Atomic beam diffraction, surface structure

Diffraction, by X-rays or neutrons, has been the standard method for determining the structures of crystals. The mean free path of X-rays and neutrons is very long, and thus is not sensitive to surfaces. To probe the structures of surfaces, the probing particles must have a very short mean free path in solids. Two methods are extensively used for determining surface structures low-energy electron diffraction (LEED) and atomic-beam diffraction. A helium... 

Atomic-beam diffraction was first demonstrated in 1930, as a verification of the concept of the de Broglie wave (Estermann and Stem, 1930). In the 1970s, it was developed into an extremely informative method for determining topography and atomic structure of solid surfaces (Steele, 1974 Goodman and Wachman, 1976). 

Fig. 4.12. Atomic-beam diffraction. A nearly monochromatic beam of helium, generated by a nozzle, falls on the solid surface with an angle of incidence. The diffracted beam is collected at an outgoing angle. The angular distribution of the diffracted helium beam contains the information about the topography and structure of the...

Lapujoulade, J., Salanon, B., and Gorse, D. (1984). Surface structure analysis by atomic beam diffraction. In The Structure of Surfaces, edited by Van Hove, M. A., and Tong, S. Y., Springer-Verlag, Berlin. 

Such waves can be used to probe the structure of crystal surfaces, through low-energy electron diffraction (LEED) or atomic beam diffraction. The latter is usually confined to He atoms, but even Ar atom diffraction can be discerned... 

RIE Rieder, K. H. Surface structural research with atom beam diffraction helium versus neon Surf. Rev. Lett. 1 (1994) 51. 

It is relatively straightforward to detemiine the size and shape of the three- or two-dimensional unit cell of a periodic bulk or surface structure, respectively. This infonnation follows from the exit directions of diffracted beams relative to an incident beam, for a given crystal orientation measuring those exit angles detennines the unit cell quite easily. But no relative positions of atoms within the unit cell can be obtained in this maimer. To achieve that, one must measure intensities of diffracted beams and then computationally analyse those intensities in tenns of atomic positions. 

Engel T and Rieder K H 1982 Structural studies of surfaces with atomic and molecular beam diffraction Structural Studies of Surfaces With Atomic and Molecular Beam Scattering (Springer Tracts in Modern Physics vol 91) (Berlin Springer) pp 55-180... 

The determination of the atomic structure of surfaces is the cornerstone of surface science. Before the invention of STM, various diffraction methods are applied, such as low-energy electron diffraction (LEED) and atom beam scattering see Chapter 4. However, those methods can only provide the Fourier-transformed information of the atomic structure averaged over a relatively large area. Often, after a surface structure is observed by diffraction methods, conflicting models were proposed by different authors. Sometimes, a consensus can be reached. In many cases, controversy remains. Besides, the diffraction method can only provide information about structures of relatively simple and perfectly periodic surfaces. Large and complex structures are out of the reach of diffraction methods. On real surfaces, aperiodic structures such as defects and local variations always exist. Before the invention of the STM, there was no way to determine those aperiodic structures. 

Determinations of the surface structure by computing the diffraction beam intensities from low energy electron diffraction are concentrated in two frontier areas at present. One is the determination of the surface structures of adsorbed molecules of ever bigger size and the other is the determination of the atomic locations in reconstructed clean solid surfaces. 

LEED studies have revealed that the atoms in this platinum surface are in the positions expected from the projection of the X-ray unit cell to the surface (5). The diffraction pattern that is exhibited (Fig. 4) clearly indicates a sixfold rotational symmetry that is expected in such a surface. Calculations of surface structure from LEED beam intensities indicate that atoms are in those positions in the surface layer (with respect to the second layer) as indicated by the X-ray unit cell within 5% of the interlayer distance (6,7). 

LEED studies of clean surfaces have revealed that most of these surfaces, if prepared under proper conditions, are ordered on an atomic scale and exhibit sharp diffraction beams and high diffraction beam intensities. Metal, semiconductor, alkali halide, inert gas, and organic crystal surfaces have been studied this way, and all exhibit ordered surface structures. 

One advantage of LEED is that the diffraction process filters out effects due to local defects or deviations from long-range order. The contribution of defects to I-V curves is proportional to the first power of the number of defects, while the contribution of the part of the surface with long-range order is proportional to the square of the number of atoms involved, so the LEED beam integrated intensity reflects the equilibrium geometry of the ordered surface structure. 

Several types of diffraction by crystals are now studied. Neutron diffraction can be used with great effectiveness to give information on molecular structure. These results complement those from X-ray diffraction studies, because there are different mechanisms for the scattering of X rays and of neutrons by the various atoms. X rays are scattered by electrons, while neutrons are scattered by atomic nuclei. Neutron diffraction is important for the determination of the locations of hydrogen atoms which, because of their low electron count, are poor X-ray scatterers. Electron diffraction, while requiring much smaller crystals and therefore being potentially useful for the study of macromolecules, produces diffraction patterns that are more complicated. Their interpretation is hampered by the fact that the diffracted electron beams are rediffracted within the crystal much more than are X-ray beams. This has limited the practical use of electron diffraction in the determination of atomic arrangements in crystals to studies of surface structure. 

Some intermetallic compounds having the diamond or zinc blende structures, such as GaSb and InSb show an altered surface structure (19) as indicated by the presence of fractional order diffraction beams after cleaning by the argon-ion bombardment and annealing technique. For crystals of this type there is asymmetry in opposite directions perpendicular to (111) planes. This asymmetry results in atoms of type A on one surface and those of type B on the opposite surface (20). The possibility of detecting effects of the asymmetry of clean surfaces by electron diffraction has been considered. However, in the cases of GaSb and InSb, no difference in the diffraction patterns from these two surfaces has been detected. 

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