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O: Oberflächenphysik

O 19: Adsorption an Oberfl
ächen (III)

O 19.3: Vortrag

Dienstag, 24. März 1998, 16:45–17:00, H36

Molecular orbital orientation as contrast mechanism in Auger and photoelectron microscopy: benzoic acid on polycrystalline Ni — •M. Zharnikov, M. Neuber, and M. Grunze — Angewandte Physikalische Chemie, Universit"at Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg

Benzoic acid (BA) can be adsorbed on Ni(110) in predominately upright geometry with the molecular plane oriented nearly parallel to the [110] in-plane direction. Molecular orientation can be probed by resonant C1s → π* excitation, which is only allowed if the electric field vector E^→ has a non-vanishing component parallel to the π* molecular orbital, the latter being directed perpendicular to the BA molecular plane. The pronounced azimuthal alignment of BA molecules on Ni(110) distinguishes this particular surface from other low-index surfaces of nickel. Following these considerations, the Auger decay signal after C1s → π* excitation was used to image azimuthally aligned BA molecules on the (110) surfaces of Ni polycrystal covered by one monolayer of BA. Whereas BA on other crystallites provide azimuthally isotropic Auger decay signal intensity, BA on the microcrystallites with (110) orientations of the surface give an anisotropic signal with the intensity depending on the relative orientation of the E^→-vector and the π* BA orbitals. The orientation contrast was found to disappear between 190^∘C and 220^∘C, which correlates with a strong decrease of the C1s → π* resonance in the NEXAFS spectra integrating over the surface of the polycrystal.