Adsorption structures of hydrogen on transition metal surfaces as detected by LEED

The capability of low energy electron diffraction to retrieve the adsorption structure of hydrogen on crystalline metallic surfaces is reviewed. We show that in most cases hydrogen atoms reside in highly coordinated adsorption sites according to a hard sphere radius of 0.5-0.6 Angstrom and induce a displacive local substrate reconstruction which dies away with increasing distance from the adsorbate. At full coverage the local displacements add up to modify the clean surface's multilayer relaxation towards a bulk like terminated surface. In some cases heavy reconstructions are induced with bond breaking and removal of surface atoms. Also, hydrogen can influence the energy balance in displacively reconstructed surfaces, so that the clean surface reconstruction is lifted or switched to another type.