Search

We have investigated the Pt(110) surface by He atom scattering. The (1 x 2) deconstruction transition at 1087 K is shown to fall in 2D Ising universality class with antiphase domain walls proliferating predominantly along the [001] direction. The surface roughening is observed at 1160 ± 10 K through a power law broadening of the anti-phase diffraction peak along the [001] direction. The two distinct phase transition confine the temperature range in which the surface is disordered but still bound to a common height level.

The angular distribution of the intensity in photoemission experiments is affected by electron diffraction patterns and by a smoothly varying ISO contribution originated by both intrumental details and physical properties of the samples. The origin of the various contributions to the ISO component has been identified since many years. Nonetheless in this work we present original developement of the ED analysis, which arises from the evolution of instrumental performance, in terms of analyzers positioning and angular resolution, as well as collimation and size of X-ray beams in third generation synchrotron sources. The analytical treatement of the instrumental factors is presented in detail for the end station of the ALOISA beamline (Trieste Synchrotron), where a wide variety of scattering geometries is available for ED experiments. We present here the basic formulae and their application to experimental data taken on the Fe/Cu3Au(001) system in order to highlight the role of the various parameters included in the distribution function. A specific model for the surface illumination has been developed as well as the overlayer thickness and surface roughness have been considered., RevTex, nine pages with five eps figures; to be published in J. Electron Spectrosc. Relat. Phenom

While the demonstrated power conversion eff i ciency of organic photovoltaics (OPVs) now exceeds 10%, new design rules are required to tailor interfaces at the molecular level for optimal exciton dissociation and charge transport in higher effi ciency devices. We show that molecular shape-complementarity between donors and acceptors can drive performance in OPV devices. Using core hole clock (CHC) X-ray spectroscopy and density functional theory (DFT), we compare the electronic coupling, assembly, and charge transfer rates at the interface between C 60 acceptors and fl at- or contorted-hexabenzocorone (HBC) donors. The HBC donors have similar optoelectronic properties but differ in molecular contortion and shape matching to the fullerene acceptors. We show that shape-complementarity drives self-assembly of an intermixed morphology with a donor/acceptor (D/A) ball-and-socket interface, which enables faster electron transfer from HBC to C 60 . The supramolecular assembly and faster electron transfer rates in the shape complementary heterojunction lead to a larger active volume and enhanced exciton dissociation rate. This work provides fundamental mechanistic insights on the improved effi ciency of organic photovoltaic devices that incorporate these concave/convex D/A materials.

The α phase of Sn/Ge(111) has been investigated from 120 K up to 500 K, using synchrotron radiation core-level photoemission. By means of photoelectron diffraction experiments, we verified that the rippled structure of the low-temperature (3×3) phase is preserved in the (√3×√3)R30° phase at room temperature, thus confirming the order-disorder character of the phase transition. We also found that at least two components are present in the Sn 4d core-level spectra up to 500 K, i.e., about 300 K above the onset of the transition from the low-temperature (3×3) phase to the (√3×√3)R30° phase, thus excluding the occurrence of any displacive transition.

Adsorption of half a monolayer of oxygen on the Rh(110) substrate gives a surface with a (2 × 2) p2mg symmetry structure, in which the substrate has a missing row (1 × 2) structure. He-beam scattering measurements show that close to 750 K this surface structure reversibly transforms into a phase of (1 × 2) symmetry. The phase transition has been classified as belonging to the order-disorder type, and falls into the 2D Ising universality class. Above the transition temperature proliferation of antiphase domain boundaries along the [001] direction is observed and discussed.

Using photoemission spectroscopy, we determine the relationship between electronic energy level alignment at a metal-molecule interface and single-molecule junction transport data. We measure the position of the highest occupied molecular orbital (HOMO) relative to the Au metal Fermi level for three 1,4-benzenediamine derivatives on Au(111) and Au(110) with ultraviolet and resonant x-ray photoemission spectroscopy. We compare these results to scanning tunnelling microscope based break-junction measurements of single molecule conductance and to first-principles calculations. We find that the energy difference between the HOMO and Fermi level for the three molecules adsorbed on Au(111) correlate well with changes in conductance, and agree well with quasiparticle energies computed from first-principles calculations incorporating self-energy corrections. On the Au(110) which present Au atoms with lower-coordination, critical in break-junction conductance measurements, we see that the HOMO level shifts further from the Fermi level. These results provide the first direct comparison of spectroscopic energy level alignment measurements with single molecule junction transport data.

The photoemission spectra of the Sn 4d electrons from the (3×3)-Sn/Ge(111) surface present two components which are attributed to inequivalent Sn atoms in T4 bonding sites. This structure has been explored by photoelectron diffraction experiments performed at the ALOISA beamline of the Elettra storage ring in Trieste (Italy). The modulation of the intensities of the two Sn components, caused by the backscattering of the underneath Ge atoms, has been measured as a function of the emission angle at fixed kinetic energies and vice versa. The bond angle between Sn and its nearest neighbor atoms in the first Ge layer ( Sn–Ge 1) has been measured by taking polar scans along the main symmetry directions and it was found almost equivalent for the two components. The corresponding bond lengths are also quite similar, as obtained by studying the dependence on the photoelectron kinetic energy with the photon polarization and the collection direction parallel to the Sn–Ge 1 bond orientation (bond emission). A clear difference between the two bonding sites is observed when studying the energy dependence at normal emission, where the sensitivity to the Sn height above the Ge atom in the second layer is enhanced. The (3×3)-Sn/Ge(111) is thus characterized by a structure where the Sn atom and its three nearest neighbor Ge atoms form a rather rigid unit that presents a strong vertical distortion with respect to the underneath atom of the second Ge layer.

The temperature-dependent behavior of the Rh(111)-(2X2)-10 phase was investigated by He-atom scattering (HAS) and low-energy electron diffraction. The adsorption system undergoes an order-disorder phase transition at T-c=280+/-5 K, with critical exponents found to be consistent with the four-state Potts model. Beyond the phase transition the HAS specular peak intensity exhibits a strong and reversible increase. This finding points toward a reduction of the surface charge-density corrugation induced by the phase transition itself. Around 160 K, hydrogen adsorbed on the Rh(111)-(2X2)-10 surface reacts with oxygen to form water, and drives the overlayer in an out-of-equilibrium condition which is characterized by a dramatic domain-wall proliferation.

The new beamline ALOISA is now working at the Elettra synchrotron facility. It is equipped with a monochromator of new design to cover the 200 eV - 8 KeV energy range: the monochromator has two interchangeable dispersive systems, one for the low energies up to 2 KeV using reflection gratings and another with crystals for the higher energies. The monochromatic beam is finally re-focused from the fixed exit slit to inside the experimental chamber by a toroidal mirror. In this paper the results of the first resolution measurements are reported for the low energy section of the monochromator: a resolving power higher than 5000, with peaks of 10000 in the 400 - 500 eV region, is obtained while using all the optical elements at full illumination.

We have studied the vapor-phase deposition of L-cysteine on the Au(110) surface by means of synchrotron-based techniques. Relying on a comparison with previous X-ray photoemission analysis, we have assigned the fine structure of the C K-shell X-ray absorption spectra to the nonequivalent carbon bonds within the molecule. In particular, the C1s --sigma* transition, where the sigma* state is mainly localized on the C-S bond, is shifted well below the ionization threshold, at approximately -5 eV from the characteristic pi* transition line related to carboxylic group. From the polarization dependence of the absorption spectra in the monolayer coverage range, the molecules are found to lay flat on the surface with both the C-S bond and the carboxylic group almost parallel to the surface. We performed in situ complementary surface X-ray diffraction, SXRD, measurements to probe the rearrangement of the Au atoms beneath the L-cysteine molecules. Since the early stage of deposition, L-cysteine domains are formed which display an intermediate fourfold symmetry along [001]. The self-assembly of molecules into paired rows, extending along the [1(-)10] direction, is fully compatible with our observations, as has been reported for the case of D-cysteine molecules grown on Au(110) [Kühnle, A. et al. Phys. Rev. Lett. 2004, 93, 086101.]

PHYS. REV. B

The effects of annealing on the structure of ultra thin Fe films (4-10 ML) deposited at 150 K on Ag(001) were studied by synchrotron radiation photoelectron diffraction (PED) and x-ray diffraction (XRD). The occurrence of a surfactant-like stage, in which a single layer of Ag covers the Fe film is demonstrated for films of 4-6 ML heated at 500-550 K. Evidence of a stage characterized by the formation of two Ag capping layers is also reported. As the annealing temperature was increased beyond 700 K the surface layers closely resembled the structure of bare Ag(001) with the residual presence of subsurface Fe aggregates. The data illustrate a film dissolution path which is in agreement with recent theoretical models [J. Roussel Phys. Rev. B 60, 13890 (1999)]

The growth of lead on Ge(1 1 1) at 120 K was investigated by various experimental methods. He atom scattering experiments were used to determine the low temperature growth morphology of the Pb/Ge(1 1 1) system. The growth modes at low and room temperature were also compared by Auger electron spectroscopy, low energy electron diffraction, and by the observation of inelastic electron scattering through image potential resonances indicating the presence of Pb islands at 300 K and of a continuous Pb film at 120 K. In contrast to the well-known room temperature Stranski–Krastanov growth mode, strong evidence for a layer-by-layer growth from three monolayers coverage onwards was found for Pb on Ge(1 1 1) at low temperature.

The experimental information accumulated in recent years on elastic and inelastic scattering of He and Ne atoms from metal surfaces is analysed by describing the atom-surface interaction as a superposition of pseudo-pairwise potentials. These are chosen according to the quite accurate potentials presently available for van der Waals pairs and take a phenomenological account for many body effects. The emerging picture is quite satisfactory and allows the physical interaction of a given atom-surface system to be predicted with excellent accuracy. In particular the coupling of He atoms to the surface vibrational modes is considered with attention focused on He inelastic scattering from Cu(001).

The electronic properties of thin metallic films deviate from the corresponding bulk ones when the film thickness is comparable with the wavelength of the electrons at the Fermi level due to quantum size effects (QSE). QSE are expected to affect the film morphology and structure leading to the low temperature (LT) ``electronic growth'' of metals on semiconductors. In particular, layer-by-layer growth of Pb(111) films has been reported for deposition on Ge(001) below 130 K. An extremely flat morphology is preserved throughout deposition from four up to a dozen of monolayers. These flat films are shown to be metastable and to reorganize into large clusters uncovering the first Pb layer, pseudomorphic to the substrate, already at room temperature. Indications of QSE induced structural variations of the growing films have been reported for Pb growth on Ge(001), where the apparent height of the Pb(111) monatomic step was shown to change in an oscillatory fashion by He atom scattering (HAS) during layer-by-layer growth. The extent of the structural QSE has been obtained by a comparison of the HAS data with X-ray diffraction (XRD) and reflectivity experiments. Whereas step height variations as large as 20 % have been measured by HAS reflectivity, the displacement of the atomic planes from their bulk position, as measured by XRD, has been found to mainly affect the topmost Pb layer, but with a lower extent, i.e. the QSE observed by HAS are mainly due to a perpendicular displacement of the topmost layer charge density. The effect of the variable surface relaxation on the surface vibration has been studied by inelastic HAS to measure the acoustic dispersion of the low energy phonons., Comment: 28 pages (laTex,elsart) and 13 figures (eps); updated references

he structure of Fe films grown on the (001) surface of a Cu3Au single crystal at room temperature has been investigated by means of grazing incidence x-ray diffraction (GIXRD) and photo/Auger-electron diffraction (ED) as a function of thickness in the (3-36)-Angstrom range. The combination of GIXRD and ED allows one to obtain quantitative information on the in-plane spacing a from the former technique, and the ratio between the vertical spacing c and a, from the latter one. At low coverage the film grows pseudomorphic to the face-centered-cubic substrate. The experimental results obtained on a film of 8 Angstrom thickness clearly indicate the overcoming of the limit for pseudomorphic growth. Above this limit the film is characterized by the coexistence of the pseudomorphic phase with another tetragonally strained phase gamma, which falls on the epitaxial line of ferromagnetic face-centered cubic Fe. Finally, the development of a body-centered phase alpha, whose unit cell is rotated by 45degrees with respect to the substrate one, has been clearly observed at similar to17 Angstrom. alpha is the dominating phase for film thickness above similar to25 Angstrom and its lattice constant evolves towards the orthomorphic phase in strict quantitative agreement with epitaxial curves calculated for body-centered tetragonal iron phases.

We report on the structural and magnetic properties of thin Fe films grown on the Cu(1 1 0) surface. In-plane grazing-incidence X-ray diffraction has been used to measure the lateral lattice spacing of the Fe films. Complementary information about the structure of the topmost layers has been obtained by means of Auger and photoelectron diffraction. The Fe film grows pseudomorphic with the substrate up to a thickness of about 0.8 nm. The diffraction feature of a new phase is observed at 1.6 nm, with a corresponding interplanar distance close to the bulk body centered cubic (bcc) Fe one, which is eventually recovered at higher thickness (6.4 nm). From comparison between X-ray diffraction and photoelectron diffraction, it is suggested that the bcc-like Fe grows on the (1 0 0) surface with its [1 1 0] axis oriented along the [0 0 1] substrate direction. The photoelectron diffraction data also indicate a strong faceting in the [1 10] substrate direction. This morphology is believed to contribute to the in-plane uniaxial magnetic anisotropy observed by Kerr effect and Brillouin light scattering from spin waves.

The Ge(001) surface is known to undergo phase transitions from c(4 x 2) to (2 x 1) at low temperature and from (2 x 1) to (I x 1) at high temperature. In the former case the phase transition is shown to occur at 240 K and to fall into the 2D Ising universality class. The high temperature phase transition takes place above 900 K and is characterized by (2 x 1) domain wall proliferation, thus indicating an order-disorder character. We have also observed an increase in the density of steps, but these are shown to be only partially involved in the disordering of the (2 x 1) phase.

Note: Univ ljubljana,jozef stefan inst,ljubljana,slovenia. univ trieste,dipartmento fis,i-34127 trieste,italy. Crottini, A, IST NAZL FIS MAT,LAB TASC,PADRICIANO 99,I-34012 TRIESTE,ITALY. ISI Document Delivery No.: XT124 Cited References: BATRA IP, 1986, PHYS REV B, V34, P8246 CIRACI S, 1986, PHYS REV B, V33, P4294 CVETKO D, 1992, MEAS SCI TECHNOL, V3, P997 EDWARDS KA, 1996, PHYSICA B, V221, P201 FEIBELMAN PJ, 1983, PHYS REV B, V27, P1991 FEIBELMAN PJ, 1984, PHYS REV B, V29, P6463 FERRER S, 1995, PHYS REV LETT, V75, P1771 HINCH BJ, 1989, EUROPHYS LETT, V10, P341 HINCH BJ, 1991, VACUUM, V42, P309 HRICOVINI K, 1991, SPRINGER SERIES SURF, V24, P589 JAKLEVIC RC, 1971, PHYS REV LETT, V26, P89 JALOCHOWSKI M, 1992, PHYS REV B, V45, P13607 JALOCHOWSKI M, 1992, PHYS REV B, V46, P4693 LAPUJOULADE J, 1992, SPRINGER SERIES SURF, V27 LENT CS, 1984, SURF SCI, V139, P121 MEYERHEIM HL, 1995, PHYS REV B, V52, P16830 SCHULTE FK, 1976, SURF SCI, V55, P427 TORRELLES X, 1996, SURF SCI, V364, P242 YANG WS, 1995, PHYS REV B, V51, P7571 ZENG H, 1995, PHYS REV LETT, V74, P582 ZHANG Y, 1993, SURF SCI, V293, L821 ZHAO RG, 1993, PHYS REV B, V48, P8462 NR 22 TC 13 PU AMERICAN PHYSICAL SOC PI COLLEGE PK PA ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA SN 0031-9007 Reference LOEQ-ARTICLE-1997-016doi:10.1103/PhysRevLett.79.1527 Record created on 2007-08-31, modified on 2017-05-12

The Fe LMM Auger and Auger vacancy satellite spectra of ultrathin Fe films grown on Cu(100) have been measured using photon excitation energies above and below the L 2 ionisation edge, and in coincidence with the Fe 2p 3/2 and Fe 2p 1/2 photoelectrons. A comparison of the relative intensities of the satellites and main Auger transitions indicates that the M 4,5 holes of L 3 M 4,5 double hole states are localised for longer than the L 3 lifetime for the 0.3 and 10 ML coverages but have a lifetime comparable to that of L 3 holes for the 1 ML coverage. The results for the 1 ML coverage are supported by the results of the coincidence experiments. The reasons for the variation in the lifetime of M 4,5 holes are discussed in terms of changes in the number of Fe–Fe neighbours and the density of states at the Fermi energy in these systems.

The atomic-like behavior of the photoionization cross-section of core levels in solids is remarkably displayed in the observation of linear magnetic dichroism in the angular distribution of photoelectrons. Structure-related effects are clearly visible as modulations of the dichroism signal, induced by photoelectron diffraction, a fact that can be exploited to obtain information from magnetic surfaces. A systematic investigation of such effects has been carried out for Fe 2p and 3p core levels on the ALOISA beamline at ELETTRA. We present results for Fe(0 0 1) with a reference calibration for amorphous RCP-Fe grown on Fe30Ni50B20, which provides the atomic-like standard, free from diffraction effects.

The temperature-dependent behavior of the Rh(111)-(2\ifmmode\times\else\texttimes\fi{}2)-1O phase was investigated by He-atom scattering (HAS) and low-energy electron diffraction. The adsorption system undergoes an order-disorder phase transition at ${\mathrm{T}}_{\mathrm{c}}$=280\ifmmode\pm\else\textpm\fi{}5 K, with critical exponents found to be consistent with the four-state Potts model. Beyond the phase transition the HAS specular peak intensity exhibits a strong and reversible increase. This finding points toward a reduction of the surface charge-density corrugation induced by the phase transition itself. Around 160 K, hydrogen adsorbed on the Rh(111)-(2x2)-1O surface reacts with oxygen to form water, and drives the overlayer in an out-of-equilibrium condition which is characterized by a dramatic domain-wall proliferation.

Using He atom scattering, layer-by-layer erosion of InSb(110) by low energy ion bombardment has been observed to proceed via nucleation of vacancy islands, island growth and coalescence. The mechanism is in full agreement with theoretical models developed for crystal growth, with diffusing adatoms substituted by diffusing vacancies. In particular it has been observed that coalescence of the vacancy islands sets in after the removal of small fractions of a monolayer and that the average terrace width increases with increasing number of sputtered monolayers.

Although extended surface defects are often described as dislocations, they are less commonly associated with Burgers vectors. The concept of linear surface dislocations and their associated surface Burgers vectors is defined and discussed in an introductory way and the main properties are summarised. The definition of the Burgers vector differs from that used in the bulk, as a closed path integral is not used. The Burgers vector is a quantity which is conserved, modulo a surface unit vector, and which adds vectorially; and a surface dislocation must form either a closed loop on a surface, or else begin and terminate at a bulk dislocation. The utility of this approach is illustrated by considering a number of applications in real space, in particular imaging with topographic or diffraction contrast, where the concept allows a general means of classifying defects. In diffraction, the surface Burgers vector provides a convenient way of quantifying the effect of defects on peak profiles.

We report measurements of the electronic structure and surface morphology of exfoliated graphene on an insulating substrate using angle-resolved photoemission and low energy electron diffraction. Our results show that although exfoliated graphene is microscopically corrugated, the valence band retains a massless fermionic dispersion, with a Fermi velocity of ~10^6 m/s. We observe a close relationship between the morphology and electronic structure, which suggests that controlling the interaction between graphene and the supporting substrate is essential for graphene device applications., Comment: 10 pages of text, 4 JPEG figures

Self-assembled layers of 3-mercaptopropionic acid deposited on Au(1 1 1) from solution have been investigated with synchrotron based, high resolution X-ray photoemission spectroscopy. Results obtained from pristine, X-ray irradiated and heated samples are reported. Pristine samples are characterized by an intense, dominant S 2p state at a binding energy of 162 eV (2p3/2 level) assigned to thiolates. A less intense doublet around 163.3 eV is tentatively attributed to physisorbed molecules forming a partially occupied upper layer. Irradiation and heating cause the development of a third S 2p doublet around 161 eV which, by comparison with the literature, is assigned to atomic sulfur generated by S–C bond scission. The decrease of the C 1s and O 1s signals with annealing and irradiation, together with the increase of the S 2p state assigned to atomic sulfur, confirms the breaking of the C–S bond.

Self assembly on suitably chosen substrates is a well exploited root to control the structure and morphology, hence magnetization, of metal films. In particular, the Cu3Au(001) surface has been recently singled out as a good template to grow high spin Fe phases, due to the close matching between the Cu3Au lattice constant (3.75 Angstrom) and the equilibrium lattice constant for fcc ferromagnetic Fe (3.65 Angstrom). Growth proceeds almost layer by layer at room temperature, with a small amount of Au segregation in the early stage of deposition. Islands of 1-2 nm lateral size and double layer height are formed when 1 monolayer of Fe is deposited on Cu3Au(001) at low temperature. We used the PhotoElectron Diffraction technique to investigate the atomic structure and chemical composition of these nanoislands just after the deposition at 140 K and after annealing at 400 K. We show that only bi-layer islands are formed at low temperature, without any surface segregation. After annealing, the Fe atoms are re-aggregated to form mainly tri-layer islands. Surface segregation is shown to be inhibited also after the annealing process. The implications for the film magnetic properties and the growth model are discussed., Revtex, 5 pages with 4 eps figures

We have measured the long-range order of the {alpha} phase of Sn on the Ge(111) surface throughout the (3 x 3){yields}(3 x 3)R30{sup o} phase transition. The transition has been found of the order-disorder type with a critical temperature T{sub c}{approx}220 K. The expected three-state Potts critical exponents are shown to be consistent with the observed power-law dependence of the (3 x 3) order parameter and its correlation length close to T{sub c}, thus excluding a charge-density wave driven phase transition.

Szoke's concept for electron holography is hampered by forward scattering that dominates electron diffraction from electron point sources below the surface top layer. Forward scattering was proposed to be suppressed if the anisotropic nature of the electron source wave is exploited [T. Greber and J. Osterwalder, Chem. Phys. Lett. 256, 653 (1996)]. Experiments show a strong suppression of forward scattering in Al(111) if Al 2s photoelectrons (E-kin = 952 eV) are measured near the nodal plane of the outgoing p wave. The holographic reconstruction from such diffraction data provides three dimensional images of atomic sites in unit cells with a size of more than 10 Angstrom.

The extensive data available for He and Ne beam elastic scattering from Cu surfaces are analysed by full quantum mechanical calculations and by describing the atom-surface interaction potential according to the present knowledge on van der Waals pair-potentials. It is shown that Ne penetrates the surface electron cloud much deeper than He and that the ratio of Ne-to-He adsorption energies exceeds the ratio of the atomic polarizabilities.

The order-disorder phase transition of the Rh(lll)-(root 3 x root 3)R30 degrees-CO phase was investigated by He scattering. The critical temperature turned out to be 330+/-5 K, while the critical exponents are consistent with a phase transition belonging to the 3-state Potts universality class. The temperature dependence below the critical temperature of both CO phases was studied using elastic He scattering and LEED. These measurements demonstrate that a simple Debye-Waller evaluation of the diffraction data is not appropriate to provide information on the vibrational properties of the CO molecules. From LEED measurements at low temperatures (70 K) and various CO partial pressures there is evidence of the formation of a disordered second CO layer in equilibrium with the gas phase. The isosteric heat of CO adsorption in the second layer is about 6 kJ mol(-1).

The UHV deposition of alpha-hexathiophene (T6) oligomers on the 1x2 reconstructed Au(110) surface has been studied in situ. The evolution of the surface morphology during the deposition is followed by the He atom scattering method. A proliferation of Au monoatomic steps is observed during deposition, indicating a strong interaction between Au substrate and T6 molecules. The build-up of a well-ordered single monolayer is obtained with a rectangular unit cell. Its dimensions indicate that T6 molecules lie on the surface, aligned along the rows of close-packed Au atoms.

Thermal energy deuterium beam scattering (TEDS) from Rh(110) surface is reported and compared to helium atom scattering (HAS). Unlike for He, we find quite strong D-2 off-specular diffraction. The D-2 diffraction intensities are fairly isotropic along the [001] and [1 (1) over bar 0], reflecting the presence of a finite dissociation barrier in the potential energy surface (PES) rather than the anisotropic corrugation of the relevant Rh(110) surface electron density. Inelastic D-2 scattering with strong J=0-->2 kinetic-to-rotational energy conversion is also presented and discussed.

The formation of an alpha-hexathienyl (T6) single layer on the (1 x 2) Au(110) surface has been investigated by means of thermal He atom scattering (THAS) in ultra-high-vacuum (UHV) conditions. The monolayer (ML) structure has a rectangular surface unit cell with the T6 molecules lying flat on the surface and aligned parallel to the Au rows. The periodicity perpendicular to the Au rows is doubled with respect to the bare Au surface, while along the rows it is 27.4 +/- 0.5 Angstrom, corresponding to the molecular length. The correlation length of the overlayer structure depends upon substrate temperature during deposition and increases when the system is heated up to 653 K. The surface structure is irreversibly lost upon heating to temperatures higher than 673 K, but T6 molecules do not completely desorb up to about 773 K.

A helium-atom scattering experiment is reported on the evolution of vacancy islands produced by low-energy ion bombardment on InSb (110). It is seen that island growth occurs through dynamical scaling during the sputtering process, in agreement with the scaling hypothesis that is the basis of nucleation and growth models. After interruption of the sputtering, the spatial distribution of steps on the surface (strictly related to the island size and distance) gradually modifies during the self-organizing surface recovery. In this case the surface reduces its free energy by decreasing the step density; the corresponding increase of the terrace width occurs in the absence of free vacancy supersaturation and the process is diffusion limited. This change of the growth mechanism causes the breaking of dynamical scaling.

The electron density of the (1×2)Pt(110) surface is determined by He reflectivity measurements and compared to the electron densities previously reported for (1×2)Au(110) and (1×2)Rh(110). With respect to the superposition of free atomic electron densities, significant charge depletion and enrichment are observed, respectively, on top of the surface atoms and in the threefold sites of the (111) microfacets.

He beam scattering measurements of the erosion of the InSb(110) surface by low-energy Ar+ bombardment are reported. Layer-by-layer erosion is observed for surface temperatures above 510 K and is found to proceed by nucleation of vacancy islands, island growth, and coalescence. The average island distance is measured at different stages of the erosion process and it is found to evolve in good agreement with the rate equations derived from models of island growth by atomic deposition. A general increase of the average terrace width is also observed with increasing number of removed layers. The activation barrier for intralayer diffusion of InSb dimer vacancies is found to be Ed=1.14±0.06 eV.

The evolution of surface defects created by low energy ion bombardment on InSb(110) is studied by He beam scattering experiments. At surface temperatures above 500 K the ion erosion is found to proceed layer-by-layer without changing the stoichiometric composition of the surface. This indicates that the nucleation of surface islands driven by vacancy diffusion is faster than the ion erosion. After different doses of Ar + ion bombardment, the surface is left in a non-equilibrium state whose time evolution is monitored by measuring the profiles of the He diffraction peaks. It is seen that the coalescence of islands proceeds with anisotropic island size distributions.

A new model potential for van der Waals atomic pairs is proposed and shown to be in excellent agreement with ab initio calculations and experimental potentials. The model allows the potential energy of a given pair to be estimated with good accuracy from ab initio density functional calculations of the free‐atom electron densities.

The (1\ifmmode\times\else\texttimes\fi{}2)-Au(110) surface has been studied using He-beam scattering with time-of-flight detection. The experiments included measurement of the diffraction pattern and the specular intensity as a function of azimuthal angle of incidence; the latter curve is found to be highly structured. The data have been modeled using the local-density approximation to calculate the charge distribution in atoms, and treating the He-surface potential as a superposition of pseudopairwise terms. The diffraction pattern and azimuthal data can be well fitted using close-coupled channel calculations and varying only three parameters, giving confidence that the calculated electron density is accurate. It is found that the peak-to-peak corrugation of the ${10}^{\mathrm{\ensuremath{-}}3}$ A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}3}$ isoelectron density surface is \ensuremath{\approxeq}1.2 \AA{}, showing a substantial smoothing with respect to the corrugation of 1.55 \AA{} found when superposing the calculated free atomic densities. In addition we have examined the sensitivity of the model calculations to geometric structure. They are consistent only with the expected missing row structure, and yield the separation between the first and second layers with an accuracy of 6%.

Adsorption of half a monolayer of oxygen on the Rh(110) substrate gives a surface with a (2 × 2) p2mg symmetry structure, in which the substrate has a missing row (1 × 2) structure. He-beam scattering measurements show that close to 750 K this surface structure reversibly transforms into a phase of (1 × 2) symmetry. The phase transition has been classified as belonging to the order-disorder type, and falls into the 2D Ising universality class. Above the transition temperature proliferation of antiphase domain boundaries along the [001] direction is observed and discussed.

Measurements are reported of the surface core level shifts of the clean Rh(110) surface, the clean metastable (1 × 2) surface, and NO and oxygen covered surfaces. The shifts are discussed in terms of a Born-Haber cycle formalism, and also within an initial state model of charge transfer after adsorption. Thermodynamic data are derived and it is shown that the heat of adsorption of NO is higher on Rh than on Pd. In addition, the valence level electronic structure of NO adlayers has been examined by Cooper minimum spectroscopy, and the molecular orbitals of NO, adsorbed oxygen and adsorbed nitrogen are reported. Near edge X-ray absorption spectra (NEXAFS) of molecular NO are reported, and the molecule is found to be adsorbed with its axis normal to the surface.

An sp type surface state has previously been reported at the Y point of the surface Brillouin zone of the Au(110) surface, and was associated with a gap in the calculated projected density of states of the unreconstructed surface occurring at the Y point of the (1 × 1) surface. More recent calculations, taking account of the true (1 × 2) structure of the surface, predict that there is no surface state at the Y point. We have carried out a detailed experimental investigation of the electronic structure of this system using angle-resolved photoemission excited by He I, Ne I and Ar I radiation and find no evidence for a surface state at the Y point, in accord with the calculations. A feature observed at the Fermi level with He I radiation is interpreted as photoemission from the d-band excited by He I β radiation. Features observed with Ne I and Ar I radiation in various parts of the Brillouin zone are interpreted as direct transitions in the bulk. This indicates that the reconstruction changes the electronic structure of the surface so that a surface state in the projection of the I gap does not occur.

The deconstruction of the Au(110) surface, i.e., the (1 × 2) → (1 × 1) phase transition, has been studied by He beam scattering. A critical temperature of 700 ± 5 K is indicated by the temperature dependence of the half-integral diffraction intensities, and the incoherent elastic intensity. Above 700 K the peak width variation is consistent with Ising-like behaviour, where the main contribution to lateral disorder comes from domain walls. It is also demonstrated that the roughening temperature is substantially larger than the deconstruction temperature. The tails of the specular peak decay as a function of parallel momentum according to a power law with exponent −1.55 ± 0.1 at 723 K, indicating that the roughening temperature is higher.

The authors describe a compact He beam apparatus developed for high resolution studies of surface structures by He diffraction and of surface vibrations by He energy-loss spectroscopy. The angular resolution is 0.135 degrees FWHM (rotation of the sample) and the He beam intensity is 1019 atoms sr-1 s-1 at the sample. The apparatus is also equipped with Auger and ultraviolet photoemission spectroscopy. Measurements of He diffraction patterns and He energy loss spectra are reported from the (1*2) Au(110) surface and provide a full characterization of the apparatus.

Coincidence measurements have been extensively performed in atomic and molecular physics since early 1970s. To apply this methodology to solids and surfaces has been a major target since early days, but the long average time needed to complete a coincidence experiment has hampered its attainment. In particular the coincidence technique has not been yet applied in an angle resolved way such for studying the momentum correlation in the ejection of electron pairs from solid surfaces. The experimental chamber at the ALOISA beamline at Elettra. by means of a set of seven homemade electron analyzers, is the first apparatus able to perform Angle Resolved - Auger Photoelectron Coincidence Spectroscopy (AR-APECS) from solid surfaces. In the typical setup ten different pairs of coincident electrons can be measured simultaneously, so reducing the acquisition time by one order of magnitude. (C) 2001 Elsevier Science B.V. All rights reserved.

We show how the anisotropy of the (1 x 2) reconstructed Au(110) surface can be effectively used to achieve spontaneous ordered planar growth of rodlike conjugated oligomers uniaxially oriented on the substrate. The growth of the first few monolayers of alpha-sexithienyl (T6) films on the (1 x 2) reconstructed Au(110) surface in ultrahigh vacuum has been investigated by He atom scattering (HAS), scanning tunneling microscopy (STM), and low-energy electron diffraction (LEED). T6 molecules lie flat on the substrate forming long chains along the [110] direction of the Au rows. The interchain spacing is coverage dependent and uniaxially commensurate with the substrate along [001]. Three ordered structures with 12, 16, and 20 Angstrom periods are formed during the deposition of the first two T6 monolayers. This T6 overlayer acts as a template for the growth of at least five monolayers which are still planar and orientationally ordered.

We report on the Structural and magnetic properties of thin Fe films grown on the Cu(1 1 0) surface. In-plane grazing-incidence X-ray diffraction has been Used to Measure the lateral lattice spacing of the Fe films. Complementary information about the structure of the topmost layers has been obtained by means of Auger and photoelectron diffraction. The Fe film grows pseudomorphic with the substrate up to a thickness of about 0.8 nm. The diffraction feature of a new phase is observed at 1.6 nm, with a corresponding interplanar distance close to the bulk body centered cubic (bee) Fe one, which is eventually recovered at higher thickness (6.4 nm). From comparison between X-ray diffraction and photoelectron diffraction, it is suggested that the bcc-like Fe grows on the (1 0 0) surface with its [1 1 0] axis oriented along the [0 0 1] substrate direction. The photoelectron diffraction data also indicate a strong faceting in the [1 1 0] substrate direction. This morphology is believed to contribute to the in-plane uniaxial magnetic anisotropy observed by Kerr effect and Brillouin light scattering from spin waves.