DIRECTED SEARCH METHODS FOR SURFACE-STRUCTURE DETERMINATION BY LEED

Conventional surface crystallography by low-energy electron diffraction employs a trial-and-error search controlled at each step by a human being. This trial-and-error approach becomes very cumbersome and unreliable when it is applied to complex surfaces with a large number of unknown structural parameters. We present an automatic optimisation procedure for LEED which combines the complementary techniques of Tensor LEED and a numerical search algorithm. The significant advantages of using Tensor LEED as the basis of a search strategy are illustrated by its application as part of an automatic directed search to the determination of two surface structures: clean Pd(111) and Pd(111)( 3 × 3)R30°-CO. This approach can reduce the computer time required for an entire structure determination by several orders of magnitude. It also allows all structural parameters to be found simultaneously; in particular complex, non-symmetric structures and adsorbate induced substrate distortions can be readily determined. © 1990.