The opening of Plutarch's Life of Themistokles

Plutarch begins the biography by selecting incidents and an epigram that anticipate the basic themes of Themistocles' life--his greatness, sometimes ambiguous, and his benefactions to all of Greece.

Oxidation of polycrystalline Ni studied by spectromicroscopy: Phase separation in the early stages of crystallite growth

Low-energy and photoemission electron microscopy enables the determination of facet planes of polycrystalline surfaces and the study of their chemical composition at the sub-m scale. Using these techniques the early oxidation stages of nickel were studied. After exposing the surface to 20 L of oxygen at 373 K a uniform layer of chemisorbed oxygen was found on all facets. After oxygen exposure at 473–673 K, small NiO crystallites are formed on all facets but not in the vicinity of all grain boundaries. The crystallites are separated by areas of bare Ni without significant oxygen coverage.

Experimental structure determination of the chemisorbed overlayers of chlorine and iodine on Au{111}

We have performed an experimental structure determination of the ordered p(sqrt[3] x sqrt[3])R30 degrees structures of chlorine and iodine on Au{111} using low-energy electron diffraction (LEED). Despite great similarities in the structure of the underlying substrate, which shows only minor deviations from the bulk positions in both cases, chlorine and iodine are found to adsorb in different adsorption sites, fcc and hcp hollow sites, respectively. The experimental Au-Cl and Au-I bond lengths of 2.56 and 2.84 A are close to the sums of the covalent radii, supporting the view that the bond is essentially covalent in nature; however, they are significantly shorter than predicted theoretically.

Alignment of a model amyloid peptide fragment in bulk and at a solid surface

The alignment of model amyloid peptide YYKLVFFC is investigated in bulk and at a solid surface using a range of spectroscopic methods employing polarized radiation. The peptide is based on a core sequence of the amyloid beta (A beta) peptide, KLVFF. The attached tyrosine and cysteine units are exploited to yield information on alignment and possible formation of disulfide or dityrosine links. Polarized Raman spectroscopy on aligned stalks provides information on tyrosine orientation, which complements data from linear dichroism (LD) on aqueous solutions subjected to shear in a Couette cell. LD provides a detailed picture of alignment of peptide strands and aromatic residues and was also used to probe the kinetics of self-assembly. This suggests initial association of phenylalanine residues, followed by subsequent registry of strands and orientation of tyrosine residues. X-ray diffraction (XRD) data from aligned stalks is used to extract orientational order parameters from the 0.48 nm reflection in the cross-beta pattern, from which an orientational distribution function is obtained. X-ray diffraction on solutions subject to capillary flow confirmed orientation in situ at the level of the cross-beta pattern. The information on fibril and tyrosine orientation from polarized Raman spectroscopy is compared with results from NEXAFS experiments on samples prepared as films on silicon. This indicates fibrils are aligned parallel to the surface, with phenyl ring normals perpendicular to the surface. Possible disulfide bridging leading to peptide dimer formation was excluded by Raman spectroscopy, whereas dityrosine formation was probed by fluorescence experiments and was found not to occur except under alkaline conditions. Congo red binding was found not to influence the cross-beta XRD pattern.

Hydrogen bond-induced pair formation of glycine on the chiral Cu{531} surface

Enantio-specific interactions on intrinsically chiral or chirally modified surfaces can be identified experimentally via comparison of the adsorption geometries of similar nonchiral and chiral molecules. Information about the effects of substrate-related and in interactions on the adsorption geometry of glycine, the only natural nonchiral amino acid, is therefore important for identifying enantio-specific interactions of larger chiral amino acids. We have studied the long- and short-range adsorption geometry and bonding properties of glycine on the intrinsically chiral Cu{531} surface with low-energy electron diffraction, near-edge X-ray absorption One structure spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption. For coverages between 0.15 and 0.33 ML (saturated chemisorbed layer) and temperatures between 300 and 430 K, glycine molecules adsorb in two different azimuthal orientations, which are associated with adsorption sites on the {110} and {311} microfacets of Cu{531}. Both types of adsorption sites allow a triangular footprint with surface bonds through the two oxygen atoms and the nitrogen atom. The occupation of the two adsorption sites is equal for all coverages, which can be explained by pair formation due to similar site-specific adsorption energies and the possibility of forming hydrogen bonds between molecules on adjacent {110} and {311} sites. This is not the ease for alanine and points toward higher site specificity in the case of alanine, which is eventually responsible for the enantiomeric differences observed for the alanine system.

Global and local expression of chirality in serine on the Cu{110} surface

Establishing a molecular-level understanding of enantioselectivity and chiral resolution at the organic−inorganic interfaces is a key challenge in the field of heterogeneous catalysis. As a model system, we investigate the adsorption geometry of serine on Cu{110} using a combination of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The chirality of enantiopure chemisorbed layers, where serine is in its deprotonated (anionic) state, is expressed at three levels: (i) the molecules form dimers whose orientation with respect to the substrate depends on the molecular chirality, (ii) dimers of l- and d-enantiomers aggregate into superstructures with chiral (−1 2; 4 0) lattices, respectively, which are mirror images of each other, and (iii) small islands have elongated shapes with the dominant direction depending on the chirality of the molecules. Dimer and superlattice formation can be explained in terms of intra- and interdimer bonds involving carboxylate, amino, and β−OH groups. The stability of the layers increases with the size of ordered islands. In racemic mixtures, we observe chiral resolution into small ordered enantiopure islands, which appears to be driven by the formation of homochiral dimer subunits and the directionality of interdimer hydrogen bonds. These islands show the same enantiospecific elongated shapes those as in low-coverage enantiopure layers.

Enantiospecific adsorption of alanine on the chiral Cu{531} surface

We have studied enantiospecific differences in the adsorption of (S)- and (R)-alanine on Cu{531}R using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. At saturation coverage, alanine adsorbs as alaninate forming a p(1 4) superstructure. LEED shows a significantly higher degree of long-range order for the S than for the R enantiomer. Also carbon K-edge NEXAFS spectra show differences between (S)- and (R)-alanine in the variations of the ð resonance when the linear polarization vector is rotated within the surface plane. This indicates differences in the local adsorption geometries of the molecules, most likely caused by the interaction between the methyl group and the metal surface and/or intermolecular hydrogen bonds. Comparison with model calculations and additional information from LEED and photoelectron spectroscopy suggest that both enantiomers of alaninate adsorb in two different orientations associated with triangular adsorption sites on {110} and {311} microfacets of the Cu{531} surface. The experimental data are ambiguous as to the exact difference between the local geometries of the two enantiomers. In one of two models that fit the data equally well, significantly more (R)-alaninate molecules are adsorbed on {110} sites than on {311} sites whereas for (S)-alaninate the numbers are equal. The enantiospecific differences found in these experiments are much more pronounced than those reported from other ultrahigh vacuum techniques applied to similar systems.

The interplay between geometry, electronic structure, and reactivity of Cu-Ni bimetallic (111) surfaces

The mutual influence of surface geometry (e.g. lattice parameters, morphology) and electronic structure is discussed for Cu-Ni bimetallic (111) surfaces. It is found that on flat surfaces the electronic d-states of the adlayer experience very little influence from the substrate electronic structure which is due to their large separation in binding energies and the close match of Cu and Ni lattice constants. Using carbon monoxide and benzene as probe molecules, it is found that in most cases the reactivity of Cu or Ni adlayers is very similar to the corresponding (111) single crystal surfaces. Exceptions are the adsorption of CO on submonolayers of Cu on Ni(111) and the dissociation of benzene on Ni/Cu(111) which is very different from Ni(111). These differences are related to geometric factors influencing the adsorption on these surfaces.

A refined LEED analysis of water on Ru{0001}: an experimental test of the partial dissociation model

Despite a number of earlier studies which seemed to confirm molecular adsorption of water on close-packed surfaces of late transition metals, new controversy has arisen over a recent theoretical work by Feibelman, according to which partial dissociation occurs on the Ru{0001} surface leading to a mixed (H2O + OH + H) superstructure. Here, we present a refined LEED-IV analysis of the (root3 x root3)R30degrees-D2O-Ru{0001} structure, testing explicitly this new model by Feibelman. Our results favour the model proposed earlier by Held and Menzel assuming intact water molecules with almost coplanar oxygen atoms and out-of-plane hydrogen atoms atop the slightly higher oxygen atoms. The partially dissociated model with an almost identical arrangement of oxygen atoms can, however, not unambiguously be excluded, especially when the single hydrogen atoms are not present in the surface unit cell. In contrast to the earlier LEED-IV analysis, we can, however, clearly exclude a buckled geometry of oxygen atoms.

Low-temperature partial dissociation of water on Cu(110)

The low-temperature reactivity of water (D2O) adsorbed on clean and oxygen pre-covered Cu(1 1 0) was studied using high resolution X-ray photoelectron spectroscopy (HRXPS) and low energy electron diffraction (LEED). On the clean surface partial dissociation to hydroxyl was observed already at 95 K. Upon annealing to 220 K hydrogen bonded water-hydroxyl chains are formed. Upon further annealing water desorbs leaving behind a layer of hydroxyl, most of which desorbs recombinatively eventually. With pre-adsorbed oxygen water reacts to hydroxyl lifting the added-row reconstruction even below 225 K. Upon annealing this adsorbate layer passes through essentially the same stages as without pre-adsorbed oxygen.

Adsorption and dissociation of benzene on bimetallic surfaces - the influence of surface geometry and electronic structure

This topical review discusses the influence of the surface geometry (e.g. lattice parameters and termination) and electronic structure of well-defined bimetallic surfaces on the adsorption and dissociation of benzene. The available data can be divided into two categories with combinations of non-transition metals and transition metals on the one side and combinations of two transition metals on the other. The main effect of non-transition metals in surface alloys is site blocking which can suppress chemisorption and dissociation of the molecules completely. When two transition metals are combined, the effects are less dramatic. They mainly affect the strength of the chemisorption bond and the degree of dissociation due to electronic and template effects.

The surface geometry of carbon monoxide and oxygen co-adsorbed on Ni{111}

Low energy electron diffraction (LEED) structure determinations have been performed for the p(2 x 2) structures of pure oxygen and oxygen co-adsorbed with CO on Ni{111}. Optimisation of the non-geometric parameters led to very good agreement between experimental and theoretical IV-curves and hence to a high accuracy in the structural parameters. In agreement with earlier work atomic oxygen is found to adsorb on fee sites in both structures. In the co-adsorbed phase CO occupies atop sites. The positions of the substrate atoms are almost identical, within 0.02 Angstrom, in both structures, implying that the interaction with oxygen dominates the arrangement of Ni atoms at the surface.