DIPR-DIP 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. 

A First Multi-dimensional Reaction Model the DIPR-DIP Model... 

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. 

Measuring the polarization of the reaction product is also an important issue in stereodynamics. A lot of the activity in this field concerns the reactivity of alkaline earth metal atoms since the corresponding reaction products are easy to probe by optical techniques. A full account of methods to measure product alignment and orientation in bimolecular collisions has been given by Orr-Ewing and Zare [18]. Such measurements, with the help of simple models such as the DIPR-DIP model considered Section 2.3.2, give insight into the shape of the reactive system at the moment where forces are released [86, 87, 184, 195, 233, 234]. 

The other important channel in the decay of the Ca-HX complex, ground state of the product CaCl, can be observed by laser-induced fluorescence and a very high vibrational excitation is detected. This high vibrational excitation arises from the sudden release at 4 A of the ground-state CaCl molecule, and thereby in a highly excited vibrational state, the distribution, maximum at u = 30, extends to v = 60. The resultant energy distribution has been interpreted with the use of the DIPR-DIP model [246]. 

Polarization of chemiluminescent products from oriented reagent has been addressed by Prisant et al, [25] using the DIPR-DIP model. As discussed in detail elsewhere [4], the electron-jump model fails to account for the enhanced reactivity of vibrationaUy excited N2O. Nevertheless, the general trends predicted by Prisant et at. are confirmed in our data. "CoUinear" approaches are predicted to result in higher product polarization than "side-on" approaches in accord with our observations. 

P2 reaction has been analysed [23] and yielded information about the product alignment. A calculation based on a modified version of the DIPR DIP model is in general qualitative agreement with experiment. 

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. 

---