Very-low-energy electron diffraction as a direct probe for unoccupied bandstructure: 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 V ery-Low-Energy Electron Diffraction (VLEED) at the energies below similar t o 40 eV. Basically, VLEED reveals the critical points in the bands with dom inant surface transmission. Recently developed methods of VLEED data analys is exploit this fact to determine the surface-parallel E(k(parallel to)) di rectly, and surface-perpendicular E(k(perpendicular to)) by a fitting techn ique. For many materials the unoccupied bands, contrary to common belief, p rove to be not free-electron-like, and significantly influenced by self-ene rgy corrections Delta Sigma. As the VLEED experimental E(k) contains all th ese effects, its use in PE band mapping in the valence band provides absolu te, accurate and approximation-free, determination of the electronic struct ure. (C) 1999 Elsevier Science B.V. All rights reserved.