Photoelektronenbeugungsuntersuchungen an Adsorbatsystemen

Abstract

The structure of atoms and molecules on surfaces can be determined by scannedenergy mode photoelectron diffraction. This is performed by measuring the angle resolved photoemission intensity of an adsorbate core level as a function of energy. This method has been applied to determine the local adsorption geometry of different adsorbates on nickel, copper and silicon surfaces. At low coverage (Θ = 0.15) C adsorbs on Ni{100} in the fourfold hollow site without causing a local reconstruction. At saturation coverage (Θ = 0.5) the (2 × 2)p4g reconstruction is induced. This is a c(2 × 2) superstructure of the C atoms in fourfold hollow sites in which the nearest-neighbour Ni atoms in the surface layer perform a clockwise or counter clockwise rotation, which does not change the C-Ni bond length significantly. There is, however, a significant increase in the Ni-Ni nearest-neighbour distance in the top layer. This implies that an adsorbate-induced Ni-Ni repulsion is the driving force for the (2× 2)p4g reconstruction. On Ni{110}, at low coverage (Θ = 0.2), ethene occupies a low-symmetry adsorption site near the half-bridge position but significantly shifted towards the atop position. The C-C axis is aligned along the 〈110〉 ridges and is a significantly tilt with respect to the surface plane by about 10◦. Neither island growth nor the adsorption of isolated molecules could be excluded by this study. For a saturation coverage of CO on Cu{210} an atop site and a (1× 1) structure was found. The C atoms bond to the Cu atoms with a bond length of 1.87±0.02 A and a polar angle, with respect to the surface normal, of 6 ± 5◦. The C-O bondlength is 1.16± 0.03 A and the molecular axis is tilted by 18± 6◦. 2-methyl-pyridine adsorbs on Cu{110} at a coverage of Θ = 0.4 with the N atom in a near-atop position. The N-Cu bond length is 2.04± 0.02 A with a tilt angle to the surface normal of 12 ± 4◦. The molecular plane is tilted by 4 ± 17◦ from the surface normal and is at an angle of about 32◦ to the 〈110〉 azimuth. The molecule is rotated in the molecular plane by about 16◦ such that the methyl group is moved away from the surface. Ethyne was found to adsorb on Si{100} with the C-C axis parallel to the Si-Si dimers and the C atoms near the atop position on the Si atoms. The C-C bond length is 1.36±0.19 A and the distance of the C to the Si atoms is 1.83±0.04 A. The value of the C-C-Si angle is 107± 9 ◦ leading to a distance of 2.44± 0.58 A between the Si dimer atoms. This shows clearly that the Si-Si dimer stays intact upon adsorption.