Specular scattering

Fig. 4.5. Schematic atomic scattering distributions for (a) specular scattering, (b) rainbow scattering and (c) diffraction...

Another important property of the specularly scattered fraction of atoms is their great sensitivity to surface disorder. On scattering from a well ordered surface, nearly 15% of the scattered helium atoms appear in the specular helium beam. This fraction decreases to 1 to 5% when the surface is disordered. Thus measurements of the fraction of specularly scattered helium can provide information on the degree of atomic disorder in the solid surface. 

The authors list other possible error sources, including minor differences between the spectra of pure components and mixtures, temperature effects, and missing minor components. Again, it is usually best to mimic the true process conditions as much as possible in an off-line set-up. This applies not only to physical parameters like flow rate, turbulence, particulates, temperature, and pressure, but also to minor constituents and expected contaminants. The researchers anticipate correcting these issues in future models, and expect to achieve approximately 0.1-mM detection limits. The models successfully accommodated issues with specular scattering from the biomass and air bubbles and from laser-power fluctuations. 

Although in the direction normal to the surface, relatively high-resolution structural information is already accessible in the plane, there is relatively little direct information on a relevant length scale available. Current developments in the technique associated with off-specular scattering will address this over the next 2-3 years, and this will transform our ability to obtain the relevant structural information in the plane of the surface. Hence, the degree of surface ordering and the extent of the bilayer or micellar patches will be accessible. 

Figure 26 The limiting cases of angular distribution of species scattered from a solid surface (a) specular scattering characteristics of short residence time, (b) cosine distribution characteristic of long residence times...

Fig. 17. Electron energy loss spectra in the vibrational regime of clean and CO-covered Cr203(0001)/Cr(l 10) surface. (Ep 7.5 eV, specular scattering) The inset shows a schematic representation of the calculated normal mode of a Cr203(0001) surface at 21.4 meV, according to ref [100].

Finally we observe that the specular scattered field is quite closely equal to the normalized scattering value of a single triad only. Similarly, the backscattered field can be obtained as the sum of the backscatter from a triad and the array factor. This shows that for a large array with adjusted edge triads the total scattered field could have been obtained quite accurately by simple pattern multiplication of a typical triad and the array factor. 

FIGURE 42 Rectilinear plot displaying the (a) specular scattering and (b) cosine angular distribution of scattered beams. The arrow indicates the angle of incidence. 

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