The Dynamics of Electronically Adiabatic Collisions

The Dynamics of ElectronicaUy Adiabatic Collisions.— There are three parts to a detailed rate theory of processes occurring in electronically adiabatic collisions. First, the potential describing the molecular interaction must be calculated or estimated. Secondly, the equations of motion have to be solved for individual, fully specified, collisions. Finally, the results of calculations on single collisions must be averaged correctly to yield the required result for example, a reactive cross-section or a detailed rate constant. The procedures for the third stage were outlined in Section 2. In the forward direction, i.e. from o(n ln 6) to ic(T), this averaging presents no problems, but it is the difficulty of reversing this process which makes it impossible to obtain detailed information about the collision dynamics or potential from experimental measurements of thermal rate constants. 

Strictly, the dynamics of intermolecular collisions should be treated quantum mechanically but there are formidable difficulties associated with three-dimensional calculations on reactive systems. Only one fully quantal study, on H -I- Hi, has been completed.One problem is that the trial solution to the Schrddinger equation is expressed as a sum of basis functions, and this should include all the rovibrational states that are coupled during the strongest part of the collision. For molecules with moments of inertia greater than that of Hi, many more states have to be included in the basis set and the size of the computation increases rapidly. This difficulty is similar to that in calculations of electronic energies in molecules, when for many-electron systems, the basis set of atomic orbitak that is required for accurate calculations becomes too large to handle. 

There is now a good deal of evidence that the QCL trajectory method provides a generally satisfactory description of reactive collision dynamics. Semi-classical calculations have been important in defining the situations where purely 

Of course, many investigations fall somewhere between the two limiting types. Thus, a potential may be chosen on limited experimental information and a Monte Carlo trajectory study carried out to predict the values of quantities that have not been observed experimentally. These results should not be accepted unreservedly, since the collision dynamics are determined by the form of the assumed potential. Therefore, the evidence on which the potential was selected should be carefully scrutinized. Recent calculations which were designed to provide information about the relaxation of HF by H atoms, i.e. 

The classical trajectory method and the results that have been obtained from its application have been reviewed in depth recently by Polanyi and Schreiber and by Port . In the next few paragraphs an attempt is made to summarize the results that are especially important from the point of view of interpreting rates of reaction and relaxation in collisions between molecules in excited internal states and potentially reactive, i.e. free radical, species. Trajectory studies that relate to particular systems will be referred to later when experimental data are under discussion. 

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