ʹMagicʹ island formation of CO coadsorbed with H on Ru(0 0 0 1)

The mutual interaction between carbon monoxide and hydrogen coadsorbed on Ru(0 0 0 1) has been studied by thermal desorption spectroscopy and thermal He-atom scattering. At 250 K, the hydrogen of a fully hydrogenated Ru surface is displaced by CO. During displacement CO segregates into islands already at very low coverages. At a low exposure pressure (1.5×10−9 mbar) small islands with a `magicʹ size of seven molecules are formed, whilst higher exposure pressures (2×10−8 mbar) lead to larger islands. This behavior is in contradiction with a mean field description of island growth introduced by Venables [Rep. Prog. Phys. 47 (1984) 399].