Direct scattering

These herbs are pungent and hot, and can directly scatter internal cold. They are used for treating excess internal cold syndrome. They can also stimulate the Yang of the internal organs and accelerate their functions so as to disperse the internal cold in both excess and deficiency conditions. 

Figure 3.15. Rotational state distributions of NO produced in direct scattering from Ag(lll) at Ts 600 as a function of incident normal energy En. Rotational populations Nj are plotted in such a way that a Boltzmann distribution characterized by a temperature T is a straight line. The different symbols correspond to rotational populations derived from the different rotational transitions as listed. From Ref. [160].

The amplitude fu is antisymmetric with respect to interchange of the nuclei, which is a direct reflection of the symmetry property of the corresponding electronic wave function. This implies that the cross section need not be symmetric about 0CM = 9O°. We can define a scattering amplitude fd(6) for direct scattering... 

The scattering amplitude defined in (4.7) characterizes the so-called direct scattering. However, when the scattered electron is slow, there can also occur processes in which the molecule captures the incident electron and emits one of its own. This sort of scattering is described by the exchange amplitude Bn0(k , k0), the formula for which differs from that for the direct amplitude (4.7) in that in the final state of the system the coordinates of the incident electron are transposed with coordinates of molecular electrons, namely,... 

If the interaction operator V can be regarded as a perturbation to the Hamiltonian H0 (this is the case for fast particles the velocity of which is much greater than those of atomic electrons), the function if>+ can be found using the perturbation theory. Such an approach was named the Born approximation. In the first Born approximation we replace the function ip+ by that of the initial state of the scattering system,48 that is, put i//+ = 0, and thereby do not have to solve Eq. (4.2). In this way, for the differential cross section of direct scattering, we get... 

If we substitute amplitude (4.32) into formula for the differential excitation cross section (4.9) and integrate it, making the substitution (1/27r) dil = q dqlk0kn, we will get a factor (hk0)6 in the total cross section. Thus, while the direct scattering cross section decreases as 1IE with decrease of the energy, the exchange cross section behaves as 1/E3, meaning that the triplet states can be excited only by the slow electrons. 

Both the structured distributions for He as partner atom and the highly inverted distributions for Ne strongly suggest that a direct scattering mechanism as discussed in Chapters 6 and 10 rather than statistical laws mentioned in Section 10.3.2 determines the final rotational state distributions. This is, at first glance, astonishing because the van der Waals complex lives for more than a thousand internal vibrational periods and... 

To account for the total directional scattering from a large sphere, effects of both diffraction and reflection must be considered. When a spherical particle is in the path of incident radiation, the diffracted intensity may be obtained from Babinet s principle, which states that the diffracted intensity is the same as that for a hole of the same diameter. The phase function for diffraction by a large sphere is given by [Van de Hulst, 1957]... 

In order to quantify the energy transfer to the e-h pairs, the energy distribution for directly scattered molecules was determined (Fig. 14b). Less than 10% of the incident kinetic energy is transfered to... 

Figure 17 Angular distributions for direct scattering of preferentially oriented NO from Pt(l 11), presented in a polar plot. E = 0.18 eV, Ts = 573 K, i = 50°. The hnes through the angular distributions are drawn to guide the eye. The arrows indicate the angle of incidence and the specular angle. In case of N-end collisions less molecules are directly scattered. Molecules with are directly scattered after an N-end collision come off closer to the surface normal than molecules with an O-end collision. From Kuipers et al. [94].

Hurst et al. attributed the second of these spectral peaks (the peak occurring at later times) to Xe atoms which had accommodated to the surface (i.e. come into equilibrium with the surface at a surface temperature of Ts) and subsequently desorbed from the surface. The other peak present in the spectra was attributed to Xe atoms that had directly scattered, without thermal accommodation, from the surface. 

Figure 17.1 Sketch of definitions gi(8) and

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