  Direct comparison of scanning tunneling microscopy and high resolution core
level photo-emission experiments provides a rationale for the mechanism of
formation of a two dimensional (2D) binary alloy (1/3 mono-layer (ML) Sn(1-
x)Six/Si(111)-\sqrt3X\sqrt3R30). In contrast with recent theoretical
predictions, the pure metal surface (x=0) results partitioned into two classes
(2/9 ML and 1/9 ML) of ad-atoms occupying non-equivalent T4 sites. During the
formation of the alloy, Si ad-atoms preferably occupy the majority type
adsorption site. This peculiar substitution mechanism leads to a mutual
arrangement of ad-atoms which is not random even at room temperature, but shows
the typical short range order universally observed in 2D and quasi 2D binary
alloys
