AUTOMATED-DETERMINATION OF COMPLEX SURFACE-STRUCTURES BY LEED

Conventional surface crystallography by low-energy electron diffractio n (LEED) employs a trial-and-error search controlled at each step by h uman effort. This trial-and-error approach becomes very cumbersome and unreliable to solve complex surfaces with a large number of unknown s tructural parameters. We discuss automatic optimization procedures for LEED, which combine numerical search algorithms with efficient method s of determining the diffracted intensities for varying structures. Su ch approaches can reduce the computer time required for an entire stru cture determination by many orders of magnitude, while fitting many ti mes more unknown structural parameters. Thereby, relatively complex st ructures, with typically 10 adjustable atoms (or 30 adjustable coordin ates), can be readily determined on today's workstations. These includ e non-symmetrically relaxed structures, surface reconstructions and ad sorbate-induced substrate distortions. We also address the theoretical and experimental requirements for an accurate structural determinatio n.