Diffuse low energy electron diffraction

HEI Heinz, K. Diffuse low-energy electron diffraction Prog. Surf. Sci. 27 (1988) 239. 

The apparatuses used for the studies of both ammonia synthesis emd hydrodesulfurization were almost identical, consisting of a UHV chamber pumped by both ion and oil diffusion pumps to base pressures of 1 x10 " Torr. Each chamber was equipped with Low Energy Electron Diffraction optics used to determine the orientation of the surfaces and to ascertain that the surfaces were indeed well-ordered. The LEED optics doubled as retarding field analyzers used for Auger Electron Spectroscopy. In addition, each chamber was equipped with a UTI 100C quadrupole mass spectrometer used for analysis of background gases and for Thermal Desorption Spectroscopy studies. 

Atoms are not rigidly bound to the lattice, but rather vibrate around their equilibrium positions. If we were able to examine the crystal over a very brief observation time, we would see a slightly disordered lattice. Incident electrons see these deviations, and this is for example the reason that in low-energy electron diffraction (LEED) the spot intensities of diffracted beams depend on temperature. At high temperatures the atoms deviate more from their equilibrium position than at low temperatures, and a considerable number of atoms is not at the equilibrium position necessary for diffraction. Thus, spot intensities are low and the diffuse background high. Similar considerations apply in other scattering techniques, as well as in extended X-ray absorption fine structure (EXAFS) and in Mossbauer spectroscopy. 

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.

Colin et aV studied the adsorption and decomposition of HMDS in the absence of oxygen on various platinum surfaces at different temperatures at low pressure by XPS, ultraviolet Photoelectron Spectroscopy (UPS), and Thermal Diffuse Scattering (TDS). They also used Auger Electron Spectroscopy (AES) and Low-Energy Electron Diffraction (LEED) as surface characterization tools. 

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