Spectator stripping

A close analogy to the localized surface interaction can be found in the field of chemical kinetics, namely, in the spectator stripping mechanism (5, 6) of the gas reactions, as evidenced by the recent crossed-molecular-beams experiments. Here the projectile seems to meet with only a part of the target molecule (that one to be transferred), while the rest of the target behaves as a spectator, in a sense not taking part in the reaction. 

Figure 13. Cartesian [center-of-mass (CM)] contour diagrams for NH+ produced from reaction of N+ with H2. Numbers indicate relative product intensity corresponding to each contour. Direction of N+ reactant beam is 0° in center-of-mass system. For clarity, beam profiles have been displaced from their true positions (located by dots and 0°). Tip of velocity vector of center of mass with respect to laboratory system is located at origin of coordinate system (+). Scale for production velocities in center-of-mass system is shown at bottom left of each diagram (a) reactant N+ ions formed by impact of 160-eV electrons on N2 two components can be discerned, one approximately symmetric about the center of mass and the other ascribed to N+(IZ3), forward scattered with its maximum intensity near spectator stripping velocity (b) ground-state N+(3/>) reactant ions formed in a microwave discharge in N2. Only one feature is apparent—contours are nearly symmetric about center-of-mass velocity.12 ...

The extreme stripping case is spectator stripping , in which A snatches the atom (group) B from molecule BC with no alteration in the velocity or internal energy of the fragment C. The molecular beam studies of the reactions of Cs, K and Rb with halogens and of Ar+ and N with D2 have been treated in this way (refs. O 10 ). 

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]... 

Opposite to rebound reactions is the reaction Na + Ch — NaCl + Cl which proceeds via the spectator stripping mechanism. In this case, the crossing between the nonreactive covalent Na-Cl2 curve and the Na+Cl ion-pair curve, which promotes the reaction, occurs at a large distance [Re = 5.22 A, when using the chlorine adiabatic electron affinity in Magee s equation). This distance increases to 22.3 A when sodium is excited to the 3p P level. One would expect an increased reaction cross-section, but this is not observed because electron transfers at such large distance are inefficient. The overlap between the sodium HOMO and the CI2 LUMO is very small at these distances. As a result, when the crossing radius increases substantially, there is only a small effect on the dynamics of the reaction [164, 165]. 

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... 

Fig. 12. Spectator-stripping cross sections for T + H2(0,0) -+ TH(h3. 73) + H, calculated from the first term of the Faddeev-Watson multiple collision expansion (from D. A. Micha and P. McGuire, 1972).

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. 

E.E.Nikitin, Optical model for spectator-stripping reaction, Chem.Phys.Lett. 1, 266 (1967)... 

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