Explicit 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.
Collisional mixing of (vib)rotational lines appearing in spectroscopic signatures of dense planetary atmospheres and combustion environments is rigorously handled for the case of two linear colliders in terms of incomplete (non-Markovian) collisions related to off-energy-shell scattering amplitudes. Contrary to the standard impact-approximation approaches valid solely in band-centre regions, a new uniform broadband spectrum description is developed on the basis of a frequency-dependent rotational relaxation matrix which accurately accounts for the influence of the extra photon energy with respect to the molecular transitions. This matrix is built using a symmetric Liouville-space metric and obeys all known fundamental rules. Its direct calculation from refined potential-energy surfaces and promising modeling methods for forthcoming practical computations are outlined. A simple preliminary test for N-N isotropic Raman line widths argues in favor of considerable effects of the internal perturber's structure on modeled spectral characteristics.
Non-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 (
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