Shadow cone

Chang C-C and Winograd N 1989 Shadow-cone-enhanced secondary-ion mass-spectrometry studies of Ag(110) Rhys. Rev. B 39 3467... 

B1.23.2.2 SHADOW CONES, BLOCKING CONES, AND STRUCTURAL ANALYSIS... 

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...

A) TIME OF FLIGHT SCATTERING AND RECOILING SPECTROMETRY (TOF-SARS)—SHADOW CONE BASED EXPERIMENT... 

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... 

An important concept is the shadow cone, which is a region where no ions can penetrate due to the ion—nucleus repulsion (see Figure 2). This effect makes ion scattering surface sensitive. The size of the shadow cone / jCan be calculated for the classical Coulomb potential as ... 

Figure 2 Schematic of the shadow cone formed by the interaction of a paraiiei beam of...

Visible edge Schlieren edge Inner shadow cone... 

The shadow cone is used by many experimenters because it is much simpler than the Schlieren techniques. Moreover, because the shadow is on the cooler side, it certainly gives more correct results than the visible cone. However, the flame cone can act as a lens in shadow measurements, causing uncertainties to arise with respect to the proper cone size. 

Figure 6a. Formation of a shadow cone, radius R, (one half shown only) at an atom in the second row for an aligned incident beam. (Reproduced, with permission, from Ref. 21. Copyright 1981, CRC Press.)...

Since the energy of the incident ion is relatively low, there is virtually no damage to the surface. In addition, because of the repulsive nature of the ion-atom interaction, a shadow cone is formed past the target surface atom and a blocking cone is generated at an adjacent surface atom these cones prevent interactions between the incident ion and subsurface which renders LEISS its unique sensitivity only to the outermost atoms. 

Figure 21 (a) Possible trajectories of an ion scattered by a target atom Mi. (b) Target atoms Ml in a surface layer. A second target atom M2, at the edge of the shadow cone due to Mi, scatters an increased flux to the detector. The target atoms in the second layer are inside the Ml shadow cones, in this case... 

MEIS, by contrast, is frequently used to achieve depth profiling information with close to monolayer resolution. The energy of the incident ion beam is in the order of 100 keV. At such energies, the shadow cone radius is relatively small and incident ions are able to channel hundreds of nanometres into the bulk of a crystalline lattice. It is possible, with careful sample alignment, to selectively illuminate a given number of surface layers (see below), in which case one may achieve layer by layer compositional information as a function of depth. 

There are several possible factors which may account for the difference between the fee (111) and (110) surfaces. Firstly, the presence of the surface layer relaxation means that, in the one layer incident geometry (Fig. 3), the shadow cone created by the ions incident on the surface atoms is sufficiently narrow at the second layer atoms that the latter become visible to the incident beam. If the surface layer relaxation is different for the (111) and (110) surfaces of CuPd, this may account for some or aU of the extra illumination. The second major factor is potentially the enhanced vibrations of surface atoms. The top layer atoms of the fee (110) surface are only 7 coordinate compared with 9 coordinate atoms on the (111) surface and 12 for atoms in the bulk. A consequence of this low coordination number is that the top layer atoms vibrate with a considerably larger amplitude than those of the bulk. As a result, even in... 

Baddeley et al. [93] also investigated the effect of the adsorbate on the number of visible atoms by measuring the total number of visible Cu and Pd atoms as a function of Cl coverage. They concluded that any effect was negligible and could be discounted in the calculations. This is another advantage of the MEIS approach compared to LEIS. The shadow cone radius varies as so the shadowing effect of H(a), C(a) and 0(a) are all relatively weak and even the effect of Cl(a) can be neglected since it is very unlikely that Cl will sit in a site which is a continuation of the bulk bimetallic lattice. 

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