c(2×2)Cl/Cu(001) adsorbate geometry and substrate-surface relaxation using low-temperature angle-resolved photoemission extended fine structure

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Abstract
A detailed structural study of the c(2×2)Cl/Cu(001) adsorbate system was made, using the angle-resolved photoemission extended-fine-structure (ARPEFS) technique at low temperature, which yields both more accurate surface structural information and near-surface structural information for deeper substrate layers. Electrons were detected along two emission directions, [001] and [011], and at two temperatures, 110 and 300 K. The Cl atoms were found to adsorb in the fourfold hollow site, 1.604(5) Å above the first copper layer, with a Cl-Cu bond length of 2.416(3) Å (in which the errors in parentheses are statistical standard deviations only). These values are in excellent agreement with a previous low-energy electron-diffraction study by Jona et al. The c(2×2)Cl-covered first copper layer showed no relaxation with respect to the bulk position. However, a small corrugation of the second copper layer was found: The second-layer copper atoms below the Cl atoms move 0.042(12) Å away from the surface, while those in open positions remain in their bulk positions. The distances from the Cl atoms to the third and fourth copper layers were found to be 5.222(25) and 7.023(22) Å, respectively, yielding a bulklike interlayer spacing. Thus the depth sensitivity of the low-temperature ARPEFS facilitated definitive referencing of near-surface atomic positions to the underlying lattice.