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The structural phases formed by atomic sulfur on Au(111) due to reaction with molecular S2 have been investigated by qualitative low-energy electron diffraction (LEED), scanning tunneling microscopy, and normal incidence X-ray standing wavefield absorption (NIXSW) combined with X-ray photoelectron spectroscopy (XPS). Three phases are identified with increasing coverage, namely, a newly identified (5 × 5) phase, a ( × )R30° phase, and a “complex” phase. The (5 × 5) phase, with a LEED pattern having the appearance of a “split-spot” ( × )R30° pattern, is interpreted in terms of local ( × )R30° ordering within a (5 × 5) ordered domain structure. The S atoms in the (5 × 5) phase occupy fcc hollow sites 1.56 Å above the outermost extended Au(111) bulk atomic scatterer plane. A specific model of the ordering in this phase is proposed that, together with the observed marginal stability of the true, long-range-ordered, ( × )R30° phase, indicates significant short-range S−S repulsion and probably compressive surface stress. The complex phase, that coexists in a poorly ordered state with the lower coverage atomic chemisorption phases, is interpreted in terms of an incommensurate long-range periodicity, but the NIXSW data shows clear evidence of local commensuration, with the S atoms mainly close to atop sites relative to the underlying Au(111) substrate; these data provide strong support for a previously proposed model based on a sulfide layer of stoichiometry AuS.

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The preparation and thermal stability of benzenethiol and benzeneselenol self-assembled monolayers (SAMs) grown on Au(111) have been investigated by electrochemical experiments and high-resolution photoemission spectroscopy. Both techniques confirm the formation of monolayers with high packing densities (θ = 0.27-0.29 ML) and good degrees of order in both cases. Despite many similarities between the two SAMs, the thermal desorption is distinctly different: whereas the benzenethiol SAM desorbs in a single steplike process, the desorption of the benzeneselenol SAM occurs with a much lower activation energy and involves the cleavage of some Se-C bonds and a change in molecular configuration from standing up to lying down. This behavior is explained by considering the different nature of the bonding of the headgroup with the metal surface and with the phenyl ring. Density functional theory calculations show that the breakage of the Se-C bond has a lower activation energy barrier than the breakage of the S-C bond.

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It is generally assumed that electrons in deep atomic core states are highly localized and do not participate in the bonding of molecules and solids. This implies well-defined core-level binding energies and the absence of any splitting and band-like dispersion, a fact that is exploited in several powerful experimental techniques, such as X-ray photoemission spectroscopy. Violations of this assumption have been found for only a few small molecules in the gas phase such as C 2 H 2 or N 2 with much stronger bonding and shorter bonding distances than present in solids 1,2 . Here we report the observation of a sizeable band-like dispersion of the C 1s core level in graphene, a single-layer honeycomb net of carbon atoms that is attracting considerable attention at present in the scientific community 3 . The dispersion is observed as an emission-angle-dependent binding-energy modulation and it is shown that under appropriate conditions only the bonding or antibonding states can be observed. A very similar dispersion is also found by ab initio calculations.The binding-energy modulations of the C 1s core state are illustrated in Fig. 1b-f. Each panel shows a group of spectra taken at a fixed polar emission angle θ as a function of azimuthal emission angle φ. Clear shifts of the peak position are observed. Figure 1g shows a comparison of the spectrum taken at normal emission and one taken at θ = 25 • together with the result of a peak fit to these two spectra.The binding-energy variation obtained from the peak fitting is given in Fig. 1h with the markers corresponding to the spectra in Fig. 1b-f. Strong changes are evident with the largest difference of binding energies spanning ≈60 meV. The variation is consistent with the point symmetry of the graphene lattice. Figure 1i shows the intensity variations of the peaks, shown as the modulation function. Strong intensity modulations are observed, caused by photoelectron diffraction in the final state. The variations also follow the point symmetry of the graphene lattice, but they do not seem to be correlated with the binding energy in Fig. 1h, with the main structures being in phase for some polar emission angles and out of phase for others.Although the data of Fig. 1 serve to illustrate the nature of the effect and the fitting procedure, they are insufficient to pin down the physical origin of the modulation. Figure 2 therefore shows a much more extensive data set measured over many polar and azimuthal angles and at different photon energies. Again, a good fit to all of the spectra in the data set was obtained for a single C 1s component using always the same line shape. Note that this excludes the existence of unresolved components, because their intensities would modulate differently, changing the shape of the peak. The left panel of the figure shows the resulting intensity modulation function and the right panel gives the binding-energy modulation. The intensity modulation function is compared to the simulation for a flat, free-standing layer of graphene. The agreement b...

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Thiourea (TU) adsorption on Ag(111) from aqueous solutions was investigated by in situ scanning tunneling microscopy operating under potential control. Hexagonal arrangements with d = 0.44, 0.38, and 0.33 nm were imaged at potentials close to −1.2 V (vs SCE), where conjugated voltammetric current peaks are observed. The analysis of in situ Fourier transform infrared reflection−absorption spectra (FT-IRRAS) shows that these current peaks mainly involve electroadsorption/electrodesorption of TU on the Ag(111) surface. Data from ex situ X-ray photolectron spectroscopy of TU-covered Ag(111) closely resemble those obtained for adsorbed alkanethiols on the same substrate, suggesting that the canonical form of TU is the species adsorbed from aqueous solutions. Experimental evidence of TU degradation into other sulfur species is also observed.