Surface structure determinations of ordered sulfur overlayers on Mo(100) and Re(0001) by low-energy electron diffraction intensity analysis

摘要

A newly developed method for surface structure determination, tensor LEED, combined with automated search was used to analyze the structures. The ordered structures of S on Mo(100) which were studied formed a c(2 (times) 2), c(4 (times) 2), and p(2 (times) l) periodicities at coverages of 0.5, 0.75, 1.0 ML (monolayers, of one sulfur atom per one molybdenum atom) respectively. A MO(sub 2)S-like overlayer, which formed at coverages greater than 1.0 ML, is also discussed. Calculations for the c(2 (times) 2) structure gave a best fit geometry with S adsorbed in a four-fold symmetric hollow site and the second layer buckled by 0.09(Angstrom). The S-Mo bond length is 2.45(Angstrom) and the Pendry R-factor is 0.21. Preliminary calculations for the c(4 (times) 2) structure did not yield an acceptable fit. The three models tried are discussed. Calculations for p(2 (times) l) data did not yield an acceptable geometry either. The types of models that were tried are discussed. Implications of this analysis are discussed along with results of a scanning tunneling microscopy (STM) investigation. The ordered structures on the RE(0001) surface studied have p(2 (times) 2) and (2(radical)3 (times) 2(radical)3)R30(degree) periodicities and occurred at S coverages of 0.25 and 0.5 ML respectively. Best fit structure for p(2 (times) 2) structure has S adsorbed in a three-fold hollow hcp site and exhibits a buckling of the first and second Re layers. The first layer is buckled by 0.05(Angstrom) and the second layer is buckled by 0.06(Angstrom). Re-S bond length is 2.32(Angstrom) and Pendry R-factor is 0.21. Preliminary results of dynamical LEED investigation of (2(radical)3 (times) 2(radical)3)R30(degree) structure show reasonable agreement with a model with a 6-S atom basis.