Atomic shadowing

Azimuthal scans obtained for tliree surface phases of Ni l 10] are shown in figure B 1.23.9 [38]. The minima observed for the clean and hydrogen-covered surfaces are due only to Ni atoms shadowing neighbouring Ni atoms, whereas for the oxygen-covered surface minima are observed due to both O and Ni atoms shadowing... 

Fig. 13. (a) An atomically resolved STM image (150 A x 150 A) of a surface prepared as in Fig. lie. Small (1x1) terminated islands and patches of connected pseudohexagonal rosettes are seen, (b) Atomic model (top and side view) for the oxygen-induced structure observed in (a). A bulk-terminated (1x1) island is shown on the right side and the unit cell is indicated. Small white balls are Ti atoms. Shadowed large balls represent oxygen atoms darker shading indicates higher z-positions. The rectangle indicates the unit cell of the (1x1) structure. The network patch ( R ) on the left side consists of an incomplete Ti02(l 10)(lxl) layer and contains only atoms at bulk position. The strands probably have a structure similar to the added Ti203 model in Fig. 1 Ic. From ref. [102].

Figure 22. The octahedral fragment of the perovskite crystal structure ABO, with the transition metal atom B at the center and six oxygen atoms (shadowed) at the apexes of the octahedron. The letters a,b,c,. .. denote the off-center positions of the atom B in the eight wells of the APES induced by the PJTE.

Figure Bl.23.2. (a) Shadow cone of a stationary Pt atom in a 4 keV Ne ion beam, appearing with the overlapping of ion trajectories as a fiinction of the impact parameter. The initial position of the target atom that recoils in the collision is indicated by a solid circle, (b) Plot of the nonnalized ion flux distribution density across the shadow cone in (a). The flux density changes from 0 inside the shadow cone, to much greater than l in the focusing region, converging to 1 away from the shadow cone edge, (c) Blocking cones...

When an ion beam is incident on an atomically flat surface at grazing angles, each surface atom is shadowed by its neighbouring atom such that only forwardscattering (FS) is possible these are large impact parameter (p) collisions. 

As a increases, a critical value " i-.iiiis reached each time the th layer of target atoms moves out of the shadow cone allowing for large-angle backscattering (BS) or small-/i collisions as shown in figure Bl.23.3. If the BS intensity 1, is monitored as a fimction of a, steep rises [36] witli well defined maxima are observed when the... 

Figure Bl.23.9. Scattering intensity of 4 keV Ne versus azimuthal angle 8 for a Ni 110] surface in the clean (1 X 1), (1 X 2)-H missing row, and (2 x l)-0 missing row phases. The hydrogen atoms are not shown. The oxygen atoms are shown as small open circles. 0-Ni and Ni-Ni denote the directions along which O and Ni atoms, respectively, shadow the Ni scattering centre.

Sometime around 1800 there came over the European mind a shadow of uncase about Newtonian science. Beauty, life, and mystery were all expiring in a desert of atoms and forces and soulless mechanisms—so the poets held, but not only they. Science also, many people felt, must seek higher themes, broader principles, deeper foundations—... 

Figure4.25. LEED pattern obtained from N atoms on Fe(l 11) at Ep = 42 eV. The surface is estimated to have a coverage of 0.96 Ml nitrogen and is reconstructed into a 5x5 overlayer structure. The dark shadow in the...

The Incident Ion beam Intensity can be measured, and there are several tabulations of cross-section calculations. ( ) Also, the analyzer parameters, T, D, and d6 can be determined. The three aspects of this equation, which are not well understood nor easily determined. Include the number of atoms of a particular kind, the Ion survival probability, and the shadowing or geometric term. The first quantity Is quite often that which you would like to determine. The second two are often difficult to separate. Shadowing can be particularly Important when trying to observe second layer effect or when trying to determine the location of adsorbates.( ) However, shadowing for polycrystalline samples, though Important, Is very difficult to deal with quantitatively. 

In scattering Intensity Is treated here. It would be wrong to Imply that other factors, such as shadowing and the target atom s electron configuration, do not play a role. Clearly, they do, but their effects do not alter our conclusions. Thus, It appears that LEISS may also be quite valuable as a qualitative probe for changes In surface morphology.(16)... 

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