SURFACE CRYSTALLOGRAPHY OF RE(0001)-(2X2)-S AND RE(0001)-(2-ROOT-3X2-ROOT) R30-DEGREES-6S - A COMBINED LEED AND STM STUDY

We analyze two structures formed by sulfur adsorbed on Re(0001) by using a combination of LEED and STM techniques. At low coverage (0.25 ML) sulfur atoms adsorb in three-fold hcp hollow sites forming a (2 x 2) structure. At higher coverages (0.5 ML) sulfur adsorbs close to three-fold hcp hollow sites and forms distorted hexamers arranged in a (2 square-root 3 x 2 square-root 3)R30-degrees pattern. A complete structural determination was performed by using automated Tensor LEED, aided by STM, showing substantial substrate relaxations that become more pronounced with increasing sulfur coverage. The sulfur height above the first rhenium layer in the case of the (2 x 2) structure agrees well with that determined by a quantitative theoretical analysis of the STM images.