Metastable methane dimers in collisions with inert gas atoms: study by the method of classical trajectories

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Abstract

The formation of collision complexes, also called quasi–complexes (QC), metastable dimers or Feshbach resonances, has been studied for CH4 – He, Ne, Ar systems by the method of classical trajectories. The calculations used exact 3D classical Hamilton equations in the action–angle variables and non-empirical surfaces of the interaction potential energy. The selection of collision parameters was carried out by the Monte Carlo method. A statistical analysis of the QCs parameters is performed. It is shown that QCs can be both short-lived and long-lived and are characterized by a variety of interparticle separations. Among the total number of collisions, the fraction of QCs increases rapidly with a decrease of temperature. Formulas are given that reveal the contribution of QCs to the cross sections of the rotational RT- relaxation of CH4. It is shown that in methane mixtures considered RT- relaxation in QC- type collisions is much more effective than in ordinary inelastic collisions.

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About the authors

S. V. Ivanov

Institute of Photon Technologies of Federal Scientific Research Centre “Crystallography and Photonics” of Russian Academy of Sciences

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Email: serg.ivanov.home@mail.ru
Russian Federation, Moscow, Troitsk

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Supplementary files

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2. 1. Radial functions V0(R) , V3(R) , V4(R) for He–CH4[36], Ne–CH4[37], Ar–CH4 [38] systems.

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3. Fig. 2. Comparison of calculated cross sections of the rotational transition J0 = 1 → J of the CH4 molecule in collisions with Ar and Ne atoms: a is the Ar–CH4 system, kinetic energy of the collision Ecoll = 256.8 cm–1:1 is the quantum CC method [44], 2 is the quantum CS method [44], 3 is the classical trajectory method [45], 4 is a real 3D calculation (the total number of trajectories is 402000); b is the Ne–CH4 system, the kinetic energy of the collision Ecoll = 716.2 cm–1. 1 is the quantum CS method [37], 2 is the classical trajectory method [45], 3 – real 3D calculation (the total number of trajectories is 1273750).

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4. 3. Examples of three trajectories with the formation of quasi–complexes in collisions of the Ar-CH4 system. Calculation using the average thermal velocity v = (8kBT/pm)1/2 at T = 100K for J0 = 5. The dependences R(t) differ in the target parameters and orientations of the angular momentum of the CH4 molecule.

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5. Fig. 4. Probability distribution functions of the lifetime tQC (a) and the number of turning points N (b) for the Ar–CH4 system. The role of the interaction potential at different temperatures: 1, 3 – a simple empirical potential [39, 40]; 2, 4 – a more accurate potential [38].

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6. 5. Distributions of QC lifetimes (a) and turning points (b) in He–CH4, Ne–CH4, Ar–CH4 collisions. T = 100 K.

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7. 6. Distributions of minimum QC sizes (closest approach distances Rmin) and maximum QC sizes (greatest particle distances Rmax) in He–CH4, Ne–CH4, Ar–CH4 collisions. T = 100 K.

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8. 7. Distributions of the aiming parameter at which QCs are formed in He–CH4, Ne–CH4, and Ar–CH4 collisions. T = 100 K.

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9. 8. Cross sections of QC formation: a is the cross section of QC formation at T = 296 K as a function of the rotational number J; b is the temperature dependence of the sQC(T) cross section (the results are averaged over all values of J under the Boltzmann distribution), bmax = 10 Å. Dark characters are all QC, light characters are only elastic QC.

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10. 9. a – sections of rotational transitions J0 = 7 → J of the CH4 molecule in collisions with He, Ne, Ar at T = 296 K; b – sections of rotational transitions J0 → J of the CH4 molecule in collisions with Ar for different J0. T = 296 K. Solid symbols represent all collisions, light symbols represent only QC. The transitions J0 → J = J0 correspond to elastic collisions.

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