LEED optics

Whereas the spot positions carry information about the size of the surface unit cell, the shapes and widths of the spots, i.e. the spot profiles, are influenced by the long range arrangement and order of the unit cells at the surface. If vertical displacements (steps, facets) of the surface unit cells are involved, the spot profiles change as a function of electron energy. If all surface unit cells are in the same plane (within the transfer width of the LEED optics), the spot profile is constant with energy. 

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. 

Fig. 3. Schematic drawing of the high pressure electron spectrometer. A, Argon ion gun D, differentially pumped region EL, electron lens G, gas cell HSEA, hemispherical electron analyzer LO, two-grid LEED optics LV, leak valve M, long travel rotatable manipulator P, pirani gauge S, sample TSP titanium sublimation pump W, window X, twin anode x-ray source.

A Physical Electronics single-pass CMA and a Phi 4-grid LEED Optics unit were used in the studies of metal films on Ti02. Pd and Au films were evaporated from high-density alumina effusion sources. 

Fig. 3. Cross-section of the combined UPS-XPS-LEED system described by Bradshaw and Menzel (25). A - Hemispherical electron energy analyser B - Slit-change mechanism C - Qua-drupole mass spectrometer D - Crystal manipulator E - LEED optics F - Assembly for electron bombardment of the sample replaceable by an argon ion gun G - Rare gas resonance lamp with axis perpendicular to the plane of the cross-section H - X-ray source. [Reproduced with permission from Ber. Bunsenges. 78, 1140 (1974)]...

All experiments were performed in an ultrahigh vacuum (UHV) STM (JEOL JSTM 4500VT) with ion guns and low-energy electron diffraction (LEED) optics. The base pressure of the chamber was less than 1 x 10-8 Pa. Eledrically etched tungsten tips were used in the STM observation, and all STM images were taken in constant current mode. 

LEED optics were used to characterize surface structures. The principle of low energy electron diffraction can be found in ref. 110. The LEED patterns were recorded on Polaroid 667 or 552 instant films using a Polaroid attachment for a Nikon F 35 mm camera. Because both LEED patterns and real space mesh structures of Ni(lll) and Ag(lll) surfaces are well known, the substrate LEED spots were used as references co determine the unit mesh of adsorbate structures. In practice the unit vectors... 

Fig. 2. Sample holder and capillary of the vacuum thin layer cell systems as seen through the view port of the work chamber, with LEED optics in the background [6],...

The film growth and electrical measurements were performed in the ultra high vacuum (UHV) chamber with a base pressure of MO Torr. It was equipped with LEED optics, evaporation unit with three sublimation sources (Si, Cr, Fe) and manipulator with samples holder and a quartz thickness sensor. In situ electrical measurements were conducted using automated UHV Hall installation [2], The p-type (lOQ cm) Si(lll) wafers were used as substrates. The native oxide and... 

Growth experiments were carried out in two ultra high vacuum (UHV) cambers with sublimation sources of Si, Fe and Cr and quartz sensors of film thickness. Optical properties of the samples were studied in UHV chamber VARIAN (210 10Torr) equipped with differential reflectance spectroscopy (DRS) facilities. The samples surface was studied in the second UHV chamber (1 -10 9 Torr) equipped with LEED optics. Si(100) and Si(l 11) wafers were used as substrates for different series of the growth experiments. For the growth of silicide islands, metal films of 0.01-1.0 nm were deposited onto silicon surface. Silicon overgrowth with the deposition rate of 3-4 nm/min was carried out by molecular beam epitaxy (MBE) at 600-800 °C for different substrates. The samples were then analyzed in situ by LEED and ex situ by HRTEM and by... 

One of several innovations in this apparatus consists of a LEED optics and a cylindrical mirror analyzer (CMA) mounted on hydraulically driven bellows, thus allowing these instruments to move back and forth relative to the sample. The optimum performance of the CMA is achieved when the end of the instrument is at a distance of 0.4 in. from the sample. Since the radius of the high-pressure cell is greater than this distance, the CMA would obstruct the cell while it was moving towards the closed position, unless the CMA were withdrawn. The LEED... 

This agreement is typical for LEED optics using a fluorescent screen to display the symmetry of the diffraction pattern. 

With D 1 mm as the primary beam diameter of conventional LEED optics on the sample, we have D t. As a consequence, the diffraction pattern is an incoherent superposition of the contribution of each coherence area, which means that their intensities just add. This is of htde importance if the structure in each domain is the same but has implications if not In particular, adsorption systems can have symmetrically equivalent but rotationaUy different small domains, so that the total diffraction pattern is a superposition of different single domain patterns. 

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