Direct interaction with product repulsion DIPR model

A widely-used model in this class is the direct-interaction with product repulsion (DIPR) model [173—175], which assumes that a generalised force produces a known total impulse between B and C. The final translational energy of the products is determined by the initial orientation of BC, the repulsive energy released into BC and the form of the repulsive force as the products separate. This latter can be obtained from experiment or may be assumed to take some simple form such as an exponential decay with distance. Another method is to calculate this distribution from the quasi-diatomic reflection approximation often used for photodissociation [176]. This is called the DIPR—DIP model ( distributed as in photodissociation ) and has given good agreement for the product translational and rotational energy distributions from the reactions of alkali atoms with methyl iodide. 

The mechanism of the electron transfer and the subsequent dynamics of the system have both been oversimplified at the moment. A more refined model of the dynamics is discussed in the next section the direct interaction with product repulsion (DIPR) model and its variant called DIPR-DIP(DIP-distributed as in photodissociation). The purpose of sections 2.4 and 2.5 is to explicit the multi-dimensional and multi-PES character of the electron jump step. 

We focus our attention on the DIPR (direct interaction with product repulsion) model and its variant, the DIPR-DIP model, mainly because it can be used to predict an entire range of dynamic observables in chemical reactions angular and recoil velocity distributions, rotational energy and orientation and vibrational energy of the reaction products. It is also able to account for the switch from the rebound to the stripping reaction mechanism for a given system when the collision energy is increased. The beauty of the model is its ability to include semiempirical parameters, each of which is related to a different physical phenomenon. 

The modified spectator stripping model (polarization model) thus appears to be a satisfactory one which explains the experimental velocity distribution from very low to moderately high energies. The model emphasizes that the long-range polarization force has the dominant effect on the dynamics of some ion—molecule reactions. However, a quite different direct mechanism based on short-range chemical forces has been shown to explain the experimental results equally satisfactorily [107, 108]. This model is named direct interaction with product repulsion model (DIPR model) and was originally introduced by Kuntz et al. [109] in the classical mechanical trajectory study of the neutral reaction of the type... 

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