XANES in addition to EXAFS C K-edge NEXAFS of polymer film

XANES in addition to EXAFS C K-edge NEXAFS of polymer film www.phwiki.com

XANES in addition to EXAFS C K-edge NEXAFS of polymer film

Burney, Jim, Music Director has reference to this Academic Journal, PHwiki organized this Journal Grazing incident X-ray Diffraction (XRD) X-rays are electromagnetic radiation with very short wavelength ( 10-8 -10-12 m), very suitable to do diffraction as long as the crystal. For photon E = h = hc/, 5000 eV photon has wavelength of 2.5 Å. XRD is a well-known technique to study bulk crystal structure. Bragg’s Law: n= 2dsin() X-ray Diffractometer X-ray Tube Sample stage Detector X-ray Diffraction measurement Grazing incident X-ray Diffraction (XRD)

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Typical spectra Besides the angle distribution, the intensity in addition to width of the peak can also gives in as long as mation about the sample, such as particle or grain size, in addition to strain, etc. Grazing incident X-ray Diffraction (XRD) As x-ray generally have rather large penetration depth than electrons, it is far from surface sensitive. The solution as long as study of the surface structure is to use grazing angle (1-3O). X-ray photoelectron diffraction (XPD) Forward scattering dominates the diffraction of the electrons with kinetic energy over several hundreds of eV.

Forward scattering Strong intensity peak along low index axis of the crystal due to as long as ward scattering at hundreds eV. This can be used to do element-specific (XPS) structure analysis with very high surface sensitivity. An application For Co film grown on fcc Cu substrate. Fcc structure have inplane lattice distance same as out-of-plane one, the (011) axis is 45 degree with respect to the sample normal. For Fct (fcc distorted) these two parameters are different, (011) is along some other angle. Magnetic structural in as long as mation from dichroism Diffraction pattern XPD: A powerful tool as long as determining the atomic structure of surfaces with Precision of bond-length measurement about 0.02 Å. The advantage over LEED/RHEED is elemental sensitivity. For low energy, multiple-scattering theory is necessary. For kinetic energy over hundreds of eV the diffraction permits a single-scattering interpretation.

Other particles scattering Atoms/Molecules such as He atom can be scattered from surface, they can be used to study surface structures with very high surface sensitivity. Thermal Helium Atom Scattering is most widely used one. The atoms have energy of 10meV – 100meV which have l of the order of Å. Neutron can be used to study surface too, although neutron scattering have very large free path length in addition to is bulk-sensitive. In some case small angle scattering is useful in addition to it has advantage to study the interfaces. XANES in addition to EXAFS X-ray Absorption Near Edge Structure in addition to Extended X-ray Absorption Fine Structure Also elemental sensitivity Extended X-ray Absorption Fine Structure (EXAFS) Outgoing photoelectrons X-ray absorption decrease with increasing photon energy except at absorption edges. After each absorption edge, the absorption undergoes oscillations due to the interference between outgoing electron wave (photoelectron) in addition to part of outgoing electron wave scattered back by neighboring atoms.

Some theoretical consideration The absorption coefficient u can be written as: m = m0K(1+c) + m0. The interference of outgoing wave in addition to backscattered waves by the neighbors leads to: c(k) = S Ai(k) sin[2kRi + ri(k)] i Here k is determined by the photon energy in addition to the binding energy of the absorption edge. ri(k) is the scattering phase shift. For simple case, after subtraction of the background due to m0K, one can determined Ri from the oscillation period DE, with Ri = (151/DE)1/2 Å A better way is to do Fourier-trans as long as m as: -does not occur as long as isolated atoms – outgoing photoelectron (KE > 50 eV) behaves like free electron – most interference from nearest neighbors – sensitive to short-range order (unlike diffraction) – works as long as amorphous, polycrystalline, glassy materials, liquids – can select absorption edge as long as one particular species (C, O, N ) Subtraction of background Calculate distance Example Different detection methods have different surface sensitivity, as long as example change of the incident angle or choose different signal resources. There is there as long as e Surface Extended X-ray Absorption Fine Structure (SEXAFS).

C K-edge NEXAFS of polymer film The change of peak height ratio of p in addition to s core excitations as the change of incident angle of the light reveals the alignment of polymer molecules are perpendicular to the surface! Ruther as long as d BackScattering (RBS): Conservation laws as long as atomic collision a) For particles with high enough energy, their wavelength are much short than the crystal lattice parameter in addition to the scattering on the surface can be treated as classic collision between the incident particles in addition to the atoms on the surface. The requiring of the conservation of both momentum in addition to energy gives the ratio between initial energy in addition to final energy as long as incident particles: E1’/E1 = {[(m22-m12sinq)1/2+m1cosq]/(m1+m2)}2 In as long as mation as long as atoms on the surface, elemental sensitive! Ruther as long as d BackScattering (RBS): scattering cross section Considering a Coulomb field in addition to recoil of the m2, one get One can write in a power series when m1 smaller than m2:

Channeling in addition to Blocking The shadow cone exists behind the scatterer. For particular low incident ion, can be broad as half a typical interatomic distance, with increasing energy, the shadow cone becomes smaller. Incident ions are steering through the channels as long as med by the rows of atoms (low index axis) with very large penetration length. Angular-dependent RBS The direction of the incident ions with respect to the crystal orientation determines the geometry as long as the scattering. Along low index axis, channeling happens in addition to scattering only at the first several layers, as long as r in addition to om direction scattering as long as all the atoms. The applications First layer atoms scatter; First in addition to second layer atoms scatter; At certain angle first in addition to second layer atoms scatter, while along normal only first layer atoms scatter; Adsorbate overlayers’ first layer atoms scatter, the first layer atoms of substrate scattering reduced. Amorphous overlayer atoms scatter, there is no peak but a bump in addition to the size of the bump depends on thickness of the overlayer.

The applications Si with As doped have same structure Yb is at interstitial positions Angular dependence Thickness in as long as . FIELD ION MICROSCOPY It consists of a sharp needle emitter in addition to a fluorescent screen. The field strength at the tip surface approaches 109 V/cm. With a gas of neutral atoms of about 10-3 torr (Ne, He, H2 in addition to Ar), gas atoms collide with tip will be ionized after many collisions, the ions then will be accelerated away to the screen to generate patterns. As ionization happens where the electric field is high (protruding atoms), the spots on the screen corresponds to the position of atoms on the tip. Resolution can reach 2.5 Å. smaller than 100 nm FIM in addition to FEM FIM gives atomic image of the surface, FEM provide current density variation emitted as long as m the surface (difference in work functions in addition to electric field on the emitter surface). Unlike LEED, FIM is more in as long as mation about 3D atomic arrangement, however, suitable to study surface migration across boundaries in addition to also surface absorbate.

Burney, Jim WSYA-AM Music Director www.phwiki.com

Burney, Jim Music Director

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