Low Energy Ion Scattering LEIS

in order to understand ion scattering with low-energy ions, we need to understand not only the scattering process but also the probabilities of neutralization and reionization. 

we discuss briefly the factors that determine the intensity of the scattered ions. During collision, a low-energy ion does not penetrate the target atom as deeply as in RBS. As a consequence, the ion feels the attenuated repulsion by the positive nucleus of the target atom, because the electrons screen it. In fact, in a head-on collision with Cu, a He+ ion would need to have about 100 keV energy to penetrate within the inner electron shell (the K or Is shell). An approximately correct potential for the interaction is the following modified Coulomb potential [1]  

A consequence of the screening of the nucleus by electrons is that the cross-section for low-energy ion scattering varies less steeply with the atomic number than in RBS, where the intensity depends on the atomic number squared. Another difference from RBS is that the scattering intensity is determined not only by the cross-section but also by the probability that an ion is neutralized. 

A second type of neutralization occurs through a resonant process, in which an electron from the sample tunnels to the empty state of the ion, which should then be at about the same energy. Resonance neutralization becomes likely if the electron affinity of the ion is somewhat larger than the work function of the 

The probability that neutralization takes place depends on the energy of the ion, simply because a slow ion spends a longer time in the vicinity of an atom. The maximum distances at which neutralization processes are thought to occur are on the order of 0.2 nm for Auger and 0.5 nm for resonance neutralization. 

The scattering cross-section is considerably different from the Rutherford cross-section, because the distance of closest approach, Ri i , is rather large at low energies. Thus, electronic screening of the interaction between the nuclei is important. The screened scattering potential V(r) reads  

When only two charge states (0, -i-l) are of relevance, there are two contributions to P  

Taking into account that neutralization means tunneling of a target conduction-band electron to the ion, the time integral can easily be replaced by integration over the distance from the surface, s, by use of the identity dt = ds/v , where Vj is the component of the ion velocity perpendicular to the surface. Prom this, the velocity-dependence of the survival probability, P , is obtained  

These theoretical predictions have been verified experimentally for numerous target materials (Fig. 3.53 [3.139]). Note that in Fig. 3.53 there is a pronounced difference between the neutralization of carbon atoms in a carbide and in graphite, respectively. This is one of the rare examples where matrix effects are observed. 

When the equipment used for RBS and LEIS is compared the following differences are apparent  

The LEIS technique owes its excellent surface sensitivity to the high neutralization probability of the rare gases. The fraction of He+ ions that survives a single collision without being neutralized is only between HT4 and 10 2. This implies automatically that the probability that a He+ ion will penetrate the surface, scatter off deeper atoms and return as an ion is practically zero. However, a finite probability exists that the backscattered neutral He atom will be ionized upon leaving the sample, and this is the reason that an LEIS spectrum still contains some information on the state of a sample below its surface. 

Charge exchange processes as discussed above are important for a good understanding of LEIS, and of SIMS as well. Unfortunately, the subject is still not yet completely understood, which forms an impediment to quantitative analysis by both techniques. Quantitative interpretation of LEIS spectra is nevertheless perfectly possible if one uses appropriate calibration standards. 

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In low energy ion scattering (LEIS, also called ion scattering spectroscopy, ISS) a beam of noble gas ions with energy of a few keV scatters elastically from a solid sur-... 

Another very important technique for fundamental consideration of multicomponent systems is low energy ion scattering (LEIS) [Taglauer and Heiland, 1980 Brongersma et al., 2007]. This is a unique tool in surface analysis, since it provides the ability to define the atomic composition of the topmost surface layer under UHV conditions. The signal does not interfere with the subsurface atomic layers, and therefore the results of LEIS analysis represent exclusively the response from the outer surface. In LEIS, a surface is used as a target that scatters a noble gas ion beam (He, Ne, ... 

Surface concentrations deduced from low-energy ion scattering (LEIS) spectra analysis showed that as erosion increases with time, the concentration of palladium increases whereas that of tin decreases. This result is in agreement with a pronounced surface enrichment by tin with respect to palladium according to the principle of the preparation method. 

Low energy ion scattering (LEIS) Composition Ions in, ions out... 

Techniques based on the interaction of ions with solids, such as secondary ion mass spectrometry (SIMS) and low-energy ion scattering (LEIS) have undoubtedly been accepted in catalyst characterization, but are by no means as widely applied as for example X-ray photoelectron spectroscopy (XPS) or X-ray diffraction (XRD). Nevertheless, SIMS, with its unsurpassed sensitivity for many elements, may yield unique information on whether or not elements on a surface are in contact with each other. LEIS is a surface technique with true outer layer sensitivity, and is highly useful for determining to what extent a support is covered by the catalytic material. Rutherford backscattering (RBS) is less suitable for studying catalysts, but is indispensable for determining concentrations in model systems, where the catalytically active material is present in monolayer (ML)-like quantities on the surface of a flat model support. 

Several studies have been reported in which model TiO thin films were used as supports, which clearly demonstrated the metal particle encapsulation [59, 60]. Linsmcicr ct al. [61] and Taglaucr and Knozingcr [62] have studied the behavior of Rh which was evaporated onto an clcctrochcmically produced TiO (anatase) film using low energy ion scattering (LEIS sec Section... 

Low-energy ion scattering LEIS gives information on the structure of the near-surface region. Low-energy ions, below 5 eV, are scattered from a surface and the ion Atomic structure... 

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