Diffracted beam, intensity

Chemical reactions of surfeces. Diffraction can be used qualitatively to identify different surface phases resulting from adsorption and chemical reaction at surfaces. Reaction rates can be investigated by following the evolution of diffracted beam intensities. 

Direct kinetic measurements from the changes in diffracted beam intensities with time during heating of the reactant are illustrated in the work of Haber et al. [255]. Gam [126] has reviewed the apparatus used to obtain X-ray diffraction measurements in thermal analysis. Wiedemann [256] has designed equipment capable of giving simultaneous thermo-gravimetric and X-ray data under high vacuum. X-Ray diffraction studies enable the presence, or absence, of topotactic relationships between reactant and product to be detected [92,102,257—260], Results are sometimes considered with reference to the pseudomorphic shape of residual crystallites. 

Determinations of the surface structure by computing the diffraction beam intensities from low energy electron diffraction are concentrated in two frontier areas at present. One is the determination of the surface structures of adsorbed molecules of ever bigger size and the other is the determination of the atomic locations in reconstructed clean solid surfaces. 

The diffraction beam intensities of the (5 x 1) surface structure are under... 

LEED studies of clean surfaces have revealed that most of these surfaces, if prepared under proper conditions, are ordered on an atomic scale and exhibit sharp diffraction beams and high diffraction beam intensities. Metal, semiconductor, alkali halide, inert gas, and organic crystal surfaces have been studied this way, and all exhibit ordered surface structures. 

Zavalij, 2003). Position-sensitive detectors (also called area detectors), based either on a gas-filled ionization chamber or an image intensifier coupled to a video camera detect and record diffraction beam intensity in two dimensions simultaneously, a feature that greatly enhances the speed of data collection (Drenth, 1999). 

Changes occur in diffracted beam intensity with time of standing in vacuum subsequent to various oxygen exposures. Figure 5 shows several plots of peak diffraction beam current as a function of log10 exposure. The curves in sets 1... 

If the diffracted intensity is unacceptably low, a quick thermal shock to the crystal may micro-shatter the crystal and thus form those domains, thus restoring a larger Bragg diffracted beam intensity. 

The results show that the first monolayer of silver contributes at least 75% of the diffracted beam intensity at 50 ev primary energy, and the first two monolayers contribute more than 90%. 

Surface diffusion is indicated by changes which occur in diffracted beam intensity with elapsed time in vacuum subsequent to various oxygen exposures. Figure 5 shows several plots of peak diffraction beam current as a function of log of exposure. The curves in sets 1 and 2 show increases, /, in beam intensities of gas structures which have occurred as a result of conversion of the adsorbed amorphous molecular oxygen into atomic oxygen in a lattice structure. The curves in set 3 show two increases and two decreases in intensity, marked and... 

Fio. 8. Diffraction beam intensities as a function of oxygen exposure obtained after a small anneal of the crystal subsequent to ion-bombardment cleaning. Curve 1 Typical beam, in the (110) azimuth at about 28 volts, from the clean nickel lattice. Multiply the ordinate scale by 2 to obtain intensity. Curve 2 Typical beam, in the (001) azimuth at about 58 volts, from the clean nickel lattice. Multiply the ordinate scale hy 6. Curve 3 Typical beam, in the (110) azimuth at about 17 volts, from a double-spaced, face-centered lattice. Curve 4 Typical beam, in the (001) azimuth at about 27 volts, from a single-spaced, simple-square lattice. Multiply the ordinate scale by 2. Curve 5 Typical beam, in the (110) azimuth at about 22 volts, from a nickel oxide lattice. [From Farnsworth and Madden (27).]... 

The Debye-Waller factor quantitatively describes the decrease of the diffracted beam intensity due to thermal vibrations of the atoms in the lattice. As the temperature increases, the root-mean-square displacements of the atoms in the lattice become larger and the diffracted beam intensity decreases. The maximum intensity should be observed at 0 K. According to Eq. 4.11, the Debye-Waller factor for LEED and XRD is... 

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