Spectator-stripping model

In the previous section, the theoretical interpretations of ion—molecule reactions have been developed on the basis of the complex mechanism, i.e. on the assumption that reactions proceed via an intermediate complex 

It has been found, however, that in many cases the calculated cross-sections based on this mechanism become smaller than the gas kinetic cross-sections at higher kinetic energies. Thus some mechanisms other than complex formation must be occurring. Indeed, the recent development of various experimental techniques, especially of the beam technique has revealed the occurrence, at higher kinetic energies, of some direct mechanism(s) in which the atom transfer reaction 

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One of the simplest models for describing direct reactions of an atom and a molecule in which the reaction products are scattered in a forward direction with respect to the reagent atom is the spectator-stripping model [167]. This has been satisfactorily applied to many ion—molecule... 

In contrast, forming AB near its dissociation limit requires the Coulomb field of the positive ion to stimulate dissociation of the molecular anion. This situation is encountered when the molecule has a large electron affinity. In this case, the crossing distance Rc is large and the dynamics of the reaction are well described by the spectator stripping model, where the reaction product is strongly forward peaked [70]. This pictorial formulation leaves unaffected the neutral moiety of the molecular reaction partner, which plays the part of a spectator in the reaction. A representative example is the reaction [71]... 

The spectator-stripping model (Henglein et al., 1965 Minturn et al, 1966) has been frequently used for reactions of type A + BC - AB + C. It is most easily described in a laboratory frame, for BC initially unexcited and at rest. Assuming that a sudden collision leaves C at rest, conservation of momentum in the centre of mass frame leads to... 

The first term corresponds to the spectator-stripping model, which arises naturally in the formalism. The second term is a displacement contribution in which atom B moves undisturbed, and so on. The first term may be conveniently calculated (Micha and McGuire, 1972) in the momentum representation, which gives... 

Most three atom ion-molecule reactions that exhibit direct mechanism behaviour do not proceed solely by a spectator stripping mechanism since product ions exist far beyond the critical energy. The migration mechanism is an attractive candidate for the direct mechanism since it contributes to the simultaneous occurrence of forward scattering, large momentum transfer to the product atom and stability to the product ion far beyond the critical energy of the spectator stripping model. It is, of course, clear that the actual mechanism is a mixture of a number of direct processes. 

In order to explain their experimental results ranging from very low to high energies in a unified manner, Herman et al. [103] proposed a direct mechanism called the modified spectator stripping model , in which the long-range intermolecular forces between the reactant and product pairs are quantitatively taken into account. 

In the modified spectator stripping model described in Section 4.2.1, these experimental observations are explained quantitatively by choosing adequate values of the reaction radius rp and rp. In other words, rp and rp are adjustable parameters which are determined by comparing the... 

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... 

As mentioned previously, the modified spectator stripping model (polarization model) of Herman et al. [103] explains the velocity distribution of products very well but does not predict the angular distribution, whereas the DIPR model explains both. Thus there had been no full comparison between the two models until Chang and Light [113] refined and extended the polarization model to yield the angular distribution of products as well. 

At high enough energies, the reaction will proceed by a direct-interaction mechanism and the many-body problem may be reduced to a consideration of pairwise interactions. Consider the reaction A BC,C)AB. On the one hand, A may interact exclusively with B this problem has been treated classically and it is implicit in the spectator-stripping model. On the other hand, sequential collisions, for example, of A with B and then C, may also yield AB here, both classical and quantum mechanical descriptions have been utilized. 

In recent model calculations these findings have been confirmed and verified for a full three-dimensional treatment [36]. Due to the weak couplings in these systems almost no V-T energy transfer occurs during the reaction process i.e. the translational energy (mainly stored in the heavy atoms) is conserved [3 ]. Hence these type of reactions provide classical examples of the traditional "spectator stripping model" [38]. 

The predominant forward scatter and the fact that it is more pronounced at the higher energy can be interpreted in terms of the spectator stripping model illustrated in Fig. 8.9a. As the Ar+ flies past the Dg at large... 

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