Grazing angle

The diffraction pattern consists of a small number of spots whose symmetry of arrangement is that of the surface grid of atoms (see Fig. IV-10). The pattern is due primarily to the first layer of atoms because of the small penetrating power of the low-energy electrons (or, in HEED, because of the grazing angle of incidence used) there may, however, be weak indications of scattering from a second or third layer. 

Experimentally, it is common for LEED and Auger capabilities to be combined the basic equipment is the same. For Auger measurements, a grazing angle of incident electrons is needed to maximize the contribution of surface... 

Illuminating the sample at grazing angles. The penetration depth of photons depends on the cosine of the incidence angle and, therefore, can be reduced by this procedure. Although such an approach has limited use, it has been successfully employed in a few instances, such as for x-ray diffraction experiments. 

When an ion beam is incident on an atomically flat surface at grazing angles, each surface atom is shadowed by its neighbouring atom such that only forwardscattering (FS) is possible these are large impact parameter (p) collisions. 

Combination techniques such as microscopy—ftir and pyrolysis—ir have helped solve some particularly difficult separations and complex identifications. Microscopy—ftir has been used to determine the composition of copolymer fibers (22) polyacrylonitrile, methyl acrylate, and a dye-receptive organic sulfonate trimer have been identified in acryHc fiber. Both normal and grazing angle modes can be used to identify components (23). Pyrolysis—ir has been used to study polymer decomposition (24) and to determine the degree of cross-linking of sulfonated divinylbenzene—styrene copolymer (25) and ethylene or propylene levels and ratios in ethylene—propylene copolymers (26). 

Though a powerfiil technique, Neutron Reflectivity has a number of drawbacks. Two are experimental the necessity to go to a neutron source and, because of the extreme grazing angles, a requirement that the sample be optically flat over at least a 5-cm diameter. Two drawbacks are concerned with data interpretation the reflec-tivity-versus-angle data does not directly give a a depth profile this must be obtained by calculation for an assumed model where layer thickness and interface width are parameters (cf., XRF and VASE determination of film thicknesses. Chapters 6 and 7). The second problem is that roughness at an interface produces the same effect on specular reflection as true interdiffiision. 

As discussed above, it is desirable to use large, almost grazing angles of incidence in RAIR spectroscopy in order to maximize the sensitivity of the technique to ultra-thin films. Using such large angles of incidence usually requires the substrates to be at least a few centimeters in length. This severely limits the spatial... 

Figure 2. Optical schematic of the chamber for grazing angle measurements. Heaters and one thermocouple are located directly behind the sample (A). Gas inlet and outlet are near the IR transmitting windows (B). The mirrors (C) can be rotated and tilted to maximize signal and eliminate stray light. The entire cell (D) can be translated to change the angle of incidence.

XANES spectroscopy is also the basis of chemically sensitive X-ray imaging, as well as qualitative and quantitative microspectroscopy [306], ptXANES is attractive for chemical analysis, with its spatial resolution down to 10 ptm. Variations on the theme are surface EXAFS (SEXAFS), grazing incidence XAS and in situ time-resolved XAS investigations. Grazing angle XAFS can be used for the study of ultrathin multilayer systems. 

Grazing angle XAS techniques (XANES) can be applied to ultrathin film systems [316]. Selected NEX-AFS, XPS and FUR spectroscopy results were obtained for plasma-polymerised films with different monomers (styrene, acetylene, ethylene and butadiene) [317]. 

Reflection-Absorption IR spectroscopy (RAIRS) where the linearly polarized IR beam is specularly reflected from the front face of a highly reflective sample, such as a metal single crystal surface (Figure 3.1(a)). This is also sometimes referred to as IRAS (IR reflection absorption). The IR beam comes in at grazing angle (i.e. almost parallel to the surface), and although absorption bands in RAIRS have intensities that are some two orders of magnitude weaker than in transmission studies on... 

In reflection ERDA the ion beam impinges on to the specimen at grazing incidence, and the recoils emerge from the front surface, generally also at a grazing angle. 

ERDA (HFS) only requires the addition of a thin foil (of carbon, mylar or aluminium) to separate forward scattered hydrogen from forward scattered primary He++ ions. The analytical information obtained consists of hydrogen concentration versus depth. The sample is tilted so that the He++ beam strikes at a grazing angle, giving a HFS depth profile resolution of about 50 nm. The surface hydrogen content... 

The problem of surface sensitivity could be overcome to a certain extent by using grazing angle incidence, however, the refractive indices of solids in... 

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