Mechanism and models for collisional energy transfer in highly excited large polyatomic molecules

Gilbert, Robert G. (1995) Mechanism and models for collisional energy transfer in highly excited large polyatomic molecules. Australian Journal of Chemistry, 48 11: 1787-1817. doi:10.1071/CH9951787


Author Gilbert, Robert G.
Title Mechanism and models for collisional energy transfer in highly excited large polyatomic molecules
Journal name Australian Journal of Chemistry   Check publisher's open access policy
ISSN 0004-9425
1445-0038
Publication date 1995
Sub-type Critical review of research, literature review, critical commentary
DOI 10.1071/CH9951787
Volume 48
Issue 11
Start page 1787
End page 1817
Total pages 31
Place of publication Collingwood, VIC, Australia
Publisher C S I R O Publishing
Language eng
Formatted abstract
Collisional energy transfer in highly excited molecules (say, 200-500 kJ mol-1 above the zero-point energy of reactant, or of product, for a recombination reaction) is reviewed. An understanding of this energy transfer is important in predicting and interpreting the pressure dependence of gas-phase rate coefficients for unimolecular and recombination reactions. For many years it was thought that this pressure dependence could be calculated from a single energy-transfer quantity, such as the average energy transferred per collision. However, the discovery of 'supercollisions' (a small but significant fraction of collisions which transfer abnormally large amounts of energy) means that this simplistic approach needs some revision. The 'ordinary' (non-super) component of the distribution function for collisional energy transfer can be quantified either by empirical models (e.g., an exponential-down functional form) or by models with a physical basis, such as biased random walk (applicable to monatomic or diatomic collision partners) or ergodic (for polyatomic collision partners) treatments. The latter two models enable approximate expressions for the average energy transfer to be estimated from readily available molecular parameters. Rotational energy transfer, important for finding the pressure dependence for recombination reactions, can for these purposes usually be taken as transferring sufficient energy so that the explicit functional form is not required to predict the pressure dependence. The mechanism of 'ordinary' energy transfer seems to be dominated by low-frequency modes of the substrate, whereby there is sufficient time during a vibrational period for significant energy flow between the collision partners. Supercollisions may involve sudden energy flow as an outer atom of the substrate is squashed between the substrate and the bath gas, and then is moved away from the interaction by large-amplitude motion such as a ring vibration or a rotation; improved experimental and theoretical understanding of this phenomenon is seen as an important area for future development.

Keyword Thermal unimolecular reactions
Random-walk model
Fall-off range
Rate coefficients
Rate constants
Low-pressures
Trajectory calculations
Multiphoton ionization
Vibrational-relaxation
So2-ar collisions
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Non-UQ
Additional Notes Invited Review.

Document type: Journal Article
Sub-type: Critical review of research, literature review, critical commentary
Collections: School of Chemistry and Molecular Biosciences
Centre for Nutrition and Food Sciences Publications
 
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Created: Mon, 07 Mar 2011, 15:43:39 EST