Surface diffraction electrons

The characterization of evaporated alloy films can be carried out at widely different levels of sophistication. At the very least, it is necessary to determine the bulk composition, probably after the film has been used for an adsorption or catalytic experiment. Then various techniques can be applied, e.g., X-ray diffraction, electron diffraction, and electron microscopy, to investigate the homogeneity or morphology of the film. The measurement of surface area by chemisorption presents special problems compared with the pure metals. Finally, there is the question of the surface composition (as distinct from the bulk or overall composition), and a brief account is given of techniques such as Auger electron spectroscopy which might be applied to alloy films. 

EM provides local structural information about the samples in both real and reciprocal space, for example local structural information about the surface and the bulk of the sample at the atomic level, together with chemical, electronic and three-dimensional structural information are now routinely available. Some of these methods are described in this chapter. Electron-sample interactions and scattering are fundamental to EM. EM is a diffraction technique in which crystals diffract electrons in accordance with Bragg s law, nX = Idhki sin 9, where X is the... 

In LEED, electrons of well-defined (but variable) energy and direction of propagation diffract off a crystal surface. Usually only the elastically diffracted electrons are considered and we shall do so here as well. The electrons are scattered mainly by the individual atom cores of the surface and produce, because of the quantum-mechanical wave nature of electrons, wave interferences that depend strongly on the relative atomic positions of the surface under examination. 

Over the past 10 years a multitude of new techniques has been developed to permit characterization of catalyst surfaces on the atomic scale. Low-energy electron diffraction (LEED) can determine the atomic surface structure of the topmost layer of the clean catalyst or of the adsorbed intermediate (7). Auger electron spectroscopy (2) (AES) and other electron spectroscopy techniques (X-ray photoelectron, ultraviolet photoelectron, electron loss spectroscopies, etc.) can be used to determine the chemical composition of the surface with the sensitivity of 1% of a monolayer (approximately 1013 atoms/cm2). In addition to qualitative and quantitative chemical analysis of the surface layer, electron spectroscopy can also be utilized to determine the valency of surface atoms and the nature of the surface chemical bond. These are static techniques, but by using a suitable apparatus, which will be described later, one can monitor the atomic structure and composition during catalytic reactions at low pressures (< 10-4 Torr). As a result, we can determine reaction rates and product distributions in catalytic surface reactions as a function of surface structure and surface chemical composition. These relations permit the exploration of the mechanistic details of catalysis on the molecular level to optimize catalyst preparation and to build new catalyst systems by employing the knowledge gained. 

Figure 8.21 Schematic of a LEED setup (left) and a LEED diffraction pattern obtained from a Pt(l 11) surface with electrons of 350 eV energy (right). The image of the diffraction pattern was kindly provided by M. Zharnikov and M. Grunze.

Fig. 6.6 The principle of low-energy electron diffraction (LEED) is that a beam of monoenergetic electrons scatters elastically from a surface. Due to the periodic order of the surface atoms, electrons show constructive interference in directions for which the path lengths of the electrons differ by an integral number times the electron wavelength. Directions of constructive interference are made visible by collecting the scattered electrons on a fluorescent screen.

The writer is greatly indebted to C. S. Smith, C. S. Barrett, and E. A. Gulbransen for many fruitful discussions pertinent to surface studies pursued over an extended period. Much helpful assistance in the preparation and characterization of sample surfaces from the viewpoint of surface structure was provided at one time or another by Joseph Cerny, Walter Bergmann, Donald Clifton, Kaye Ikeuye, and James Hess. Without their assistance the tedious assignment involved in the meticulous preparation of sample surfaces could not have been achieved. The useful collaboration with L. P. Schulz on the characterization of metal surfaces by electron microscopy and electron diffraction techniques was an essential part of the surface studies. Assistance in the design and construction of techniques for the controlled evaporation of metals was provided by H. E. Shaw. 

A quantitative measurement of the depth of penetration of the diffracted electrons has been made previously by the author (1) by depositing silver vapor onto a gold crystal surface, using a calibrated silver source. Since the lattice structures are the same and the lattice constants differ by less than 0.4%, the silver was found to deposit as a thin crystal on the gold surface. Because of the different indices of refraction and certain fine-structure characteristics for the two metals, the diffraction beams from silver and gold were readily distinguished. 

The technique of low energy electron diffraction (LEED) has been the most widely used tool in the study of surface structure. LEED experiments involve the scattering of monoenergetic and collimated electrons from a crystal surface and detection of elastically diffracted electrons in a backscattering geometry (Figure 2). The characteristic diffraction pattern in LEED arises from constructive interference of electrons when scattered from ordered atomic positions. The diffraction pattern represents a reciprocal map of surface periodicities and allows access to surface unit cell size and orientation. Changes in the diffraction pattern from that of a clean surface can be indicative of surface reconstruction or adsorbed overlayers. 

Optical microscopy. Interferometry, various electron microscopies, atomic force techniques (AFM, STM), surface diffraction, BET (+t-plot), poroslmetry. 

While collapsed films of this polymer can be lifted off the surface on electron microscope grids, viewed under the light microscope they are seen to break under the action of surface forces within a few minutes. Electron diffraction observations are evidently not feasible, but good polarized IR spectra are obtainable (Figure 2). The parallel dichroism of the Amide A band (3300 cm ) and the Amide I band (1660 cm ), and the perpendicular dichroism of the Amide II band (1555 cm ) is strong evidence that the collapsed monolayer is in the a-helical conformation with the molecules aligned on the water surface more or less parallel to the barrier. There is not sufficient dichroism in the bands associated with the n-decyl side chain for it to be orientated predominantly either parallel or perpendicular to the backbone. Since the side chains are very flexible it is probable that during collapse of the monolayer the side chains fold to form a more compact non-dichroic structure. 

Several types of diffraction by crystals are now studied. Neutron diffraction can be used with great effectiveness to give information on molecular structure. These results complement those from X-ray diffraction studies, because there are different mechanisms for the scattering of X rays and of neutrons by the various atoms. X rays are scattered by electrons, while neutrons are scattered by atomic nuclei. Neutron diffraction is important for the determination of the locations of hydrogen atoms which, because of their low electron count, are poor X-ray scatterers. Electron diffraction, while requiring much smaller crystals and therefore being potentially useful for the study of macromolecules, produces diffraction patterns that are more complicated. Their interpretation is hampered by the fact that the diffracted electron beams are rediffracted within the crystal much more than are X-ray beams. This has limited the practical use of electron diffraction in the determination of atomic arrangements in crystals to studies of surface structure. 

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