# Dresden 2006 – wissenschaftliches Programm

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# O: Oberflächenphysik

## O 46: Adsorption VI

### O 46.3: Vortrag

### Freitag, 31. März 2006, 11:45–12:00, TRE Phys

**H-Induced Symmetry Break of the Hexagonal Surface Reconstruction of Ir(100)** — •D. Lerch^{1}, A. Schmidt^{1}, A. Klein^{1}, L. Hammer^{1}, S. Müller^{1}, K. Heinz^{1}, H.-C. Poon^{2}, and D. Saldin^{2} — ^{1}Festkörperphysik, Universität Erlangen-Nürnberg, Staudtstr. 7, D-91058 Erlangen — ^{2}Univ. Wisconsin-Milwaukee

Upon H adsorption the (quasi-)hexagonal 5×1-reconstruction of clean Ir(100) is lifted. Yet, as this process is thermally activated the hexagonally close-packed surface Ir-layer persists as a metastable phase for sample temperatures below 200 K. We have investigated this 5×1-hex-H phase by the combined application of quantitative LEED and DFT. Using LEED spectra as a structural fingerprint we find that hydrogen considerably modifies the complex reconstruction pattern of the surface. Yet, though the clean surface structure could be reliably solved by quantitative LEED, the method fails to produce a satisfactory theory-experiment fit as long as all symmetry elements of the clean surface are enforced. Only by breaking the mirror symmetry of the 5×1 unit cell the reproduction of measured intensity data is possible. DFT calculations corroborate this symmetry break as well as the experimentally estimated H coverage of 0.6 ML. The calculations show that the symmetry break is due to only one of the three hydrogen atoms per unit cell which assumes an asymmetric hollow site near the centre of the troughs of the surface reconstruction. The resulting 33 (!) geometrical parameters describing the surface and subsurface Ir structure are in quantitative agreement with those determined by the LEED analysis. The H-induced structure can be interpreted as a precursor phase for the reconstruction lifting transition.