Digital LEED

In this method, the electrode snrface is snbjected to a beam of low-energy (50 to 500 eV) electrons, and the elastically back-scattered electrons are collected onto a phosphor screen. The appearance of distinct diffraction spots (LEED patterns) on the screen indicates an ordered near-surface region. For known monatomic and small-molecule adsorbates, the adlayer stractural symmetry may be deduced readily from the LEED pattern, especially when information on the surface coverage is available from other experiments. For complex molecules, extraction of the substrate-adsorbate interfacial stracture from digitized LEED data is a nontrivial computational task. °... 

LEED patterns of pyridine adsorbed at Pt(lll) are shown in Figure 7-A. Best clarity of the pattern occurs when the PYR concentration is ImM. Measurement of the lengths and directions of LEED vectors in Figure 7-A, followed by conversion of the LEED vectors to real space by means of standard formulas, reveals that the structure is approximately (3.3x4.7) with an included angle of 77°. Digital simulation of the LEED pattern demonstrates that the best fit between theory and experiment occurs when the model structure is Pt(lll)(3.324x4.738, 77.1°)R34.0°-PYR. In matrix notation this is ... 

Fig. 18. Scheme of LEED experiment with a fluorescent screen, three-grid system, Faraday cup, and Vidicon system. The diffraction spots are observed visually directly or via the camera and a monitor. The beam intensities can be measured using the Faraday cup or a digitized Vidicon system [1]. Reprinted with permission firom M. Schleberger, S. Speller, and W. Heiland, Exp. Meth. in the Phys. Sci. 30,241 (1998), 1998, Academic Press. 

The results presented were calculated on the Amdahl 5860 digital computer at the University of Leeds. There are two parameters in the problem, the dimensionless fluid flow rate Q and the dimensionless plate speed U. Normally the normalised plate velocity (n.p.v.) was chosen to be 1.0 and the normalised fluid flow rate was varied. The exception to this was that when the pressure on the moving plate was calculated an n.p.v. of 0.0 was desired, as well as an n.p.v, of 1.0, for a fixed flow rate. 

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