Pressure effects on N 2 –N 2 rototranslational Raman spectra predicted from leading spectral moments

International audienceNon‐Markovian effects having a strong influence on far‐wing intensities of spectroscopic signatures by molecular gases are analyzed theoretically with the use of a non‐Markovian relaxation matrix derived for rapidly colliding linear rotators (J. Chem. Phys. 149, 044305 [2018]) for the benchmark case of rototranslational Raman spectra of molecular nitrogen recorded at high densities up to very far wings (Phys. Lett. A 157, 44 [1991]). This matrix is built here on the base of the translational‐spectrum model of Birnbaum and Cohen and the recently computed, from known potential energy surfaces, two leading classical spectral moments (J. Ram. Spectrosc. 2020, DOI: 10.1002/jrs.5923). Theoretical intensity computations, going beyond the commonly used impact approximation, give much less overestimated values in the far wing and constitute a promising tool for getting accurate theoretical description of broad‐band spectra

Non-Markovian rotational relaxation matrix for fast collisions between two linear molecules in high-pressure gaseous media. I. General formalism and preliminary testing

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Double vibrational collision-induced Raman scattering by SF6–N2: Beyond the point-polarizable molecule model

International audienceCollision-induced Raman bandshapes and zeroth-order spectral moments are calculated both for the depolarized spectrum and for the extremely weak isotropic spectrum of the SF6(ν1) +N2(ν1) double-Raman-scattering band. A critical comparison is made with experiments conducted recently by the authors [ Phys. Rev. A 81 012702 (2010) 81 042705 (2010)]. The study of this transition, hitherto restricted to the model framework of two point-polarizable molecules, is now completed to incorporate effects beyond the point-molecule approximation. Whereas the extended model offers a few percent improvement in the depolarized spectrum, it reveals a huge 80% increase in the isotropic spectrum and its moment, owing essentially to the polarizability anisotropy of N2. For both spectra, agreement between quantum-mechanical calculations and our experiments is found, provided that the best ab initio data for the (hyper)polarizability parameters are used. This refined study shows clearly the need to include all mechanisms and data to a high level of accuracy and allows one to decide between alternatives about difficult and controversial issues such as the intermolecular potential or the sensitive Hamaker force constants.</p

Buffer-gas effect on the rotovibrational line intensity distribution: Analysis of possible mechanisms

Line intensities Am (|m| ≤ 4) of the HF fundamental band (T=293 K) are found to decrease linearly with the buffer-gas (Xe) density (DXe = 2.6-16 Amagat). The obtained slopes Δ1(m) of the Am(d)/ Am(0) vs. dXe plots are maximum at |m| =1 (Δ1(1) ≈ Δ1(-1) = 1.7(4) * 10-2Amagat-1) and rapidly drop with |m| . Many possible mechanisms are considered; the most effective one appears to be the HF-Xe bimer formation, with the equilibrium constant strongly depending on the rotational quantum number. The rigid-rotator approximation used gives the density derivatives considerably smaller than the measured ones. The disbalance may be lessened for vibrating rotators by allowance for the interband intensity transfer induced by the vibrational modulation of short-range forces

Evidence for double incoherent Raman scattering in binary gas mixtures: SF6-N2

International audienceWe report a collision-induced Raman band by room temperature gas mixtures of sulfur hexafluoride and nitrogen. The band is centered at the sum of the frequencies of the symmetric-stretching ν1 transition of SF6 and the fundamental transition of N2, and its intensity scales as the product of the partial densities of the gases. The observed process is evidence of double incoherent Raman scattering (DRS) by SF6-N2, in which both molecules simultaneously undergo two Raman-allowed transitions. The band was found to be almost fully depolarized, in agreement with previous observations in other systems and with theoretical predictions. Its integrated intensity is about one-third higher than the total area predicted by the leading-order dipole-induced dipole model. This discrepancy suggests that DRS is a practical means of assessing the quality of intermolecular potential models, which, in the case of SF6-N2, is still believed to be not good enough. Our work is expected to open the door to a multitude of studies involving complicated processes encountered in nonpolar gases and their mixtures, which are of direct relevance to atmospheric research.</p

Leading spectral moments for easy computation of pressure effects on rotational lines

International audienceExplicit classical‐limit formulae for two leading spectral moments characterizing the anisotropy of the interaction potential are derived for fast collisions of linear rotators in the binary regime. With the help of these moments, model Fourier‐transforms of the time‐correlation functions comprising non‐Markovian effects are restored, which enables a novel and straight approach to pressure transformation of broad spectral bands and pressure‐broadening characteristics of (vib)rotational spectral lines, as demonstrated on a simple Markov‐limit example of isotropic and anisotropic Raman line widths of molecular nitrogen