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Zusammenfassung:
Chemisorption of CO on Ni(110) results at saturation in a well ordered (2 × 1) p2mg structure observed in LEED. The strong lateral interaction between the CO molecules leads to the formation of a two-dimensional band structure, which has been investigated in great detail by angle resolved photoelectron spectroscopy using monochromatized light from the storage ring BESSY at Berlin. Due to the low symmetry and the existence of two CO molecules in the unit cell eight valence bands have been observed, namely the 4σ+, 4σ-, 5σ+, 5σ-, 1πx+, 1πx-, 1πy+ and 1πy- bands. The experimentally determined band structure can be well reproduced by tight binding calculations of an unsupported CO overlayer of p2mg symmetry, if the interaction with the substrate is included so as to account for 4σ-5σ mixing upon chemisorption as well as for the 5σ-1π hybridization. Certain deviations between experiment and theory allow us to deduce information about the possible influence of indirect intermolecular interactions. The dispersion of the 1πx bands turned out to be very sensitive to the tilt angle of the CO molecules. An inclination of 17 ± 2° from the normal along the [001] direction alternating to both sides has been evaluated, consistent with a structure of p2mg symmetry rather than p1g1 symmetry.
