IMAGING GA TETRAMERS ON AG(001) BY SCANNING-TUNNELING-MICROSCOPY - THEORY AND EXPERIMENT

Very recently, we obtained well-resolved scanning tunneling microscopy (STM) images of what appear to be Ga tetramers on Ag(001). To clarify this finding, we apply two related semiempirical theoretical techniqu es [atom superposition electron delocalization (ASED) and electronic s cattering quantum chemistry (ESQC)], previously applied with success t o other adsorbate systems, to single Ga adatoms and Ga tetramers adsor bed on Ag(001). The dependence of STM images on the bonding site and o n various tip configurations is computed by the ESQC method. These sim ulations agree with the observed STM images confirming the tetramer mo del, but show no clearcut bonding-site dependence, and hence do not al low a determination of the Ga adsorption site by comparison with exper imental STM images alone. However, ASED calculations for a single Ga a datom yield an adsorption energy for the fourfold hollow site which is about 1 eV higher than for the bridge and top sites, indicating that the hollow site is also the preferred bonding site for Ga adatoms form ing a tetramer. [S0163-1829(98)05116-9].