Dresden 2006 – scientific programme
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O: Oberflächenphysik
O 24: Adsorption III
O 24.5: Talk
Tuesday, March 28, 2006, 16:45–17:00, TRE Phys
Origin of anomalous surface lattice expansion in Pd(001) — •Holger L. Meyerheim1, S. H. Kim1, J. H. Seo2, J.-S. Kim3, J. Barthel1, and J. Kirschner1 — 1MPI f. Mikrostrukturphysik, 06120 Halle — 2Dept. of Ophthalmic Optics, Chodang Univ. (Korea) — 3Dept. of Physics, Sook-Myung Women’s Univ. (Korea)
We present a systematic study of the hydrogen induced anomalous expansion of the top layer spacing, d$_{12}$, using low-energy electron diffraction and surface x-ray scattering. After exposure of 6 Langmuirs of hydrogen at 150~K sample temperature, the hydrogen atoms occupy the surface hollow sites and a lattice expansion of $∖Delta$$d_{12}$=+4.7∖% is determined in agreement with theoretical predictions (5.2∖%). Heating the sample above the hydrogen desorption temperature (T$_{D}$$∖approx$340~K), leads to an almost complete relaxation of d$_{12}$ to the bulk value of 1.945~∖AA. Similarly, no expansion is observed for clean Pd(001) prepared by rapid cooling after thermal treatment to remove hydrogen. Hydrogen re-adsorption from the residual gas atmosphere and possibly hydrogen agglomeration in the near surface region leads to an expansion of d$_{12}$ in the 2-3∖% range as observed in previous experiments. On the basis of our results, surface magnetism as a mechanism to trigger lattice expansion in Pd(001) needs not to be invoked.