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We present an accurate measurement of the weak quadrupole S(2) 2-0 line in self-perturbed D 2 and theoretical ab initio calculations of both collisional line-shape effects and energy of this rovibrational transition. The spectra were collected at the 247–984 Torr pressure range with a frequency-stabilized cavity ring-down spectrometer linked to an optical frequency comb (OFC) referenced to a primary time standard. Our line-shape modeling employed quantum calculations of molecular scattering (the pressure broadening and shift and their speed dependencies were calculated, while the complex frequency of optical velocity-changing collisions was fitted to experimental spectra). The velocity-changing collisions are handled with the hard-sphere collisional kernel. The experimental and theoretical pressure broadening and shift are consistent within 5% and 27%, respectively (the discrepancy for shift is 8% when referred not to the speed averaged value, which is close to zero, but to the range of variability of the speed-dependent shift). We use our high pressure measurement to determine the energy, ν 0 , of the S(2) 2-0 transition. The ab initio line-shape calculations allowed us to mitigate the expected collisional systematics reaching the 410 kHz accuracy of ν 0 . We report theoretical determination of ν 0 taking into account relativistic and QED corrections up to α 5 . Our estimation of the accuracy of the theoretical ν 0 is 1.3 MHz. We observe 3.4 σ discrepancy between experimental and theoretical ν 0 . [ABSTRACT FROM AUTHOR]

Abstract: Classical Molecular Dynamics Simulations (MDS) are carried for the calculation of the collision kernels describing the velocity changes of H2 molecules induced by interactions with Ar atoms. These MDS results are first used for the test of a previously proposed three dimensional bi-parametric extension (KS-3D) of Keilson and Storer velocity-changing model. Semi-classical and close-coupling calculations of the speed-dependent collisional-broadening γ coll(v) and -shifting δ coll(v) coefficients are also made for the Q(1) line, using the best available H2–Ar potential energy surface. These data are compared with the corresponding values previously extracted from measured Raman spectra. Furthermore, the calculated values of , γ coll(v) and δ coll(v) are used to calculate the density dependence of the Q(1) line-width from the Doppler to the collisional regime and compare the results with experiments. The conclusions of this exercise are three fold. The first is that the KS-3D approach gives a satisfactory description of the velocity changes. The second is that the procedure used in a previous study in order to extract γ coll(v) and δ coll(v) from measured spectra was correct. The last, consistent with the findings of other independent studies, is that the H2–Ar interaction potential used needs refinements at short distances. [Copyright &y& Elsevier]

• New method for populating line-by-line spectroscopic databases with beyond-Voigt lineshape parameters. • He-perturbed H2. • Ab initio quantum scattering calculations of beyond-Voigt line-shape parameters. • Simple and compact formula that allows the speed-dependence parameters to be calculated directly from the generalized spectroscopic cross sections. • Dataset compatible with the HITRAN database. • Temperature dependences of the beyond-Voigt line-shape parameters covering the range from 20 to 1000 K. 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 H 2 (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 line-shape 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. [ABSTRACT FROM AUTHOR]

• Ab initio quantum scattering calculations. • Temperature dependences of the pressure broadening, pressure shift, speed dependence and the complex Dicke parameter. • Comparison of the four-temperature-ranges (4TR) and double-power-law (DPL) temperature dependence models. • Hartmann-Tran profile (HTP) parameterization. Temperature dependences of molecular line-shape parameters are important for the spectroscopic studies of the atmospheres of the Earth and other planets. A number of analytical functions have been proposed as candidates that may approximate the actual temperature dependences of the line-shape parameters. In this article, we use our ab initio collisional line-shape calculations for several molecular systems to compare the four temperature ranges (4TR) representation, adopted in the HITRAN database [J. Quant. Spectrosc. Radiat. Transfer 2017;203:3] in 2016, with the double-power-law (DPL) representation. Besides the collisional broadening and shift parameters, we consider also the most important line-shape parameters beyond Voigt, i.e., the speed dependence of broadening and shift parameters, and real and imaginary parts of the complex Dicke parameter. We demonstrate that DPL gives better overall approximation of the temperature dependencies than 4TR. It should be emphasized that DPL requires fewer parameters and its structure is much simpler and more self-consistent than the structure of 4TR. We recommend the usage of DPL representation in HITRAN, and present DPL parametrization for Voigt and beyond-Voigt line profiles that will be adopted in the HITRAN database. We also discuss the problem of the Hartmann-Tran profile parametrization in which the correlation parameter, η , and frequency of the velocity-changing collisions parameter, ν vc , diverges to infinity when collisional shift crosses zero; we recommend a simple solution for this problem. [ABSTRACT FROM AUTHOR]

We report revisited experimentally retrieved and theoretically calculated He-broadened Lorentz half-width coefficients and He- pressure-shift coefficients of 45 carbon monoxide transitions in the 1←0 band. The spectra analyzed in this study were recorded over a range of temperatures between 79 and 296 K. The He-broadened line parameters and their temperature dependences were retrieved using a multispectrum nonlinear least squares analysis program. The line shape models used in this study include Voigt, speed dependent Voigt, Rautian (to take into account confinement narrowing) and Rautian with speed dependence, all with an asymmetric component added to account for weak line mixing effects. We were unable to retrieve the temperature dependence of line mixing coefficients. A classical method was used to determine the He-narrowing parameters while quantum dynamical calculations were performed to determine He-broadening and He-pressure shifts coefficients at different temperatures. The line mixing coefficients were also derived from the exponential power gap law and the energy corrected sudden approximation. The current measurements and theoretical results are compared with other published results, where appropriate. [ABSTRACT FROM AUTHOR]

• The R(0) line of CO perturbed by Ar is studied experimentally in a wide pressure range. • Line shape parameters are calculated ab initio using two potential energy surfaces for a wide temperature range. • Theoretical model reproduces experimental spectra at a sub percent level of uncertainty. • Significance of the line-mixing effect for accurate modeling of spectra is demonstrated. We use three independent spectroscopic techniques, operating in the millimeter-wavelength range, to study molecule-atom collisions, and validate our quantum-scattering calculations on two recent potential energy surfaces. We study the first pure rotational transition in a CO molecule perturbed by Ar. This molecular system is a good prototype of atmospherically relevant cases. It is, on the one hand, affordable for calculation of the line shape parameters by modern ab initio methods, and on the other hand, is very convenient for experimental studies because of its regular, well spaced rotational spectrum having a moderate intensity. We show that the simulated collision-perturbed spectra, which are based on our fully ab initio calculations, agree with the experimental line profiles at sub-percent level over a wide range (more than four orders of magnitude) of pressures. We demonstrate that the agreement between theory and experiment can be further improved if the model accounts for the collisional transfer of an optical coherence between different rotational transitions (the line-mixing effect). We show that the two surfaces tested in this work lead to a very similar agreement with the experiment. Capability of calculating line shape parameters in a broad range of temperatures is demonstrated. [ABSTRACT FROM AUTHOR]