X-ray standing wave studies of surface adsorption structures

Soft X-ray Photoelectron Spectroscopy of the S 2p levels at relatively high spectral resolution has been used to characterise the interaction of methanethiol, CH3SH, ethanethiol, C2H5SH and dimethyl disulphide, (CH3S)2 with a Cu(111) surface at temperatures from approximately 130 K to 500 K. The results are consistent with reports of the formation of a surface thiolate species at intermediate temperatures, but also provide clear evidence for two distinct surface intermediates in addition to the intact molecules and chemisorbed atomic sulphur reported previously for this surface. Prior structural studies of the Cu(111)/CH3S- surface at room temperature showed the surface to be reconstructed under these conditions, and the lower temperature species identified here is assigned to a surface thiolate species on an unreconstructed surface, reconstruction being hindered at low temperatures. Additional evidence is found for two different atomic sulphur states in a narrow temperature range. The interaction of methanethiol, CH3SH, with Cu(100) has been studied by S 1s photoemission, S K-edge near-edge x-ray absorption fine structure (NEXAFS) and normal-incidence standing x-ray wavefield absorption (NIXSW) at both (200) and (111) reflections. The results indicate that a single methanethiolate species, CH3S-, is formed and this is bonded to the unreconstructed surface via the S atoms which adopt the fourfold symmetric hollow site. This conclusion confirms the results of a recent near-edge and surface extended x-ray absorption fine structure study, but contrasts with the established adsorbate-induced reconstruction produced by this species on Cu(111). The asymmetry parameter Q arising from dipole-quadrupole interference in core-level photoemission from F 1s and P 1s states at photon energies 3430 eV and 2975 eV have been determined to be 0.31 ± 0.02 and 0.18 ± 0.02, respectively, via NIXSW experiments conducted on multilayer films of perfluorohexane, C6F14 and phosphorous tribromide, PBr3, grown on Cu(100). The structure of the PF3 molecule adsorbed on Cu(100) (dosed with the crystal kept at around 110 K and then annealed briefly to 180 K) has been studied by P K-edge NEXAFS and NIXSW at both (200) and (111) reflections using absorption at both the P and F atoms of the adsorbate molecule. Analysis of the F 1s data utilised the dipole-quadrupole interference correction parameter obtained from the multilayer calibration experiment. The results show that PF3 bonds to Cu(100) via the P atom and occupies an atop site with a Cu-P distance equal to 2.33 ± 0.04 A but with some evidence for a large amplitude of frustrated transnational vibration parallel to the surface. Although NEXAFS result indicates a perpendicular orientation (with an accuracy of ± 10 deg) of the C3ν axis of the molecules to the surface, the F NIXSW result is reconciled with a tilt of the molecule of 13 deg from the surface normal. The three F atoms appeared to deviate from the gas phase geometry by having an increased F-P-F angle. (author)