'Magic' island formation of CO coadsorbed with H on Ru(0001)

The mutual interaction between carbon monoxide and hydrogen coadsorbed on Ru(0001) 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 x 10(-9) mbar) small islands with a 'magic' size of seven molecules are formed, whilst higher exposure pressures (2 x 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]. (C) 2002 Elsevier Science B.V. All rights reserved.