Scattering, hydrogen

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... 

Many molecules contain chemically equivalent atoms, which, though in a different crystal environment, have, to a good approximation, the same electron distribution. Such atoms may be linked, provided equivalent local coordinate systems are used in defining the multipoles. In particular, for the weakly scattering hydrogen atoms, abundant in most organic molecules, this procedure can lead to more precisely determined population parameters. 

A preliminary knowledge of the crystal structure is important prior to a detailed charge density analysis. Direct methods are commonly used to solve structures in the spherical atom approximation. The most popular code is the Shelx from Sheldrick [26] which provides excellent graphical tools for visualization. The refinement of the atom positional parameters and anisotropic temperature factors are carried out by applying the full-matrix least-squares method on a data corrected if found necessary, for absorption and diffuse scattering. Hydrogen atoms are either fixed at idealized positions or located using the difference Fourier technique. 

Another effect of hydrogen in crystalline silicon is to break Si—Si bonds. After exposure of the surface to atomic hydrogen, extended defects are found in the surface region, typically to a depth of about 1000 A (Johnson, Ponce, Street and Nemanich 1987). These defects have no Burgers vector and are therefore not dislocations, but rather appear to be microcracks, in which the (111) planes of the crystal are pushed apart. A plausible explanation of the crack is that the silicon atoms are terminated by hydrogen and so are pushed apart. The presence of Si—H bonds is confirmed by Raman scattering. Hydrogen therefore can break Si—Si bonds and has a tendency to disorder the crystal. 

Keywords Proton dynamics, neutron scattering, hydrogen bond, proton transfer, decoherence-... 

HFS Hydrogen in thin films (quantitative) Forward-scattered hydrogen atoms... 

For hydrogen depth profiling in polymers, He is the most commonly used incident ion. ERDA experiments are somewhat more challenging for the novice user than are backscattering experiments, as the incident beam is also scattered toward the detector with comparable energy to the forward scattered hydrogen nuclei that are of interest. There are usually some orders of... 

Marzocchi M P, Mantini A R, Casu M and Smulevich G 1997 Intramolecular hydrogen bonding and excited state proton transfer in hydroxyanthraquinones as studied by electronic spectra, resonance Raman scattering, and transform analysis J. Chem. Phys. 108 1-16... 

Let us take two polymers (one deuterated and one hydrogenated) and dissolve them in a solvent (or another polymer) having a scattering length b. The coherent scattered intensity can be derived from (B 1.9.117), which gives... 

Figure Bl.23.9. Scattering intensity of 4 keV Ne versus azimuthal angle 8 for a Ni 110] surface in the clean (1 X 1), (1 X 2)-H missing row, and (2 x l)-0 missing row phases. The hydrogen atoms are not shown. The oxygen atoms are shown as small open circles. 0-Ni and Ni-Ni denote the directions along which O and Ni atoms, respectively, shadow the Ni scattering centre.

The major role of TOF-SARS and SARIS is as surface structure analysis teclmiques which are capable of probing the positions of all elements with an accuracy of <0.1 A. They are sensitive to short-range order, i.e. individual interatomic spacings that are <10 A. They provide a direct measure of the interatomic distances in the first and subsurface layers and a measure of surface periodicity in real space. One of its most important applications is the direct determination of hydrogen adsorption sites by recoiling spectrometry [12, 4T ]. Most other surface structure teclmiques do not detect hydrogen, with the possible exception of He atom scattering and vibrational spectroscopy. 

Roux C D, Bu H and Rabalais J W 1992 Hydrogen adsorption site on the Ni 110 -p(1 time-of-flight scattering and recoiling spectrometry (TOF-SARS) Surf. Sc/. 271 68-80... 

Figure Bl.24.1. Schematic diagram of the target chamber and detectors used in ion beam analysis. The backscattering detector is mounted close to the incident beam and the forward scattering detector is mounted so that, when the target is tilted, hydrogen recoils can be detected at angles of about 30° from the beam direction. The x-ray detector faces the sample and receives x-rays emitted from the sample.

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