Very-low-energy electron diffraction as a direct probe for unoccupied band structure: principles, results, implications in photoemission

Excited-state unoccupied band structure E k. above the vacuum level, which forms the final states in photoemission PE., may be directly accessed by Very-Low-Energy Electron Diffraction VLEED. at the energies below ;40 eV. Basically, VLEED reveals the critical points in the bands with dominant surface transmission. Recently developed methods of VLEED data analysis exploit this fact to determine the surface-parallel E k .directly, and surface-perpendicular E k . 5 H by a fitting technique. For many materials the unoccupied bands, contrary to common belief, prove to be not free-electron-like, and significantly influenced by self-energy corrections DS. As the VLEED experimental E k. contains all these effects, its use in PE band mapping in the valence band provides absolute, accurate and approximation-free, determination of the electronic structure. q1999 Elsevier Science B.V. All rights reserved.