Electron scattering by atomic chains: Multiple-scattering effects

Multiple-scattering effects of electrons traveling along atomic chains are shown to be important and useful for surface structure determination, especially in the medium-energy range (200--1000 eV). This applies to techniques based on diffraction of elastic electrons, such as medium-energy electron diffraction, as well as to techniques based on the angle-resolved detection of ``secondary'' electrons such as photoelectrons, Auger electrons, and other electrons. Two new methods for computing multiple-scattering amplitudes for chains are derived and used to show how multiple scattering along chains of atoms produces focusing, defocusing, amplification, and layer-dependent enhancements that can be put to effective use in structure determination. Strong forward-focusing peaks along internuclear axes are dominant under many conditions: this is a single-scattering effect. However, multiple scattering can totally defocus such forward-focused peaks, while other interference effects produce additional scattering peaks that do not have the direction of an internuclear axis. In addition, it follows that surface composition analysis by conventional Auger-electron spectroscopy can be quite sensitive to the incident direction of the electron beam used to produce Auger emission.