Low-energy electron diffraction pattern

H. Ohtani, C.-T. Kao, M.A.V. Hove, and G. Somorjai, A tabulation and classification of the stmctures of clean solid surfaces and of adsorbed atomic and molecular monolayes as determined from low energy electron diffraction patterns, Progress in Surface Science 23(2,3), 155-316 (1986) and reference therein. 

Figure 10.12 (A) STM image (100 x 100 nm) of a clean Si02 thin film grown on Mo(112) (V = -1.7 V and / = 0.18 nA). (B) STM image (25 x 25 nm) ofthe Si02thin film. (C) Low-energy electron diffraction pattern ofthe Si02 thin film showing c(2x2) periodicity (V = 56 eV).

EXAMPLE 9.7 Comparison Between Bulk and Surface Structures Using Low-Energy Electron Diffraction Patterns. If we accept that an LEED pattern has the same symmetry as the net of surface atoms responsible for its formation, what additional information is needed to complete the comparison between bulk and surface structures for the tungsten surface shown in Figure 9.16a ... 

C. IR/Raman is a computational instrument that predicts IR/Raman spectra. C. LEED/RHEED helps interpret low-energy electron diffraction patterns and reflection high-energy electron diffraction from surfaces. 

Fig. 6. Low energy electron diffraction patterns at normal incidence from clean tungsten surfaces, (a) Ball model of W(llO) face. Some of the net lines (hk) are indexed in terms of a centered rectangular unit mesh (outlined), (b) Clean W(llO), 75 V. Diffuse brightness and central bright spot are caused by light from electron gun filament, (c) Clean W(llO), 300 V. (d) Ball model of (112) surface, the third densest of the boo lattice, (e) Clean W(112) at 90 V. Note the asymmetric intensities of the A/c and hA beams. The unit mesh contains only a single mirror plane perpendicular to surface. There is a strong scattering contribution from the exposed second layer which is asymmetrically positioned.

These first LEED experiments have not yet led to understanding of NHg decomposition or synthesis on W. Effects of H2 or Ng coadsorbing with NHg or NHj still remain to be investigated and one can anticipate many more interesting experiments before the work is completed. It is appropriate to mention briefly that LEED study of NHg decomposition on a Si(lll) surface (293) reveals some similarities to the tungsten experiments. Low energy electron diffraction patterns attributed to N atoms on the surface are developed by heating a room temperature deposit above 700°C. These patterns could not be interpreted in terms of thin layers of silicon nitride. 

Fig. 11. Low-energy electron diffraction patterns taken at four different energies of the reconstructed Si(l 11) crystal face exhibiting a (7 x7) surface structure...

Sputtering the iron at 873 K for a prolonged period of time removes the sulfur, but treatment of the sample in about 1 x lO torr of oxygen at 673 K is needed to rid the sample of carbon. The iron surface is considered clean if Auger electron spectroscopy (AES) shows no impurities and if a low-energy electron diffraction pattern (LEED) is obtained which is representative of the bulk crystal orientation. 

Adsorbate-adsorbate repulsion at high coverage is reflected in the low-energy electron diffractions patterns, in the occurrence of temperature programmed desorption peaks, and in the enthalpy of chemisorption. 

On single crystals, N2 adsorbing on Fe(l 00) forms c(2 x 2) [229, 466, 474, 484]. Detailed interpretation of the low-energy electron diffraction patterns for N/Fe(l 00) shows that N is located in sites with C4 symmetry 0.27 A above the outermost plane of Fe-atoms [485]. The distance between the first and the second layer of Fe atoms is expanded by 7.7% compared to the bulk lattice of Fe [485]. 

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