Forward scattering mechanism

Figure 1 Simplistic schematic illustration of the scattering mechanism upon which X-ray photoelectron diffraction (XPD) is based. An intensity increase is expected in the forward scattering direction, where the scattered and primary waves constructively interfere.

Obviously, experiments designed to measure cross-sections as a function of energy are needed. At present, tandem experiments are not capable of high precision at low energies because one must assume details of collision mechanics and because it is difficult to estimate collection efficiencies in forward scattering geometry (15). The extension of all known techniques to lower energy (64, 65) and the further development of pulse methods (58) offer the possibility for advances in this area. 

Fig. 37. Reaction mechanism for the forward scattering product from the H + HD H2 + D reaction at the collision energy of 1.200 eV.

Figure 12, Schematic mechanism for impulsive reaction of thermal energy reaction of K with oriented CF3I. The electron is assumed to be transferred at large distance to the molecule irrespective of orientation. The molecular ion is formed in a repulsive state that promptly dissociates, ejecting the T ion in the direction of the molecular axis, and the K is dragged off by the departing T resulting in backward scattering for heads orientation and forward scattering for tails as observed.

Since the critical configuration is reached when P is far from M, then the reaction will have a large cross section. The velocity of the products will be low, and the products will initially be in excited vibrational levels. The molecular beam contour diagram will show predominantly forward scattering typical of a stripping mechanism. 

The reactions of Sn with Cl2 and Br2 show forward scattering of the SnX product, with about 30—45% of the reaction energy appearing as translation of the products in the case of Sn + Cl2 [424]. The exact contribution to the reaction of the various spin-orbit states of tin, Sn(3P012), is unknown. Similarities between the results for Sn + Cl2 and those for Li + Cl2 [297] suggest an electron-jump mechanism, although the ionic—covalent curve crossing radius is quite small for Sn + Cl2 (— 2.9 A). 

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. 

Sometimes, it is difficult to accurately say resonance effect of a chemical reaction system is from which of the above-mentioned mechanisms. The difference between them is sometimes not very clear. The F -h H2 reaction product HF (v = 3) forward scattering found by Yuan T. Lee et al. [13] was presumably from the Feshbach resonance. Experimental and theoretical research in recent years attributed the forward scattering to time delay effect and shape resonance [14]. 

Wang XG, Dong WR, Qiu MH et al (2008) HF (v = 3) forward scattering in the F -I- H2 reaction shape resonance and slow-down mechanism. Proc Natl Acad Sci USA 105 6227-6231... 

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