Overlay model

Figure 2. Substrate/overlayer model and schematic of angles involved in XPS...

For an inhomogeneous sample the situation which is most often encountered is that shown in Figure 2, in which a modified surface layer of thickness d overlays an unmodified bulk. This is usually referred to as the substrate/overlayer model. The signal intensity for a core level in the surface layer is given by equation (2). 

Beyond similarity-based applications, machine learning techniques may pick the specific descriptor elements that appear to correlate with the observed activity trends throughout a training set. Unlike in overlay models, where there is an obvious link between pharmacophore spheres or fields in space and their source atoms, the actual pairs (triplets, etc) of atoms in molecules that incarnate the picked descriptor elements must be first established, to gain any potential insights into the binding mechanisms. 

Figure 17 Two possible structures for the c(2 x 2) Sn-Pt(lOO) surface, (a) Overlayer model with the Sn atoms located above the Pt(l 11) surface plane in threefold hollow sites, (b) Surface alloy model with the Sn atoms replacing every second Pt atom in the surface plane, (c) ALISS is ideally suited to distinguish between these two structures with high accuracy, as indicated by the shift in the critical angle for Sn-scattering upon alloying between 720-760 K. (From Ref. 73.)...

In an overlay model the student knowledge is viewed as a subset of the expert knowledge, and the learning process consists in an enlargement of such subset. Errors are considered as deviations from the correct knowledge. 

The student module, the whom part, stands for the accurate definition of the student is current knowledge state (what he knows - either correct or incorrect - and does not know). It can be a (static) data stmcture, representing a subset of the domain knowledge mastered by the learner (an overlay model), or it can be a set of (possibly wrong) procedures describing the student past behaviour (a buggy model). 

TABLE 13.3 Overall Relative Thickness of BOx and PEI Layers (1-3) Determined from Overlayer Model... 

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