Geometrical evidence for long-range coupling in strongly anisotropic adsorbate systems: Sr and Li on Mo(211)

Alkali and earth alkali metals like Li and Sr adsorbed on the furrowed subs trates Mo(211) and W(211) tend to form chain structures (1 x n). These chai ns are oriented normal to the furrows, and n can reach values up to 8. In o rder to clarify the nature of the coupling along the furrows, we carried ou t a quantitative LEED-IV study in the energy range between 50 and 180 eV. W e investigated the adsorption geometry of the Sr(1 x 5) structure on Mo(211 ) including the relaxations in the substrate. These results are compared wi th those obtained recently for the Li(1 x 4) structure on the same surface. The long-range indirect interactions between both types of adsorbate atoms that are responsible for ordering of the chains in the [0 (1) over bar 1] direction are correlated with substantial lateral substrate relaxations by up to 0.2 A along the Mo(211) [(1) over bar 11] furrow direction. For both adsorbates, the relaxations form wave-like density modulations in the subst rate that seem to be caused by Friedel oscillations of the substrate conduc tion electrons. These screen the local adsorbate-induced distortion of the electron density. The variation of periodicity and the corresponding change of effective screening length between the two adsorbates may be due to sur face states involved in the bond formation. Whereas for Sr, the Langmuir-Gu rney (LG) model still provides a consistent picture of bond formation compa red with the observed bond lengths and changes of work function, the predic tions of the LG model are inconsistent with the experimental findings for L i. (C) 2000 Elsevier Science B.V. All rights reserved.