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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Close-coupling calculations of generalized spectroscopic cross-sections • Ab initio investigation of pressure broadening and shift coefficients • Ab initio investigation of Dicke parameters• Data for electric dipole R (j=0-5) and P (j=1-6) lines for purely rotational transitions, fundamental band and first four overtonesHighlights for Ab initio calculations of collisional line-shape parameters and generalized spectroscopic cross-sections for rovibrational dipole lines

We demonstrate a new method for populating line-by-line spectroscopic databases with beyond-Voigt line-shape parameters, which is based on ab initio quantum scattering calculations. We report a comprehensive dataset for the benchmark system of He-perturbed H2 (we cover all the rovibrational bands that are present in the HITRAN spectroscopic database). We generate the entire dataset of the line-shape parameters (broadening and shift, their speed dependence, and the complex Dicke parameter) from fully ab initio quantum-scattering calculations. We extend the previous calculations by taking into account the centrifugal distortion for all the bands and by including the hot bands. The results are projected on a simple structure of the quadratic speed-dependent hard-collision profile. We report a simple and compact formula that allows the speed-dependence parameters to be calculated directly from the generalized spectroscopic cross sections. For each line and each lineshape parameter, we provide a full temperature dependence within the double-power-law (DPL) representation, which makes the dataset compatible with the HITRAN database. The temperature dependences cover the range from 20 to 1000 K, which includes the low temperatures relevant for the studies of the atmospheres of giant planets. The final outcome from our dataset is validated on highly accurate experimental spectra collected with cavity ring-down spectrometers. The methodology can be applied to many other molecular species important for atmospheric and planetary studies.

Information about molecular collisions is encoded in the shapes of collision-perturbed molecular resonances. This connection between molecular interactions and line shapes is most clearly seen in simple systems such as the molecular hydrogen perturbed by a noble gas atom. We study the H2-Ar system by means of highly-accurate absorption spectroscopy and ab initio calculations. On the one hand, we use the cavity-ring-down-spectroscopy technique to record the shapes of the S(1) 3-0 line of molecular hydrogen perturbed by argon. On the other hand, we simulate the shapes of this line using ab initio quantum-scattering calculations performed on our accurate potential energy surface (PES). In order to validate the PES and the methodology of quantum-scattering calculations separately from the model of velocity-changing collisions, we measured the spectra in experimental conditions in which the influence of the latter is relatively minor. Our theoretical collision-perturbed line shapes reproduce the experimental spectra at the percent level. However, the collisional shift, δ0, differs from the experimental value by 20%. Compared to other line-shape parameters, collisional shift displays much higher sensitivity to various technical aspects of the computational methodology. We identify the contributors to this error and find the inaccuracies of the PES to be the dominant factor. With regards to the quantum scattering methodology, we demonstrate that treating the centrifugal distortion in a simple, approximate manner is sufficient to obtain the percent-level accuracy of collisional spectra.

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