Anion replacement at Au(110)/electrolyte interfaces.

A characteristic reflection anisotropy spectrum (RAS) is observed from a Au(110) surface in a wide range of electrolytes and combinations of pH and applied potentials. It is suggested that this common RAS profile arises from an interaction between the potential applied to the Au(110) electrode and the dipole moments of oxidized species that locates the Fermi level at a common position with respect to the electronic band structure of Au. Rapid changes in this RAS profile are observed for Au(110)/H2SO4 as the potential is switched between 0.3 V and 0.6 V, a potential range in which the surface is not reconstructed and below the potential range of surface oxidation. The spectral changes are completed in less than 10 ms, are reversible and are attributed to the replacement of adsorbed anions by an oxygenated species.

The stability of the Au(1 1 0)-(1 × 3) surface reconstruction in electrochemical environments.

Changes in the reflection anisotropy (RAS) profile of the Au(1 1 0)-(1 × 3)/Na2SO4 interface over 25 h are attributed to the slow accumulation of impurities on the Au(1 1 0) surface which reduce the intensity of a transition involving a surface state that makes a positive contribution to the RAS profile at 1.8 eV. The growth in the intensity of a feature that makes a negative contribution to the RAS profile at 2.6 eV and the reduction in the intensity of contributions to higher energy is attributed to shifts in the energy of the surface band structure relative to the Fermi level caused by the accumulation of impurities. There is no clear explanation of the subsequent decay of the 2.6 eV feature or the long term reduction in intensity to high energy of the RAS profile.

The reflection anisotropy spectroscopy of the Au(1 1 0) surface structures in liquid environments.

The reflection anisotropy (RAS) profiles of the Au(1 1 0)-(1 × 1), (1 × 2) and (1 × 3) surface structures in electrochemical environments are shown to arise mainly from surface dipole transitions directed along the principal axes of the Au(1 1 0) surface. There are weak contributions to the RAS profiles of the Au(1 1 0)-(1 × 1) and (1 × 3) surfaces in the region of 4.0 eV which probably arise from (1 1 1) facets that are either intrinsic to the surface structures or are associated with steps. A transition involving a surface state just above the Fermi level, E F, contributes to the RAS profiles of the (1 × 2) and (1 × 3) surfaces but not to the RAS profile of the (1 × 1) surface. A strong feature at 2.5 eV in the RAS profiles of the Au(1 1 0)-(1 × 1) and (1 × 2) surfaces is attributed to a transition in the vicinity of the L point of the Brillouin zone between the 5d band and the [Formula: see text] band at E F. It is argued that the applied potential of -0.6 V, which creates the Au(1 1 0)-(1 × 3) surface, lifts E F above the [Formula: see text] band causing it to become occupied and quenching this contribution to the RAS profile.

Conformational change in cytochrome P450 reductase adsorbed at a Au(110)-phosphate buffer interface induced by interaction with nicotinamide adenine dinucleotide phosphate.

Changes observed in the reflection anisotropy spectroscopy (RAS) profiles of monolayers of cytochrome P450 reductase adsorbed at Au(110)-electrolyte interfaces at 0.056 V following the addition of nicotinamide adenine dinucleotide phosphate (NADP(+)) are explained in terms of a simple model as arising from changes in the orientation of an isoalloxazine ring located in the flavin mononucleotide binding domain of the protein. The model also accounts for the changes observed in the RAS as the potential applied to the Au(110) surface is varied and suggests that differences in the dependence of the RAS profile of the adsorbed protein on the potential applied to the electrode in the absence and presence of NADP(+) are explicable as arising from a competition between the applied potential acting to reduce the protein and the NADP(+) to oxidize it.

The influence of the structure of the Au(110) surface on the ordering of a monolayer of cytochrome P450 reductase at the Au(110)/phosphate buffer interface.

The reflection anisotropy spectra (RAS) observed initially from Au(110)/phosphate buffer interfaces at applied potentials of -0.652 and 0.056 V are very similar to the spectra observed from ordered Au(110) (1 × 3) and anion induced (1 × 1) surface structures respectively. These RAS profiles transform to a common profile after cycling the potential between these two values over 72 h indicating the formation of a less ordered surface. The RAS of a monolayer of a P499C variant of the human flavoprotein cytochrome P450 reductase adsorbed at 0.056 V at an ordered Au(110)/phosphate buffer interface is shown to arise from an ordered layer in which the optical dipole transitions are in a plane that is orientated roughly normal to the surface and parallel to either the [11̄0] or [001] axes of the Au(110) surface. The same result was found previously for adsorption of P499C on an ordered interface at -0.652 V. The adsorption of P499C at the disordered surface does not result in the formation of an ordered monolayer confirming that the molecular ordering is strongly influenced by both the local structure and the long range macroscopic order of the Au(110) surface.

Conformational change induced by electron transfer in a monolayer of cytochrome P450 reductase adsorbed at the Au(110)-phosphate buffer interface.

The reflection anisotropy spectroscopy profiles of a variant of cytochrome P450 reductase adsorbed at the Au(110)-phosphate buffer interface depend on the sequence of potentials applied to the Au(110) electrode. It is suggested that this dependence arises from changes in the orientation of the isoalloxazine ring structures in the protein with respect to the Au(110) surface. This offers a method of monitoring conformational change in this protein by measuring variations in the reflection anisotropy spectrum arising from changes in the redox potential.

The nature and stability of the Au(110)/electrochemical interface produced by flame annealing.

It is shown using reflection anisotropy spectroscopy (RAS) that following flame annealing and immersion in pure water, the Au(110) surface adopts a (1×1) structure and that this structure is preserved in a 0.1 M H(2)SO(4) environment. The surface transforms to the (1×2) reconstruction following the application of a potential of 0.0 V versus SCE (a saturated calomel electrode). This surface is unstable and the RAS profile changes over periods of 15 min and 1 h in a manner which suggests that changes are occurring in the structure and distribution of [11(-)0] steps. Over longer periods the RAS transforms towards a profile attributed to a surface associated with the specific adsorption of anions.

Controlling the formation of a monolayer of cytochrome P450 reductase onto Au surfaces.

The conditions necessary for the formation of a monolayer and a bilayer of a mutated form (P499C) of human cytochrome P450 reductase on a Au(110)/electrolyte interface have been determined using a quartz crystal microbalance with dissipation, atomic force microscopy, and reflection anisotropy spectroscopy (RAS). The molecules adsorb through a Au-S linkage and, for the monolayer, adopt an ordered structure on the Au(110) substrate in which the optical axes of the dipoles contributing to the RAS signal are aligned roughly along the optical axes of the Au(110) substrate. Differences between the absorption spectrum of the molecules in a solution and the RAS profile of the adsorbed monolayer are attributed to surface order in the orientation of dipoles that contribute in the low energy region of the spectrum, a roughly vertical orientation on the surface of the long axes of the isoalloxazine rings and the lack of any preferred orientation in the molecular structure of the dipoles in the aromatic amino acids. Our studies establish an important proof of principle for immobilizing large biological macromolecules to gold surfaces. This opens up detailed studies of the dynamics of biological macromolecules by RAS, which have general applications in studies of biological redox chemistry that are coupled to protein dynamics.

Optical reflectance anisotropy of the growth of Fe monolayers on W(110).

We report measurements of the optical anisotropy of Fe layers grown on the W(110) surface using reflection anisotropy spectroscopy (RAS). As the first monolayer of Fe is deposited onto W(110), the resonance-like RAS profile of the clean surface is reduced in intensity. We find evidence for the surface state on W(110) surviving as an interface state following Fe deposition. We observe an anisotropic optical response from Fe layers grown on top of the first two monolayers, where a broad peak at 3 eV dominates the RAS response. The results are simulated in terms of a layered Fresnel reflection model incorporating either a strained Fe overlayer or an Fe overlayer whose dielectric properties are approximated by a simple Lorentzian oscillator. Both approaches are found to produce simulated RA spectra that are in good agreement with experiment. The former approach provides evidence that RAS can detect anisotropy in strained overlayers and that 7 ML films have bulk-like electronic and optical properties.

Prevention of surface reconstruction at the Au(110)/electrolyte interface by the adsorption of cytosine.

It is shown that the adsorption of cytosine at the Au(110)/liquid interface at a potential of 0.0 V "freezes" the Au(110) surface in the (1x1) structure and that the molecule does not change its orientation on the surface as the potential is varied. In contrast the adsorption of adenine does not freeze the Au(110) surface even though both molecules adopt a base stacking structure with individual molecules oriented in a plane vertical to the Au(110) surface with their long axes along [110] rows. It is suggested that cytosine bonds to three Au atoms through the NH(2) group, the N(3) and O(8) sites, and that this arrangement stabilizes the Au(110) surface and prevents its reconstruction to the more open (1x2) and (1x3) structures as the applied voltage is varied. The weaker bonding of the adenine molecule with the gold surface is unable to prevent the voltage induced reconstruction of the Au(110) surface.

Insight into the local density of states at Si sites at the submonolayer Si/Ge(001)-2 × 1 interface from Si KLV Auger spectroscopy.

An analysis of the differences observed between the Si KLV Auger spectra of the Si/Ge(001)-2 × 1 interface and pure Si indicates that the electronic structure of the interface is characterized by a reduction in the local p DOS at the Si sites and a transfer of p valence charge from Si to Ge. As a result, the screening of core-ionized Si sites at the interface is significantly shifted towards s screening compared with the situation for pure Si. It is possible that there is an increase in the on-site electron correlation energy, UP, for Si sites at the interface as compared with pure Si.

Spectral signatures of the surface reconstructions of Au(110)/electrolyte interfaces.

It is demonstrated that the (1 × 1) structure and the (1 × 2) and (1 × 3) surface reconstructions that occur at Au(110)/electrolyte interfaces have unique optical fingerprints. The optical fingerprints are potential, pH and anion dependent and have potential for use in monitoring dynamic changes at this interface. We also observe a specific reflection anisotropy spectroscopy signature that may arise from anions adsorbed on the (1 × 1) structure of Au(110).

Determination of the structure of adenine monolayers adsorbed at Au(110)/electrolyte interfaces using reflection anisotropy spectroscopy.

Reflection anisotropy spectroscopy (RAS) has been used to show that at saturation coverage adenine adsorbs on the Au(110)/electrolyte interface in a base-stacking configuration with the plane of the bases orientated vertically on the surface and with the long axis of the molecules parallel to the [110] direction. Changes in the RAS observed from adsorbed adenine as a result of changes in the potential applied to the Au(110) electrode could arise from slight changes in the orientation of the molecules in the vertical plane.

Reflection anisotropy spectroscopy of the oxidized diamond (001) surface.

We report reflection anisotropy spectroscopy (RAS) measurements of the oxidized (001) surface of a type IIb natural diamond. These measurements were made possible due to recent developments in diamond surface preparation. We compare RAS results from the hydrogenated, clean and oxidized C(001) surface and demonstrate that RAS is sensitive to the structural transition of the surface from the 2 × 1 reconstruction of the clean surface to the 1 × 1 reconstruction of the oxidized surface.

Evidence for protein conformational change at a Au(110)/protein interface.

Evidence is presented that reflection anisotropy spectroscopy (RAS) can provide real-time measurements of conformational change in proteins induced by electron transfer reactions. A bacterial electron transferring flavoprotein (ETF) has been modified so as to adsorb on an Au(110) electrode and enable reversible electron transfer to the protein cofactor in the absence of mediators. Reversible changes are observed in the RAS of this protein that are interpreted as arising from conformational changes accompanying the transfer of electrons.

Orientation of ordered structures of cytosine and cytidine -monophosphate adsorbed at Au(110)/liquid interfaces.

It is demonstrated using reflection anisotropy spectroscopy that the adsorption of cytosine and cytidine -monophosphate at the Au(110) 1 x 2/electrolyte interface gives rise to ordered structures in which the base is oriented vertical to the surface and parallel to the [110] axis of the Au(110) plane.

Reflectance anisotropy spectra of the diamond (100)-(2x1) surface: evidence of strongly bound surface state excitons.

We compare the results of ab initio calculations with measured reflection anisotropy spectra and show that strongly bound surface-state excitons occur on the clean diamond (100) surface. These excitons are found to have a binding energy close to 1 eV, the strongest ever observed at a semiconductor surface. Important electron-hole interaction effects on the line shape of the optical transitions above the surface-state gap are also found.

Auger energy shifts in fcc AgPd random alloys from complete screening picture and experiment.

We extend the complete screening picture to ab initio calculations of Auger kinetic energy and Auger parameter shifts in metallic alloys. Experimental measurements of the L(3)M(4,5)M(4,5) Auger transition in fcc AgPd random alloys are compared with first-principles calculations and the results are in excellent agreement for both the Ag and Pd Auger shifts over the whole concentration range. We discuss the Auger kinetic energy shifts in terms of single-hole states for the 2p(3/2) core level and double-hole states for the 3d(5/2) level.

Reflection anisotropy spectroscopy: A new probe for the solid-liquid interface

Introducing reflection anisotropy spectroscopy (RAS) as a new probe for solid-liquid interfaces, we present results for the Au(110)/electrolyte interface which serves as a model system. We demonstrate that RAS is sensitive to surface phase transitions, step morphology, and electronic surface states. Using an empirical approach, the RA spectra are reproduced and features are identified which reflect the known character of the bias voltage driven (2x1) to (1x1) phase transition. RAS is established as an experimental technique to probe the electronic structure of solid-liquid interfaces in real time to study a wide range of interface properties.