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,d12, are compared. These are taken from geometrical structures which, except for very stable (O, S and Cs) layers, have been determined using a LEED data acquisition system based on a slow scan CCD camera, constructed to minimize the total dose necessary for the accumulation of full IV data without radiation damage. The reproducibility is such that a relative accuracy ofd12of better than 0.02 Å is likely. For the overall (center of mass) changes ofd12, we find that in most cases the sign of the change can be correlated with the character of the adsorbate: electronegative adsorbates tend to increased12, while electropositive or strongly polarizable adsorbates such as noble gases lead to a further contraction beyond that seen for the clean surface(-2.5%). The local changes ofd12are 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 the bonding of adsorbates. Some existing ideas and arguments directed at a conceptual understanding of the observed changes are discussed.