Ions, scattering at surfaces

NIehus H, Helland W and Taglauer E 1993 Low-energy Ion scattering at surfaces Surf. Sc/. Rep. 17 213-304 An excellent review of ion scattering. 

Niehus H, Heiland W, and Taglauer E (1993) Low-energy ion scattering at surfaces. Surface Science Reports 17 213-303. 

NIE Niehus, H., Heiland, W., Taglauer, E. Low-energy ion scattering at surfaces Surf. Sci. 

This qualitative account serves to underline the complexity of the physical processes involved in forming the ion image, and the difficulty of arriving at a detailed interpretation. Neither the theory of atom scattering at surfaces, nor the description of field ionization are sufficiently advanced to allow a quantitative account of image intensities. However, such a quantitative evaluation is not really necessary in... 

It is clear that if Ei is measured and mo and Eo are known, the mass of the surface atom, /Mg, can be determined and the element identified. The method involves an indirect mass measurement. The number of ions scattered at a certain intensity is proportional to the amount of the element present. 

However, LEIS (ISS) offers one advantage an extreme surface sensitivity can be obtained with He ions because ions scattered at subsurface atoms will neutralize at the exit path. If only scattered ions are detected, the signal will therefore originate exclusively from the surface monolayer of atoms. This property is extremely useful in, for example, studies of surface segregation. 

Low energy noble gas ion scattering, a surface sensitive method, is generally incapable of probing surface hydrogen directly. However, if pulsed rare gas ion beams at grazing incident angle are used to bombard the surface. 

If a beam of monoenergetic ions of mass A/, is elastically scattered at an angle 6 by surface atoms of mass Mg, conservation of momentum and energy requires that... 

In many materials, the relaxations between the layers oscillate. For example, if the first-to-second layer spacing is reduced by a few percent, the second-to-third layer spacing would be increased, but by a smaller amount, as illustrated in figure Al,7,31b). These oscillatory relaxations have been measured with FEED [4, 5] and ion scattering [6, 7] to extend to at least the fifth atomic layer into the material. The oscillatory nature of the relaxations results from oscillations in the electron density perpendicular to the surface, which are called Eriedel oscillations [8]. The Eriedel oscillations arise from Eenni-Dirac statistics and impart oscillatory forces to the ion cores. 

The dynamics of ion surface scattering at energies exceeding several hundred electronvolts can be described by a series of binary collision approximations (BCAs) in which only the interaction of one energetic particle with a solid atom is considered at a time [25]. This model is reasonable because the interaction time for the collision is short compared witii the period of phonon frequencies in solids, and the interaction distance is shorter tlian the interatomic distances in solids. The BCA simplifies the many-body interactions between a projectile and solid atoms to a series of two-body collisions of the projectile and individual solid atoms. This can be described with results from the well known two-body central force problem [26]. 

Earlier studies of ion scattering were directed primarily at gas-ion interactions. As studies of ion—solid surfaces became common the energy of the scattered ions was eventually related mathematically to a simple binary elastic event involving a single atom on a surface element and a single probe ion. 

The main experimental techniques used to study the failure processes at the scale of a chain have involved the use of deuterated polymers, particularly copolymers, at the interface and the measurement of the amounts of the deuterated copolymers at each of the fracture surfaces. The presence and quantity of the deuterated copolymer has typically been measured using forward recoil ion scattering (FRES) or secondary ion mass spectroscopy (SIMS). The technique was originally used in a study of the effects of placing polystyrene-polymethyl methacrylate (PS-PMMA) block copolymers of total molecular weight of 200,000 Da at an interface between polyphenylene ether (PPE or PPO) and PMMA copolymers [1]. The PS block is miscible in the PPE. The use of copolymers where just the PS block was deuterated and copolymers where just the PMMA block was deuterated showed that, when the interface was fractured, the copolymer molecules all broke close to their junction points The basic idea of this technique is shown in Fig, I. 

The Newns-Anderson approximation successfully accounts for the main features of bonding when an adsorbate approaches the surface of a metal and its wave functions interact with those of the metal. It can also be used to describe features of the dynamics in the scattering of ions, atoms and molecules on surfaces. In particular the neutralization of ions at surfaces is well understood in this framework. The subject is beyond the scope of this book and the reader is referred to the literature [J.K. N0rskov, J. Vac. Sci. Technol. 18 (1981) 420],... 

In all of the studies described above, the CuaSi samples were prepared by ion bombardment at 330 K followed by cooling of the surface to 180 K before adsorbing the methyl radicals and chlorine. AES studies as well as ion scattering results in the literature [7, 15] show that this procedure produces a surface that is enriched in silicon compared with the Cu3Si bulk stoicWometry. We have found that surfaces with less Si enrichment (possibly even copper enriched relative to the bulk stoichiometry) can be prepared by ion bombardment at temperatures below 300 K. Specifically, Cu(60 eV)/Si(92 eV) Auger peak ratios of 1.2 - 1.7 compared with a ratio of 0.5 at 400 K can be obteiined by sputtering at 180 K. 

The group at Aarhus have reported carbon-induced structures at Ni(lll) and Ni(110) surfaces resulting from the dissociation of ethylene at high temperatures.27 Between 400 and 500 K, the Ni(l 10) surface is seen to form two carbidic structures with (4 x 3) and (4 x 5) domains present arising from surface reconstruction with substantial transport of nickel taking place. At higher temperatures (560 K), the surface becomes dominated by the (4 x 5) structure, which is well ordered and can be observed clearly by LEED. Ion scattering studies provide additional information which enables models to be constructed for both the (4 x 3) and (4 x 5) phases. 

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