The atomic structures of clean and hydrogen-adsorbed Pd(001) are investigated by low energy electron diffraction (LEED) I/V analysis. Clean Pd(001) shows little surface relaxation in sharp contrast to previous reports. Adsorbing 1 monolayer of hydrogen on Pd(001), we observe sizable expansion of the interlayer spacing of the first two surface layers, d12 by 4.7% of the corresponding one of bulk Pd. Both experimental observations are in excellent agreement with the predictions of recent {\it ab initio} calculations. A series of experiments with varying coverages of hydrogen adsorbed on Pd(001), reveals that d12 monotonically increases with the increasing coverage. Such an observation strongly supports the contention that the previous observation of expanded d12 in clean Pd(001) results from contamination of the surface by residual hydrogen.
@article{arxiv.cond-mat/0411685,
title = {Effect of hydrogen on the atomic structure of Pd(001)},
author = {S. H. Kim and J. Barthel and H. L. Meyerheim and J. Kirschner and Jikeun Seo and J. -S. Kim},
journal= {arXiv preprint arXiv:cond-mat/0411685},
year = {2007}
}