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

Abstract

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 ⊥) 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 Δ Σ. 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.