Structure and chemistry of Pd(100)-p(2 x 2)-p4g-Al surface alloy

Atomic structure and hydrogen adsorption on the Pd(100)-p(2 x 2)-p4g-Al sur face alloy have been studied using scanning tunneling microscopy and electr on energy loss spectroscopy. It was found that the topmost layer of this su rface alloy is reconstructed into the p(2 x 2)-p4g structure, which is in g ood agreement with the recent crystallographic results based on low-energy electron diffraction and ion scattering. The initial hydrogen adsorption on this surface alloy gives rise to an unusual desorption peak, which does no t shift with increasing coverage as reported previously. The vibrational sp ectrum for adsorbed hydrogen corresponding to this desorption state shows a peak at 77 meV, which is assigned to H at four-fold hollow sites on the cl ock-reconstructed p(2 x 2)-p4g-Pd layer. (C) 1999 Elsevier Science B.V. All rights reserved.