite working in the Doppler-limited regime, we reached 40 kHz of statistical uncertainty and 161 kHz of absolute accuracy, achieving the highest accuracy for homonuclear isotopologues of molecular hydrogen. The accuracy of our measurement corresponds to the fifth significant digit of the leading term in QED correction. We observe 2.3σ discrepancy with the recent theoretical value. * mzab@doktorant.umk.pl † piotr.wcislo@fizyka.umk.pl

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Fully quantum ab initio calculations of the collision-induced shapes of two rovibrational H2 lines perturbed by He provide an unprecedented subpercent agreement with ultra-accurate cavity-enhanced measurements. This level of consistency between theory and experiment hinges on a highly accurate potential energy surface and a realistic treatment of the velocity changing and dephasing collisions. In addition to the fundamental importance, these results show that ab initio calculations can provide reference data for spectroscopic studies of planet atmospheres at the required accuracy level and can be used to populate spectroscopic line-by-line databases.

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 H2 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.

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