SHORT-RANGE-ORDER INVESTIGATION BY LOW-ENERGY ELECTRONS

During the last few years a number of experimental techniques have been developed with the general purpose of obtaining structural information from EXAFS‐like spectra without resorting to synchrotron radiation. The aim of this paper is to show the physical principles and the experimental results concerning two of these techniques, which basically use low‐kinetic‐energy electrons instead of x‐rays to excite the investigated system. EELFS (extended energy loss fine structure) and EXFAS (extended fine Auger structure) spectroscopies refer to the detection of the features that appear in the secondary electron distribution diffused inelastically from a solid surface. These features are located in energy a few hundreds of electron‐volt above a core edge ionization and above a few selected Auger transitions, respectively. The satisfactory agreement in the structural determination (position of the nearest‐neighbour distance, back‐scattering amplitude, total phase shift…) obtained by the EELFS technique allows us to conclude that the EXAFS formalism is a valid and correct method of analysis. Moreover, we present a complete calculation of the inelastic cross‐section actually measured in an EELFS spectrum, taking into account the various weights of the final‐state angular momenta. The main result of this computation ensures the predominance of the dipole channel over the other multipole contribution, even for very low primary beam energies. The interpretation of the EXFAS singal is closely related to that of the EELFS singal, both arising from an interference effect between the core excited and back‐scattered components of the final‐state wave function. Although the proposed EXAFS‐like origin awaits a more complete theoretical support, the EXFAS technique provides a very attractive extension of Auger spectroscopy to study also the local surface structure. Copyright © 1990 John Wiley & Sons Ltd.