Molecular orientation of CO adsorbed on clean and Al-modified Ru(0 0 0 1) surfaces

Chemisorption of CO on the bare and Al precovered Ru(0 0 0 1) surface as well as on the Al₂Ru surface alloy, was studied by means of angle-resolved photoelectron spectroscopy using unpolarised He II radiation. Whereas CO molecules stand upright on the clean Ru(0 0 0 1) surface, as deduced by applying the symmetry selection rules of photoemission, they adsorb in a tilted geometry on the Al-precovered Ru(0 0 0 1) surface. On an Al₂Ru surface alloy, produced by annealing the Al-covered Ru(0 0 0 1) surface up to 1100 K, the CO molecules are again found to stand upright, as on the clean Ru(0 0 0 1) surface. The Al atoms, incorporated into the topmost Ru layer, do not have a significant influence on the orientation of the axis of the CO molecules.     

Specular reflectance studies of bromide adsorption on gold

Specular reflectance changes have been used to examine the specific adsorption of bromide on gold in the presence of a large excess of supporting electrolyte (NaF) which is not specifically adsorbed. A linear relation has been demonstrated between the reflectance changes and the surface excess of bromide through the examination of the time dependence of the reflectance under conditions where the rate of adsorption of the bromide is diffusion controlled and hence known. The adsorption isotherms have been found to follow Temkin behavior. The electrosorption valency has been evaluated from the charge and surface excess at constant potential and found to be ?0.49 to ?0.59, depending on the potential. Various mechanisms for the subtantial changes in reflectance attending the specific adsorption of anions are discussed. The observed effects cannot be explained on the basis of changes in the charge on the electrode and corresponding changes in the contribution of the conduction band to the surface optical properties. The principal mechanism is proposed to be modifications in the surface electronic states of the metal electrode through direct orbital interactions between the adsorbed anions and the metal.     

Si(1 1 1)-(6 × 6)Au surface morphology and nucleation of Pb at low temperature

The morphologies of the Si(1 1 1)-(6 × 6)Au surface and the nucleation of Pb have been investigated at temperatures below room temperature using scanning tunnelling microscopy (STM). The STM scans of the clean Si(1 1 1)-(6 × 6)Au surface revealed exceptionally rich and strongly bias-dependent details within the unit cell. After deposition of about 0.01 ML of Pb(1 1 1), at temperatures above 180 K, the single Pb atoms formed a two-dimensional mesh with (6 × 6)-Au periodicity. In this early stage of deposition, the Pb atoms occupied predominantly one specific site of the (6 × 6) unit cell. The local symmetry of this site determined the further growth of the Pb layer. As a consequence, the structure of the Pb layer on Si(1 1 1)-(6 × 6)Au is rotated of 30° relative to that of the Pb layer on Si(1 1 1)-(7 × 7).     

Vibrational spectroscopy for glycine adsorbed on silicon clusters: Harmonic and anharmonic calculations for models of the Si(1 0 0)-2 × 1 surface

The vibrational spectroscopy of a glycine molecule adsorbed on a silicon surface is studied computationally, using different clusters as models for the surface. Harmonic frequencies are computed using density functional theory (DFT) with the B3LYP functional. Anharmonic frequency calculations are carried out using vibrational self-consistent field (VSCF) algorithms on an improved PM3 potential energy surface. The results are compared with experiments on Glycine@Si(1 0 0)-2 × 1.

The main findings are: (1) Agreement of the computed frequencies with experiment improves with cluster size. (2) The anharmonic calculations are generally in better agreement with experiment than the harmonic ones. The improvements due to anharmonicity are most significant for hydrogenic stretching. (3) An important part of the anharmonic effects is due to anharmonic coupling between different normal modes of the system. (4) The anharmonic coupling between glycine vibrational modes is much larger than the anharmonic coupling between glycine and “phonon” (cluster) modes.

Implications of the results for surface vibrational spectroscopy are discussed.     

Surface electronic structure of Pt(1 1 0): comparison with Ni and Pd

The unoccupied electronic structure of Pt(1 1 0) was investigated by inverse photoemission. The results were compared with the data for Ni(1 1 0) and Pd(1 1 0) with particular emphasis on surface states. The surface states in the gap of Pt(1 1 0) are shifted upwards relative to Ni and Pd, as a consequence of the (1 × 2) missing-row reconstruction. In contrast, the surface state at is only weakly affected, which indicates that it is essentially a one-dimensional state, localized on the densely-packed atomic chains on the Pt(1 1 0) surface.     

The lipid-mediated hypothesis of fumonisin B1 toxicodynamics tested in model membranes

The disruption of lipidic metabolism was considered a good candidate to explain FB1 toxicity mechanism. In the present work we investigated molecular organizational changes induced by FB1–biomembrane interaction possibly involved in mycotoxic effects.

FB1 was self-aggregated with a critical micellar concentration of 1.97 mM. FB1 (0–81.4 μM), decreased in a dose-dependent manner, the fluorescence anisotropy of TMA-DPH (from 0.349 ± 0.003 to 0.1720 ± 0.0035) in dpPC bilayers, whilst no differences were registered with DPH. At 5.6 μM in the subphase, FB1 increased the lateral surface pressure (π) of a Langmuir film to an extent that depended on the monolayer composition (Δπ_{dpPC:DOTAP 3:1} > Δπ_dpPC:dpPA3:1 > Δπ_dpPC), the molecular packing (Δπ decreased linearly as a function of the initial π) and the subphase pH (Δπ_{pH 2.6} > Δπ_{pH 7.4} and maximal π allowing the drug penetration π_cut-off was 34.3 and 27.7 mN/m at pH 2.63 and 7.4, respectively). FB1 increased the surface potential of dpPC and dpPC:DOTAP monolayers and decreased that of dpPC:dpPA. This suggested that FB1 acquired different orientations and/or foldings depending on the surface electrostatics and the toxin charge state. Moreover, FB1–lipid interactions were transduced into long-range effects at the mesoscopic level affecting the lipidic self-separated lateral domains shape and density.     

Self-assembled parallel mesoscopic Pb-wires on Au-modified Si(5 3 3) substrates

Arrays of extremely long and perfectly parallel mesoscopic Pb-wires are formed and studied in UHV conditions. Au-modified Si(5 3 3) substrate modified by deposition of sub-monolayer amount of Au are used as templates. A uniform distribution of monoatomic steps and terraces on well-oriented Si(5 3 3) is induced by formation of Au-chains running along step edges. Real-time surface imaging with LEEM shows that the wires growing on substrates held at temperatures close to the room temperature are all aligned parallel to azimuth, along the step edges. After nucleation of elongated islands, the 1 ML thick wetting layer remains on the vicinal Si surface. RHEED and low-temperature STM experiments show that the wires have triangular cross-section, limited by (1 1 1) and (1 0 0) facets of Pb. The width of the wires is 60 nm, whereas their length is up to 8 μm. The observed growth anisotropy leading to the formation of mesoscopic wires is attributed to enhanced one-dimensional diffusion along the parallel grooves and trenches that form vicinal surfaces. An additional factor, contributing to the anisotropic growth, is probably the anisotropic strain, due to the large misfit between Pb and Si lattices.     

Electronic band structure and optical properties of titanium oxyphosphates Li0.50Co0.25TiO(PO4) single crystals: An ab-initio calculations

From the refined atomic positions obtained by Belmal et al. (2004) [1] using X-ray diffraction for Li_0.50Co_0.25TiO(PO₄), we have performed a structural optimization by minimizing the forces acting on the atoms keeping the lattice parameters fixed at the experimental values. With this relaxed (optimized) geometry we have performed a comprehensive theoretical study of electronic properties and dispersion of the linear optical susceptibilities using the full potential linear augmented plane wave (FP-LAPW) method. The generalized gradient approximation (GGA) exchange-correlation potential was applied. In addition, the Engel-Vosko generalized gradient approximation (EVGGA) was used for comparison with GGA because it is known that EVGGA approach yields better band splitting compared to the GGA. We have calculated the band structure, and the total and partial densities of states. The electron charge densities and the bonding properties were analyzed and discussed. The complex dielectric optical susceptibilities were discussed in detail.     

Optical properties of alkaline-earth metal oxides from first principles

This study reports the results of an ab initio electronic and optical calculation of alkaline-earth metal oxides (MgO, CaO, SrO and BaO) in the NaCl crystal structure using the full potential linearized augmented plane wave (FP-LAPW) method within the density functional theory. The exchange-correlation potential is treated by the generalized gradient approximation within the Perdew et al scheme. Moreover, the Engel–Vosko GGA formalism is applied so as to optimize the corresponding potential for band structure calculations. The real and imaginary parts of the dielectric function ?(ω), the optical absorption coefficient I(ω), the reflectivity R(ω) and the energy loss function are calculated by random phase approximation (RPA). The calculated results show a qualitative agreement with the available experimental results in the sense that we can recognize some peaks qualitatively, those due to single particle transitions. Furthermore the interband transitions responsible for the structures in the spectra are specified. It is shown that the oxygen 2p states and metal d states play the major role in optical transitions as initial and final states respectively. The effect of the spin–orbit coupling on the optical properties is also investigated and found to be quite small, especially in the low energy region. The dielectric constants are calculated and compared with the available theoretical and experimental results.     

Influence of the substrate electronic structure on metallic quantum well state dispersions in ultrathin metal films

We have studied ultrathin Cu films grown on three related ferromagnetic (FM) layers to clarify the role of the substrate in determining the two-dimensional dispersion of metallic quantum well (MQW) states in the overlayer. The dispersions with parallel momentum of Cu MQW states above the Fermi level E_F were measured in the Cu/fccFe/Cu(1 0 0) and Cu/fccCo/Cu(1 0 0) systems using inverse photoemission, and below E_F in the Cu/fccNi/Cu(1 0 0) system using angle resolved photoemission spectroscopy. This study focussed on the direction of the two-dimensional Brillouin zone. For states away from the projected band gaps of the FM layer, the identity of the FM layer (i.e., Fe or Co) has little influence on Cu MQW states and their dispersions closely resemble those of a free standing Cu film. In contrast, all three systems exhibit nondispersing MQW states when the Cu bands overlap the minority spin projected band gaps of the FM metal. A phase accumulation approach gives a very simple explanation of this behavior, showing that the flat dispersion occurs because the phase shift upon reflection from the FM layer has a strong energy dependence in the projected gap.     

Modelling of aqueous solvation of eosin Y at the rutile TiO2(1 1 0)/water interface

Molecular dynamics simulations at 298 K are used to study an aqueous dissolved dye (eosin Y) adsorbed at the TiO₂(1 1 0) surface to extract static and dynamic information of solvation. Differences in the physical behaviour of the dye at the surface and in bulk water are compared with recent transient absorption and photon echo experiments within the limits of linear response. The calculated solvent dynamics features fast contributions, which change very little at the surface as well as a slow component, which slows down by a factor of two at the interface, in good agreement with experimental data.     

Initial stage of Si(001) surface oxidation from first-principles calculations

A comprehensive density-functional theory (DFT) study of the atomic structure, electronic properties, and optical response of the Si(001) surface at the initial stages of oxidation is presented. The most favored adsorption position of a single O atom on top of the (4 x 2)-reconstructed Si(001) surface is found at the back-bond of the "down" Si dimer atom. There is no energy barrier for oxygen insertion into this bond. The ionization energy of the surface reaches a maximum when the oxidation of the second Si monolayer starts. Oxidation leads to an increase of the energy gap between occupied and empty surface states. The calculated reflectance anisotropy spectroscopy (RAS) data in comparison with experiment suggest a considerable amount of surface disorder already after oxidation of the first monolayer.     

Er2O3纳米晶光伏特性及谱带解析

纳米做教是介于体相材料和分子或原子之间的新型材料，表现出许多超常的电、热、光、磁及催化的特性，因此引起人们高度的重视［1，2］．目前对其光学、磁学、力学和比热等研究已有大量的报导[3]．但是，对于稀土氧化物纳米材料的合成及性质的研究，尤其对它们的电学性质、电子输运方式和光电行为报导较少.本文利用表面光电压谱（SPS）技术对不同粒径的Er2O3纳米晶进行了表面光伏特性的研究，并结合漫反射吸收光谱对其谱带进行了解析，发现，除f→f跃迁外，在300～550nm区间有一定吸收边带，此带可归属于O2p→Er4f电荷转移跃迁．随着样品…     

Effect of electrolytes on the surface behavior of rhamnolipids R1 and R2

The surface behavior of solutions of the rhamnolipids, R1 and R2, were investigated in the absence and presence of an electrolyte (NaCl) through surface tension measurements and optical microscopy at pH 6.8. The NaCl concentrations studied are 0.05, 0.5 and 1 M. Electrolytes directly affect the carboxylate groups of the rhamnolipids. The solution/air interface has a net negative charge due to the dissociated carboxylate ions at pH 6.8 with strong repulsive electrostatic forces between the rhamnolipid molecules. This negative charge is shielded by the Na⁺ ions in the electrical double layer in the presence of NaCl, causing the formation of a close-packed monolayer, and a decrease in CMC, and surface tension values. The maximum compaction is observed at 0.5 M NaCl concentrations for R1 and R2 monolayers, with the R1 monolayer more compact than R2. The larger spaces left below the hydrophobic tails of R1 with respect to that of R2, due to the missing second rhamnosyl groups are thought to be responsible for the higher compaction. The rigidity of both R1 and R2 monolayers increases with the electrolyte concentration. The rigidity of the R1 monolayer is greater than that of R2 at all NaCl concentrations due to the lower hydrophilic character of R1. The variation of CMC values as a function of NaCl concentration obtained from the surface tension measurements and critical packing parameter (CPP) calculations show that spherical micelles, bilayer and rod like micelles are formed in the rhamnolipid solutions as a function of the NaCl concentration. The results of optical microscopy supported these aggregation states indicating lamellar nematic liquid crystal, cubic lamellar and hexagonal liquid crystal phases in R1 and R2 solutions depending on the NaCl concentration.     

Experimental characterization of the weakly anisotropic CN X 2Σ+ + Ne potential from IR-UV double resonance studies of the CN-Ne complex

IR-UV double resonance spectroscopy has been used to characterize hindered internal rotor states (n(K) = 0(0), 1(1), and 1(0)) of the CN-Ne complex in its ground electronic state with various degrees of CN stretch (ν(CN)) excitation. Rotationally resolved infrared overtone spectra of the CN-Ne complex exhibit perturbations arising from Coriolis coupling between the closely spaced hindered rotor states (1(1) and 1(0)) with two quanta of CN stretch (ν(CN) = 2). A deperturbation analysis is used to obtain accurate rotational constants and associated average CN center-of-mass to Ne separation distances as well as the coupling strength. The energetic ordering and spacings of the hindered internal rotor states provide a direct reflection of the weakly anisotropic intermolecular potential between CN X (2)Σ(+) and Ne, with only an 8 cm(-1) barrier to CN internal rotation, from which radially averaged anisotropy parameters (V(10) and V(20)) are extracted that are consistent for ν(CN) = 0-3. Complementary ab initio calculation of the CN X (2)Σ(+) + Ne potential using MRCI+Q extrapolated to the complete one-electron basis set limit is compared with the experimentally derived anisotropy by optimizing the radial potential at each angle. Experiment and theory are in excellent accord, both indicating a bent minimum energy configuration and nearly free rotor behavior. Analogous experimental and theoretical studies of the CN-Ne complex upon electronic excitation to the CN B (2)Σ(+) state indicate a slightly more anisotropic potential with a linear CN-Ne minimum energy configuration. The results from these IR-UV double resonance studies are compared with prior electronic spectroscopy and theoretical studies of the CN-Ne system.     

Isotropic and anisotropic scattering from polyethylene during crystallization

A light-scattering method for determining the optical anisotropy of large molecules is considered in detail. An application of the method is demonstrated to linear polyethylene while isothermally crystallizing in p-xylene solution. The polyethylene crystals formed are characterized as anisotropic thin discs having an optical anisotropy of 0.4–0.6.     

Formation of π-coupled organic wire on the Si(0 0 1)[2 × 1] surface

The stability and electronic properties of highly packed 1-hexyl-naphthalene (HNap) molecular wire on Si(0 0 1) have been studied with first principles DFT method. HNap assembles into a 1D arrangement on the Si(0 0 1)[2 × 1] surface on which molcules adopt a commensurate structure along a dimer row with an intermolecular distance of 3.8 Å. HNap is attached to the surface through the hexyl chain, and stands normal to the surface. This highly packed structure leads to the formation of delocalized π-orbitals over the entire wire but essentially localized on the naphthalene counterpart, and well separated from the Si surface states. Cohesion energy within the wire arises from a significant attraction between hexyl chains, and to a weaker stabilizing π–π interaction between naphthalenes.     

Addition reactions of nitrones on the reconstructed C(1 0 0)-2 × 1 surfaces

In this paper, the reactions of nitrone, N-methyl nitrone, N-phenyl nitrone and their hydroxylamine tautomers (vinyl-hydroxylamine, N-methyl-vinyl-hydroxylamine and N-phenyl-vinyl-hydroxylamine) on the reconstructed C(1 0 0)-2 × 1 surface have been investigated using hybrid density functional theory (B3LYP), Møller–Plesset second-order perturbation (MP2) and multi-configuration complete-active-space self-consistent-field (CASSCF) methods. The calculations showed that all the nitrones can react with the surface “dimer” via facile 1,3-dipolar cycloaddition with small activation barriers (less than 12.0 kJ/mol at B3LYP/6-31g(d) level). The [2+2] cycloaddition of hydroxylamine tautomers on the C(1 0 0) surface follows a diradical mechanism. Hydroxylamine tautomers first form diradical intermediates with the reconstructed C(1 0 0)-2 × 1 surface by overcoming a large activation barrier of 50–60 kJ/mol (B3LYP), then generate [2+2] cycloaddition products via diradical transition states with negligible activation barriers. The surface reactions result in hydroxyl or amino-terminated diamond surfaces, which offers new opportunity for further modifications.     

Photolysis wavelength dependence of the translational anisotropy and the angular momentum polarization of O2(a 1Deltag) formed from the UV photodissociation of O3

The translational anisotropy and angular momentum polarization of the O(2)(a (1)Delta(g),v = 0;J = 15-27) molecular photofragment produced from the UV photodissociation of O(3) in the range from 270 to 300 nm have been determined using resonance-enhanced multiphoton ionization in conjunction with time-of-flight mass spectrometry. At the shortest photolysis wavelengths used, the fragments exhibit the anisotropic vector correlations expected from a prompt dissociation via the (1)B(2) <--(1)A(1) transition. Deviations from this behavior are observed at longer photolysis wavelengths with, in particular, the angular momentum orientation showing a significant reduction in magnitude. This indicates that the dissociation can no longer be described by a purely impulsive model and a change in geometry of the dissociating molecule is implied. This observation is substantiated by the variation of the translational anisotropy with photolysis wavelength. We also observe that the bipolar moments describing the angular momentum polarization of the odd J states probed are consistently lower in magnitude than those of the even J states and that this variation is observed for all photolysis wavelengths.     

Nonlinear response of aluminum surface to electric field

The static limit of the perpendicular second-harmonic response of the exposed single-crystal planes of aluminum to applied electric field is studied using the variant of stabilized-jellium model which allows to account for the anisotropy of surface potential. The self-consistently calculated linear and second-order induced charge density distributions are used to determine the normal component of the polarization vector. The surface structure is found to have a pronounced effect on the anisotropy of second-harmonic response. © 1997 John Wiley & Sons, Inc.