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 substrates Mo(211) and W(211) tend to form chain structures (1×n). These chains are oriented normal to the furrows, and n can reach values up to 8. In order to clarify the nature of the coupling along the furrows, we carried out a quantitative LEED–IV study in the energy range between 50 and 180 eV. We investigated the adsorption geometry of the Sr(1×5) structure on Mo(211) including the relaxations in the substrate. These results are compared with those obtained recently for the Li(1×4) 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 [01̄1] direction are correlated with substantial lateral substrate relaxations by up to 0.2 Å along the Mo(211) [1̄11] furrow direction. For both adsorbates, the relaxations form wave-like density modulations in the substrate that seem to be caused by Friedel oscillations of the substrate conduction 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 surface states involved in the bond formation. Whereas for Sr, the Langmuir–Gurney (LG) model still provides a consistent picture of bond formation compared with the observed bond lengths and changes of work function, the predictions of the LG model are inconsistent with the experimental findings for Li.