Adsorption on active surfaces studied by the combined methods of STM and surface XAFS

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An ultrahigh vacuum chamber for surface analysis including a STM instrument was exploited. LEED and Auger electron spectra can be measured using a reverse-view four-grid LEED optics. X-ray photoelectron spectra are also obtainable employing a Mg and Al twin-anode X-ray tube and a concenric hemispherical electron energy analyzer. We have examined a performance of the system and successfully obtained good STM images of (5ROO<3>*2) -rect. S/Ni (111), the quality of which is comparable to the previously reported data. Next we have investigated surface structure of (ROO<17>*ROO<17>) R14ﾟS/Cu (100) by means of STM and S K-edge SEXAFS.The combined methods of STM and SEXAFS are quite useful for the determination of surface structure, since STM gives information on the locations of adsorbate atoms as well as a long-range structure, while SEXAFS clarifies relative locations between adsorbate and substrate, these information being fully complementary to each other. On (ROO<17>*ROO<17>) R14ﾟS/Cu (100) STM has revealed the lateral positions of two kinds of S atoms. Combining the results of SEXAFS, we have proposed a new surface structure model in which the surface Cu layr is significantly reconstructed. In the unit cell four S atoms locate on the rearranged hollow site of the surface Cu layr and other four embedded S atoms are on the normal fourfold hollow site of the second Cu layr.