Scattering backward/forward

The experimental dichroism is seen to have its greatest magnitude some 5 eV above threshold, where 0.10. This corresponds to an asymmetry factor in the forward-backward scattering of y 20%. Such a pronounced PECD asymmetry from a randomly oriented sample looks to comprehensively better the amazingly high 10% chiral asymmetry recorded with highly ordered nanocrystals of tyrosine enantiomer [25] or the spectacular 12.5% asymmetry reported from an oriented single crystal of a cobalt complex [28]. 

It follows once again thatthe breakup of a long-lived complex must show forward-backward scattering symmetry. 

For pure S-wave scattering, the difFerential cross section (DCS) is isotropic. For pure P-wave scattering, tlie DCS is symmetric about 0 = n/2, where it vanishes the DCS rises to equal maxima at 0 = 0, ti. For combined S- and P-wave scattering, the DCS is asynnnetric with forward-backward asynnnetry. 

Symmetry oscillations therefore appear in die differential cross sections for femiion-femiion and boson-boson scattering. They originate from the interference between imscattered mcident particles in the forward (0 = 0) direction and backward scattered particles (0 = 7t, 0). A general differential cross section for scattering... 

Momentum conservation implies that the wave vectors of the phonons, interacting with the electrons close to the Fermi surface, are either small (forward scattering) or close to 2kp=7i/a (backward scattering). In Eq. (3.10) forward scattering is neglected, as the electron interaction with the acoustic phonons is weak. Neglecting also the weak (/-dependence of the optical phonon frequency, the lattice energy reads ... 

Figure 2. Probability density plots of the ethyl cation product, (a) from the unlabeled reaction, (b) CH2CH3 from the labeled reaction, and (c) CD3CH2 from the labeled reaction. The backward scattered ethyl cation is more probable in (b), while the forward scattered ethyl cation is more probable in (c). Reprinted from [39] with permission from Elsevier.

Because the sense, or sign, of chiral asymmetry in the forward-backward electron scattering asymmetry depends on the helicity of the photon and of the molecule, it is essential that these variables are properly specified in any study to permit meaningful comparisons to be made. Discussing and comparing quantitative asymmetry factors, y [Eq. (8)] and dichroism [Eq. (9)] likewise requires agreement on the convention adopted in the definition of these terms. 

Fig. 17. The angle-dependent integrated opacity function dan(00 —> v = 0,1, f = 0 6, Eq, Jmax) versus Jmax computed for the experimental energy Eq = 1.200eV. This quantity is computed by restricting the partial wave sum in the DCS to the terms J < Jmax- The result is shown for forward and backward scattering to illustrate the J-contributions to scattering at different 0.

Fig. 29. The CM product translational energy distributions at the forward and backward scattering direction for the 0(1D) +D2 — OD + D reaction at two collision energies (a) 2.0 kcal/mol, and (b) 3.2 kcal/mol.

Figure 6. Calculated absorbed energy density due to forward and backward scattered electrons. Zq is the penetration depth in a 0.4-nm PMMA film which is coated on an Al substrate. The electron energy used is 20 keV. (Reproduced with permission from Ref. 4j...

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.

Figure 13.3 Polar plots of angular scattering by spheres with m = 1.33. Note the great change in scale as x increases by 20 forward-to-backward scattering increases by about 1000.

Hu results from the effects of impurities with random potential strength Ui and positions x. The potential strength is characterized by / = 0 and UiUj = U mp5itj, and includes a forward and a backward scattering term proportional to po and pi, respectively. The disorder average of the impurity potential U(x) follows then to be given by U(x) = 0 and... 

If u is finite, the action of the system has a forward and a backward scattering part. With the decomposition (22), the phase correlation function divides into two parts ... 

Cl + CH2 = CHBr -> CH2 = CHC1 + Br forward and backward scattering are roughly equal in amount. 

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