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Collisional Energy Transfer

How does collisional energy transfer between a highly vibrationally excited reactant molecule and a thermal medium molecule depend on the nature of the collision event? (e.g. strength of the intermolecular interaction, IVR, timescale). The provision of data on and a theoretical description of intermolecular energy transfer is an area of much active research. How reactant molecules acquire and lose their energy via collisions is of much relevance to reaction rate theory [1]. We have studied statistical theories of intermolecular energy transfer, and found them to have the correct qualitative trends, but to be in quantitative disagreement with experiment [1-4]. In order to get more detailed data on the energy transfer process, I recently performed an extensive numerical simulation of the collisional energy transfer experienced by a highly energetically (microcanonically or ro-vibrationally) excited triatomic molecule (SO2) upon collision with a thermal atom (Ar) [5,6]. We found indications that energy transfer did not just involve transfer between vibration and translation, but that rotations played an important role as well.

Further simulation studies need to be performed to check that this is a general result. Systematic variation of model potential parameters would lead to a better understanding of the important mechanisms. The next stage would involve the use of progressively more realistic and more accurate potential energy surfaces to yield more quantitative predictions [7]. A long-term goal would be the comparison of experimental results with semiclassical and full quantum scattering calculations on accurate ab initio potential surfaces.

References

[1] S. Nordholm and H. W. Schranz, Collisional Energy Transfer in Unimolecular Reactions. Statistical Theory and Classical Simulation, in Advances in Chemical Kinetics and Dynamics, edited by J. R. Barker (JAI Press, 1995) pp 245-281.

[2] H. W. Schranz and S. Nordholm, Unimolecular activation-deactivation: Impulsive collision theory, Int. J. Chem. Kin. 13, 1051 (1981).

[3] S. Nordholm, H. W. Schranz, B. C. Freasier, and N. D. Hamer, Diatomic dissociation rate theory. I. Angular momentum conservation and impulsive collisions in the low pressure limit, Chem. Phys. 129, 351 (1989).

[4] H. W. Schranz, B. C. Freasier, N. D. Hamer, and S. Nordholm, Diatomic dissociation rate theory. II. Extensions and comparison with experiment, Chem. Phys. 129, 363 (1989).

[5] H. Hippler, H. W. Schranz, and J. Troe, Trajectory calculations of intermolecular energy transfer in SO2-Ar collisions. I. Method and representative results, J. Phys. Chem. 90, 6158 (1986).

[6] H. W. Schranz and J. Troe, Trajectory calculations of intermolecular energy transfer in SO2-Ar collisions. II. State specific rate coefficients, J. Phys. Chem. 90, 6168 (1986).

[7] Terry J. Frankcombe, Rob Stranger, Harold W. Schranz, The intermolecular potential energy surface of CO2-Ar and its effect on collisional energy transfer, ECCC4, 1997.

Collisional Energy Transfer Topics and Resources

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Last revised Wednesday 18 February 1998 EST - Harold W. Schranz Email: Harold.Schranz@anu.edu.au