ADSORBATE-INDUCED GLOBAL AND LOCAL EXPANSIONS AND CONTRACTIONS OF A CLOSE-PACKED TRANSITION-METAL SURFACE

For over 20 adsorbate and coadsorbate systems on Ru(0001), the changes of the distance between the first and the second metal layer, d(12), are compared. These are taken from geometrical structures which, excep t for very stable (O, S and Cs) layers, have been determined using a L EED data acquisition system based on a slow scan CCD camera, construct ed to minimize the total dose necessary for the accumulation of full I V data without radiation damage. The reproducibility is such that a re lative accuracy of d(12) of better than 0.02 Angstrom is likely. For t he overall (center of mass) changes of d(12), we find that in most cas es the sign of the change can be correlated with the character of the adsorbate: electronegative adsorbates tend to increase d(12), while el ectropositive or strongly polarizable adsorbates such as noble gases l ead to a further contraction beyond that seen for the clean surface (- 2.5%). The local changes of d(12) are very complex and corroborate the view that even the close-packed surface of a highly refractory metal responds very flexibly to the local electron rearrangement caused by t he bonding of adsorbates. Some existing ideas and arguments directed a t a conceptual understanding of the observed changes are discussed.