XRD-TED-comp_short copy

XRD-TED-comp_short copy - Scattering of xwrays and...

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Unformatted text preview: Scattering of xwrays and electrons (fast ones with velocity equal to significant fractions of the terminal velocity, speed of light) by crystals XRD Vis—auvis El) X—Ray Diffraction: * Photon is the quantum of the eleetto=magnetie field rest mass is zero, tax I 0 Charge neutrala qx : 0 0 Deeeletation of fast eleetrons in a target material is typieally used in an Xwiay tube for the generation ofx«rays suitable for diffraction studies oferystals. X»— ray photons with maximum energies on the ordet of l0==40leV are produced, leading to the short wavelength limit Mb usually on the order of l0”10 mg, the An range. (Cheek on the geiietation of X-~f€iy8 if your eurreat understanding of the process and its implications are shaky or opaque. Cullity arid Steele treat it very well in their text.) Characteristic X~rays emitted li‘om suitable X—tay tubes typically have wavelengths between f\ else 3 a 252% l ‘ The interaetioa between thays and atoms in crystals during scattering is effectively limited to the scattering by the atomic: electrons or more generally the ei‘ystal electrons; Hence, the scattering ot‘Xerays by atoms in crystals is eonsidered a weak interaction; which results in large penetration depths due te relatively little absorption. Recall that quite heavy metals (e:th Pb) are used. fer shieldiag agaiest harmfial effeets from expesute t0 Xa‘aysi The above is already geaeral knowledge ef the students at this point. Electron Diffraction: ' Electron is the quantum rest mass is flake? m6 :3 9.1l 2410'“ kg Charge is limits and negativm qe : ~ 1.602 x l()49 C * Acceleratien of fast electrons through a petential drop, U [V] produces highly energetic beam. of electrons emitted from. an slectmn gun? which can be lbcused thmugh electm—static and electl‘ondynamiC(magnetic) lenses to very small dimensians: E: qc U r: (me/2) V2 : h "v 2 h c/ A : pCZ/(cmc) For fast electmns the resulting wavelength must include rslatlvlstic censidsrafions 21136 is very shaft cmnpared with that oftypical X-rays usecl in diffraction. studies Qf crystals. Eagl U [W] kc [A] 10 0.122 50 0086 ms M37 200 91125 400 6.0165 * Consequences form the short electron wavelength important fer diffractien of crystals a) The Eweld sphere radius is very large RES 3 1/ 9% b) The Limiting sphere radius is very large RLS == 2 RES The minimum d~spacing that can be resolved using fast electrons is below the size at a single stem! c) The Bragg angles? (3), diffractlen angles are very small l Eggs, AL cigmethlA and usng EQOkV electrens with A=Ofl25fs we final thet (9 200(Al) : sin"l {0025/4} 1: 036° 2 0.36 (it/180) radian = 6.25 mixed 3 0.025/4 sadism since fer small 6*), sin ‘8 2“: G) DES!GN NGTE‘: 80, clearly, a]! the isformetion relevant to the study of the crystal structure: to be deceded from the ED patterns using the modified Bragg Law, 20’ a 2/69, and the structure faster expression, can be Gel/ected Within about a few degrees ef the incident beam direction. This would suggest to the clever engineer that a transmissiomtype geometry WOLild be the best choice for the instrumentation for the diffraction experiments, Hence, the trensmisssion electrerr microscope (TEA/7) for ED studies of crystal. * The scattering of fast electrons by stems is considered a streng interaction and comprises centribution form betli scattesing by the nucleus and by the electmss of the stems» Thus, electrons are absorbed very strengly by crystalline substances and penetratien {lepth tbreugb typical crystalline substances, such as metals, are 011 the Order at a few mlcmns {2: l_0“6m) Only, This sbeut twe it) four antlers ef magnitude less than fer X—rsys. Hence, "l‘lvllN FOIL SPEClMENS MUST BE USED ESE F'l‘llANSMlSSlON “l‘YlT’E‘ EXPERIEMENTS in the ’l‘lil‘s’l, Tlseei‘eticsl ttestmests beyend the scepe {if this cesrse shew that the scstteriiig length (stresgtb) fer electrons by stems? {‘95 has 2:. magnitude want. that (“if the scattering length (strength) for ways, fix, by the same type of atom that is about four orders of magnitude larger, that, x fxt s: $4 Note: The structure factor for XRE) and ED are different for a given crystal, since Pg 2 fx, 2;; 104. The strong scattering of electrons by crystals and the large absorption again indicate that the design of the instrument for the diffraction studies should function in a transmission type geometry and that very thin specimens/samples have to be used GEGMETRY of TEM in real space and in reciprocal space The shape factor (thin specimen effect) and the reciprocal lattice treatment of electron diffraction in the TEM Types of selected area diffraction patterns in the TEM interpretation and decoding the selected area diffraction patterns in the 'lllEM ...
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XRD-TED-comp_short copy - Scattering of xwrays and...

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