Low-energy electron diffraction methods

METHODS OF OBTAINING atomically-clean surfaces of solids are listed with comments on their advantages and limitations. The method of argon-ion bombardment is reviewed with a discussion of the operating conditions and precautions necessary for successful results. The low-energy electron-diffraction method is used to determine the condition of the surface. Experimental results indicate that the relative positions of the atoms in the clean (100) surface planes of germanium and silicon are not the same as those of similar planes in the bulk crystals. 

We will, in the latter part of this discussion, focus only on those few methods that have been the most productive, with low-energy electron diffraction (FEED) receiving the most attention. Indeed, LEED has been the most successfiil surface stmctiiral method in two quite distinct ways. First, LEED has become an almost universal characterization... 

This chapter contains articles on six techniques that provide structural information on surfaces, interfeces, and thin films. They use X rays (X-ray diffraction, XRD, and Extended X-ray Absorption Fine-Structure, EXAFS), electrons (Low-Energy Electron Diffraction, LEED, and Reflection High-Energy Electron Diffraction, RHEED), or X rays in and electrons out (Surfece Extended X-ray Absorption Fine Structure, SEXAFS, and X-ray Photoelectron Diffraction, XPD). In their usual form, XRD and EXAFS are bulk methods, since X rays probe many microns deep, whereas the other techniques are surfece sensitive. There are, however, ways to make XRD and EXAFS much more surfece sensitive. For EXAFS this converts the technique into SEXAFS, which can have submonolayer sensitivity. 

The methods of X-ray diffraction usually were used to determine the orientation of crystal faces. Low-energy electron diffraction (LEED) gives more accurate results. However, such measurements provide an exact characterization only of the initial surface state of the electrodes. It is more difficult to determine the surface state after the electrochemical studies, and even more so during these studies. 

Diffraction, by X-rays or neutrons, has been the standard method for determining the structures of crystals. The mean free path of X-rays and neutrons is very long, and thus is not sensitive to surfaces. To probe the structures of surfaces, the probing particles must have a very short mean free path in solids. Two methods are extensively used for determining surface structures low-energy electron diffraction (LEED) and atomic-beam diffraction. A helium... 

The determination of the atomic structure of surfaces is the cornerstone of surface science. Before the invention of STM, various diffraction methods are applied, such as low-energy electron diffraction (LEED) and atom beam scattering see Chapter 4. However, those methods can only provide the Fourier-transformed information of the atomic structure averaged over a relatively large area. Often, after a surface structure is observed by diffraction methods, conflicting models were proposed by different authors. Sometimes, a consensus can be reached. In many cases, controversy remains. Besides, the diffraction method can only provide information about structures of relatively simple and perfectly periodic surfaces. Large and complex structures are out of the reach of diffraction methods. On real surfaces, aperiodic structures such as defects and local variations always exist. Before the invention of the STM, there was no way to determine those aperiodic structures. 

In this section we discuss the adsorption on crystal surfaces. First, we begin with low-energy electron diffraction (LEED)—an experimental method for examining crystal surfaces—and introduce some basic crystallographic concepts needed to interpret the experimental measurements. Then we look at the implication of the adsorbate structure to adsorption and the structure of adsorbed layers using LEED measurements. 

Allied with the diffraction methods, such as low-energy electron diffraction (LEED) and photoelectron diffraction (PED), which can also be applied in single-crystal research, these advances have led to much better interpretations of the vibrational spectra of chemisorbed hydrocarbons in terms of the structures of the surface species. The new results have in turn led to the possibility of reassessing more reliably earlier interpretations of the infrared or Raman spectra of adsorbed hydrocarbons on the finely divided metal samples (usually oxide supported) that are more closely related to working solid catalysts. Such spectra are more complicated because of the occurrence of a variety of different adsorption sites on the metal particles, with the consequence that the observed pattern of absorption bands frequently arises from overlapping spectra from several different surface species. 

As with all surface analytical methods, surface preparation is critical to obtaining reproducible SHG from metallic surfaces and single crystals in particular. For surfaces prepared in UHV and then transferred to an electrochemical cell, sputtering and heating or annealing followed by Auger analysis of impurities should proceed inert transfer. Low energy electron diffraction (LEED) can also be used to check surface order. Metal electrode surfaces, particularly for the rotational anisotropy ex-... 

---