Atomic arrangements, investigation technique

Optical characterization of materials is a vast field of research that remains extremely active. Many new methodologies are developed each year that explore the interaction between photons of light and atoms to interrogate all kinds of atomic arrangements, from simple diatomic molecules to complex metamaterials. In general, optical techniques are non-destructive, relatively fast, and can be used to investigate innu-merous materials properties, from electronic to chemical, to morphological. 

No investigation of a solid, such as the electrode in its interface with the electrolyte, can be considered complete without information on the physical structure of that solid, i.e. the arrangement of the atoms in the material with respect to each other. STM provides some information of this kind, with respect to the 2-dimensional array of the surface atoms, but what of the 3-dimensional structure of the electrode surface or the structure of a thick layer on an electrode, such as an under-potential deposited (upd) metal At the beginning of this chapter, electrocapillarity was employed to test and prove the theories of the double layer, a role it fulfilled admirably within its limitations as a somewhat indirect probe. The question arises, is it possible to see the double layer, to determine the location of the ions in solution with respect to the electrode, and to probe the double layer as the techniques above have probed adsorption Can the crystal structure of a upd metal layer be determined In essence, a technique is required that is able to investigate long- and short-range order in matter. 

When X-rays are passed through a crystal of sodium chloride, for example, you get a pattern of spots called a diffraction pattern (Figure 3.15b). This pattern can be recorded on photographic film and used to work out how the ions or atoms are arranged in the crystal. Crystals give particular diffraction patterns depending on their structure, and this makes X-ray diffraction a particularly powerful technique in the investigation of crystal structures. 

The measurement of daPs/dQ is not an easy task and, as we shall see, there is little experimental information presently available. We note here that many experimental arrangements which could be used to measure differential positronium formation cross sections sum over all the possible quantum states (nPs, lPs) of the positronium, whereas calculations usually refer to one particular state. Some differentiation between positronium states with different values of nPs can be achieved if the time of flight of the positronium, and hence its kinetic energy, is measured, and such a technique has been used to investigate beams of positronium atoms produced in positron-gas collisions (see section 7.6). 

More directly, solid state physics contributed to the emergence of materials science, because of one of its foci. Spencer Weart identified three pillars on which solid state physics was erected First, X-ray diffraction techniques provided precise atomic picture of solids second, quantum mechanics provided the theoretical foundations for the description of solids and the third, more subtle pillar was the attempt to discriminate between properties depending on the idealized crystal pattern and properties dependent on accidents of either the inner arrangement or the surface of the solid. [13] This focus on structure-sensitive-properties can be seen as the main investigative pathway, to resume Frederic L. Holmes s concept, which lead to materials science. 

Until recently, analytical investigations of surfaces were handicapped by the lack of suitable methods and instrumentation capable of supplying reliable and relevant information. Electron diffraction is an excellent way to determine the geometric arrangement of the atoms on a surface, but it does not answer the question as to the chemical composition of the upper atomic layer. The use of the electron microprobe (EMP), a powerful instrument for chemical analyses, is unfortunately limited because of its extended information depth. The first real success in the analysis of a surface layer was achieved by Auger electron spectroscopy (AES) [16,17], followed a little later by other techniques such as electron spectroscopy for chemical analysis (ESCA) and secondary-ion mass spectrometry (SIMS), etc. [18-23]. All these techniques use some type of emission (photons, electrons, atoms, molecules, ions) caused by excitation of the surface state. Each of these techniques provides a substantial amount of information. To obtain the optimum Information it is, however, often beneficial to combine several techniques. 

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