Reaction channels energy dependence

If the barrier between the left- and right-hand sides is lower (as depicted by the dashed line), then all of the possible reactants have greater access to the entire potential surface, but to differing degrees depending on the energy contained within the reactants. Then the relative importance of the various reaction channels and their occurrence will give information on that accessibility and thus on the structure of the surface. 

See the reviews by McMahon16and by Dunbar17 for a discussion of this mechanism. Note that the reaction channels 36a and 36b are unlikely to be the only channels, since a considerable amount of energy is available in the recombination ( 130 kcal mol-1, depending on the isomer) and some further fragmentation probably occurs. For example, the recombination of H30+ has recently been shown to yield the channels,21... 

The different threshold behaviors for heads and tails orientations shown in Figures 7 and 8 indicate that, at very low energies, reaction is restricted to only one end of the molecule. We directly observe that there is no reaction for attack in the unfavored orientation. The different thresholds for attack at different ends of these molecules requires the final state of the system, at the respective thresholds, to be somehow different for attack at the opposite ends of the molecule. For CFjBr, we believe that different products may be formed, depending on the end attacked, but the same species in different internal states could also be a possibility. In these experiments there are two likely low-energy reaction channels. 

As the positron energy is raised above the positronium formation threshold, EPs, the total cross section undergoes a conspicuous increase. Subsequent experimentation (see Chapter 4) has confirmed that much of this increase can be attributed to positronium formation via the reaction (1.12). Significant contributions also arise from target excitation and, more importantly, ionization above the respective thresholds (see Chapter 5). In marked contrast to the structure in aT(e+) associated with the opening of inelastic channels, the electron total cross section has a much smoother energy dependence, which can be attributed to the dominance of the elastic scattering cross section for this projectile. 

The prevailing reaction channel depends on the energy of the projectiles, because the excitation energy transferred to the compound nucleus determines the way it is transmuted. With increasing excitation energy, the number of reaction channels increases markedly. 

The determination of the microcanonical rate coefficient k E) is the subject of active research. A number of techniques have been proposed, and include RRKM theory (discussed in more detail in Section 2.4.4) and the derivatives of this such as Flexible Transition State theory. Phase Space Theory and the Statistical Adiabatic Channel Model. All of these techniques require a detailed knowledge of the potential energy surface (PES) on which the reaction takes place, which for most reactions is not known. As a consequence much effort has been devoted to more approximate techniques which depend only on specific PES features such as reaction threshold energies. These techniques often have a number of parameters whose values are determined by calibration with experimental data. Thus the analysis of the experimental data then becomes an exercise in the optimization of these parameters so as to reproduce the experimental data as closely as possible. One such technique is based on Inverse Laplace Transforms (ILT). 

Similarly detailed studies have been made of the photodissociation of isocyanic acid (HNCO). With this molecule, three reaction channels compete fragmentation into NH and CO, with the NH in either of two electronic states, and fragmentation into H and NCO. Experiments in which HNCO was excited to the S state in expansion jets have shown that the competition depends on the photon energy and the excitation conditions. The photodissociation dynamics of... 

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