Methyl on Cu(111) - structural determination including influence of co-adsorbed iodine

The adsorption geometry of the methyl species on Cu(111) with and without coadsorbed iodine has been determined using scanned energy mode photoelectron diffraction. Under all circumstances the three-fold-coordinated hollow sites are occupied. At saturation coverage of pure methyl species only the 'fcc' site, directly above a third layer Cu atom is occupied, whereas at half saturation coverage similar to70% of the methyl species occupy these fcc hollows and similar to30% occupy the 'hcp' sites above second layer Cu atoms. Best agreement between theory and experiment corresponded to a methyl group adsorbed with C-3v symmetry, but the possibility that the species was tilted on the surface could not be excluded. The height of the C above the surface in a pure methyl layer was 1.66 +/- 0.02 Angstrom, but was reduced to 1.62 +/- 0.02 Angstrom in the presence of co-adsorbed iodine, suggesting that iodine may increase the strength of adsorption. Iodine was also found to occupy the fcc hollow sites with a Cu-I bondlength of 2.61 +/- 0.02 k (C) 2002 Elsevier Science B.V. All rights reserved.