Surface scattering specular

Diffuse surface scattering Specular surface scattering... 

Studies of inelastic scattering are of considerable interest in heterogeneous catalysis. The degree to which molecules are scattered specularly gives information about their residence time on the surface. Often new chemical species appear, whose trajectory from the surface correlates to some degree with that of the incident beam of molecules. The study of such reactive scattering gives mechanistic information about surface reactions. 

For semiconductor work, either whole wafers or small pieces are used, the latter often being necessary for insertion into a cryostat. Bulk solids may be analyzed in any form, but scattered light may be reduced and the signal increased if the emitting surface is specular. 

Origin and measurement of reflectance spectra. For mineral identification purposes, reflected light that has penetrated inside regolith materials (volume or diffuse reflectance), rather than surface-scattered light (or specular reflectance), is measured spectrally. The diffuse reflectance spectra give profiles in which minima correspond to peak maxima in absorption spectra but with much loss of spectral resolution. 

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...

Because the surface light scattering method is less well known than the bulk scattering one, we will give some details about its principle. In the surface scattering method, a laser beam impinges on the surface. Because of thermal motion, the surface is not perfectly flat and scatters light in all directions around the specularly reflected beam. The surface displacement C can be described as a superposition of sinusoidal deformations C = where r is the position in the equilibrium plane of the surface and q is... 

The increase in resistivity at narrow fine widths has been attributed to surface scattering and grain-boundary scattering. The Fuchs and Sondheimer (FS) model attributes the resistivity increase in thin and narrow fines to diffuse scattering of electrons at the exterior surfaces with a probability of 1 — p, where p is the specular scattering coefficient. The length scales in the FS model are the thickness and line width of the conductor and the mean free path A. The simplified expression for resistivity as a function of thickness (T) and linewidth (W) of the conductor is given by ... 

The CTRs are classed as specular when the scattering vector Q is normal to the surface, and nonspecular when there is a component of Q parallel to the surface. The specular CTRs provide information about the structure of the electrode and the electrolyte perpendicular to the surface, whilst the nonspecular CTRs only provide information about the in-plane structure of the electrode. The collection of experimental data involves collecting the scattering intensity along a CTR to yield a so-called rocking curve [11], as shown in Figure 14.4... 

Standards can be roughly grouped into two major groupings, specular, or mirror-like, and diffuse, or scattering. Specular standards are usually highly polished surfaces, with the reflecting layer a metal, typically silver, aluminum, gold, or beryllium. In many cases, this metal surface is over-... 

Beer s law applies where refractive index, scattering specular reflection at infinite-finite numbers of surfaces all obey the Fresnel formulas. A definite reflectance theory does not exist, as the convolution of an infinite number of integrals would be required to describe all the combined light interaction effects at all surfaces under varying conditions. Thus, Beer s law is often shown to illustrate the properties of NIR spectroscopy for lack of an ideal model. 

No real surface behaves in exactly this way, although some surfaces approach it. Some simultaneously exhibit specular and diffuse reflectance that of a new automobile provides a familiar example. The hard, highly polished outer surface exhibits specular reflectance of some of the incident light. The remainder is refracted into the layers below, which contain diffusely reflecting, spectrally selective absorptive pigments suspended in a binding matrix. Light not absorbed is scattered within this layer with an intensity pattern that may approximate that of a perfectly diffuse reflector. Because of the absorptive properties of... 

Scattering to the diffraction channel (n,m) = (0,0) is called specular scattering. The Schrodinger equation for atom-surface scattering is... 

Figure A3.9.4. The ratio of specular reflectivity to incident beam intensity ratio for D2 molecules scattering from a Cii(lOO) surface at 30 K [21],...

Figure Bl.25.12 illustrates the two scattering modes for a hypothetical adsorption system consisting of an atom on a metal [3]. The stretch vibration of the atom perpendicular to the surface is accompanied by a change m dipole moment the bending mode parallel to the surface is not. As explained above, the EELS spectrum of electrons scattered in the specular direction detects only the dipole-active vibration. The more isotropically scattered electrons, however, undergo impact scattering and excite both vibrational modes. Note that the comparison of EELS spectra recorded in specular and off-specular direction yields infomiation about the orientation of an adsorbed molecule.

Figure Bl.25.12. Excitation mechanisms in electron energy loss spectroscopy for a simple adsorbate system Dipole scattering excites only the vibration perpendicular to the surface (v ) in which a dipole moment nonnal to the surface changes the electron wave is reflected by the surface into the specular direction. Impact scattering excites also the bending mode v- in which the atom moves parallel to the surface electrons are scattered over a wide range of angles. The EELS spectra show the higlily intense elastic peak and the relatively weak loss peaks. Off-specular loss peaks are in general one to two orders of magnitude weaker than specular loss peaks.

Several practical issues of the scatterometer must be considered in the case of characterizing nominally smooth surfaces. The incident laser beam may be collimated, but more commonly it is brought to a focus at a distance defined by the arc in which the detector rotates. In addition, a deflection mirror or an optical fiber might be used to direct light to the detector element. These features permit measurements close to the specular and transmitted beams, and this is critical to folly characterize the scattered light. This is especially significant since the scattered light intensity... 

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