Quantum Theory of Scattering and Unimolecular Breakdown

Quantum Theory of Scattering and Unimolecnlar Breakdown.—From the theoretical viewpoint it would appear natural to compute lifetimes and cross-sections for unimolecular processes like equation (34) by one of the existing methods for the solution of the set of coupled equations of the scattering problem. There have been, however, hardly any calculations for experimental examples or at least realistic model systems. The present status of the quantum theory of unimolecular reactions is still rather in the domain of formal theories or hi y simplified models, which are not of immediate interest to the experimentalist. We shall, nevertheless, review some of the recent developments, because one may hope that in the future the detailed dynamical theories will provide a deeper understanding of unimolecular dynamics than the statistical theories presently do. 

The relation of unimolecular breakdown to predissociation processes (with or 

Slater, Theory of Unimolecular Reactions, Cornell University Press, New York, 1959. S. A. Safron, Mol. Phys., 1975, 30, 493. 

The second term plays the role of a transmission coefiicient (maximum value = 1). In the limit of small couplings (Fp 1), one gets the Breit-Wigner lineshape with an exponential decay  

In other situations the decay law is, however, non-exponential. It should be stressed that such simplified models, if they describe the basic physics correctly, may introduce some artifacts which disappear if one uses more realistic assumptions (e.g. random spacings, random coupling ). Assuming independent decay into many channels a , one may sum equation (63) to obtain an RRKM-type formula (65), where W is the number of accessible dissociation continue or open 

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