Diffraction
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Diffraction

Diagramme de diffraction d'électrons lents (LEED) sur un monocristal de Cuivre Cu(001). On remarque la symétrie d'ordre 4 de la surface.

Les techniques de diffraction permettent de sonder l'ordre dans la matière. Pour ceci, on fait interagir une onde dont la longueur d'onde λ est comparable à la taille du motif cristallin élémentaire de l'échantillon. Ce peut être une onde électromagnétique (rayons X, lumière Laser) ou des particules (électrons, neutrons ou des atomes d'hélium).

La présence de pics intenses dans le diagramme de diffraction montre la présence d'ordre. Les pics sont séparés de 2π/a en nombre d'onde (2π/λ) où a est le pas du réseau. La forme des pics de diffraction traduit la qualité de l'ordre : par exemple, la diffraction sur un domaine ordonné de taille L se traduit par un élargissement des pics de diffraction en (2π/L).


Diffraction techniques are used to probe the order in the matter. For this,  a wave with wavelength (λ) comparable with the elementary domain size interact with a sample. It can be an electromagnetic wave (x-rays, Laser light) or particles (electrons, neutrons or helium atoms).

The presence of intense peaks in the diffraction pattern shows the presence of order. The peaks are separated from 2π/a (2π/λ in wave number) where a is the crystal lattice parameter. The shape of the diffraction peaks traduces the quality of the order. Diffraction on an ordered domain of size L results in a broadening of the diffraction peaks in (2π/L).

 
#116 - Màj : 03/01/2013
 
News December 2023 The ICONE project and HiCANS in general were presented to the French user community during the Journées de la Neutronique in Erquy. HiCANS High Current Accelerator-driven Neutron Sources - State of the art  (F. Ott) PRESTO: diffraction on ICONE (X. Fabrèges) Spectroscopy : state of the art, and possibilities in HiCANS (Q. Faure) The HERMES - LLB outstation at the JULIC Neutron Platform (M.
Technique (desc. gen.)
X-ray Photoelectron Diffraction is one surface technique used to study the surface structure of sample. The intensity of excited photoelectron as a function of azimuthal or polar angle of sample is recorded by changing the sample position. XPD provides information on surface relaxation and also the local atomic structure down to the monolayer. Quantitative information is given combined with the electron scattering simulation. Besides, XPD is also the basis of the photoelectron holography.
La diffraction d’hélium est une technique encore peu répandue, une dizaine de laboratoires seulement la possède à l'échelle mondiale, mais cette technique a déjà permis des progrès importants dans la compréhension de la structure et de la dynamique des surfaces et des films minces. Son principe consiste à envoyer sur la surface à étudier un faisceau d’atomes d’hélium et d’analyser le faisceau réfléchi et diffracté par celle-ci.
La diffraction d'électrons lents (LEED : Low Energy Electron Diffraction) est une technique qui permet d'étudier la structure et la symétrie des surfaces. Cette technique repose sur la nature ondulatoire des électrons et sur la forte interaction de ceux-ci avec la matière.
Le groupe GOC de l'IRAMIS/SPEC possède un diffractomètre pour poudres D8 Advance (Bruker-axs) équipé d’une chambre en température MRI, permettant de caractériser les matériaux synthétisés (identification des phases cristallines, évaluation de la cristallinité des échantillons et calcul des paramètres cristallins) et étudier leurs structures de l’ambiante à 1100°C.
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