Electron diffraction, LEED

There are two major features of the electron-solid interaction evidenced in the 1-V profiles and in other scattering data in LEED electron diffraction that the theory must provide for ... 

LEED Low-energy electron diffraction [62, 75, 105] Elastic backscattering of electrons (10-200 eV) Surface structure... 

The technique of low-energy electron diffraction, LEED (Section VIII-2D), has provided a considerable amount of information about the manner in which a chemisorbed layer rearranges itself. Somotjai [13] has summarized LEED results for a number of systems. Some examples are collected in Fig. XVlII-1. Figure XVIII-la shows how N atoms are arranged on a Fe(KX)) surface [14] (relevant to ammonia synthesis) even H atoms may be located, as in Fig. XVIII-Ih [15]. Figure XVIII-Ic illustrates how the structure of the adsorbed layer, or adlayer, can vary wiA exposure [16].f There may be a series of structures, as with NO on Ru(lOTO) [17] and HCl on Cu(llO) [18]. Surface structures of... 

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... 

As the table shows, a host of other teclmiques have contributed a dozen or fewer results each. It is seen that diffraction teclmiques have been very prominent in the field the major diffraction methods have been LEED, PD, SEXAFS, XSW, XRD, while others have contributed less, such as NEXAFS, RHEED, low-energy position diffraction (LEPD), high-resolution electron energy loss spectroscopy (HREELS), medium-energy electron diffraction (MEED), Auger electron diffraction (AED), SEELFS, TED and atom diffraction (AD). 

The diffraction pattern observed in LEED is one of the most connnonly used fingerprints of a surface structure. Witii XRD or other non-electron diffraction methods, there is no convenient detector tliat images in real time the corresponding diffraction pattern. Point-source methods, like PD, do not produce a convenient spot pattern, but a diffrise diffraction pattern that does not simply reflect the long-range ordermg. 

HEED = high energy electron diffraction IILE = ion-induced light emission INS = ion-neutralization spectroscopy IRS = infrared spectroscopy ISS = ion-scattering spectroscopy LEED = low energy electron diffraction LEIS = low energy ion scattering ... 

Low-Energy Electron Diffraction, LEED 252 Reflection High-Energy Electron Diffraction, RHEED 264... 

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. 

Alternatives to XRD include transmission electron microscopy (TEM) and diffraction, Low-Energy and Reflection High-Energy Electron Diffraction (LEED and RHEED), extended X-ray Absorption Fine Structure (EXAFS), and neutron diffraction. LEED and RHEED are limited to surfaces and do not probe the bulk of thin films. The elemental sensitivity in neutron diffraction is quite different from XRD, but neutron sources are much weaker than X-ray sources. Neutrons are, however, sensitive to magnetic moments. If adequately large specimens are available, neutron diffraction is a good alternative for low-Z materials and for materials where the magnetic structure is of interest. 

To measure the goodness of fit, and to quantify the structural determination, a reliability (i -factor) comparison is used. In comparing the data and simulation of the experiment for many trial structures, a minimum R factor can be found corresponding to the optimal structure. In this way atomic positions can be determined in favorable cases to within a few hundredths of an A, comparable to the accuracy achieved in Low-Energy Electron Diffraction (LEED). 

Edited by H. Bubert and H. Jenett Copyright 2002 Wiley-VCH Verlag GmbH ISBNs 3-527-30458-4 (Hardback) 3-527-60016-7 (Electronic) 2.4 Low-energy Electron Diffraction (LEED)... 

Low-energy Electron Diffraction (LEED) 73 crystal lens... 

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