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Fig. 2. Filled (a,c) and empty (b,d) state images (25 Å ´ 25 Å) of the structures involved in the observed mechanism for epitaxial island formation. Figures 2(a) and 2(b) show the cross structure, and 2(c) and 2(d) show the tie structure. The ball and stick model for the tie structure is a proposed arrangement, based on the appearance of the structure.