Search

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 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.

We present new measurements of He atom inelastic scattering from the (001) surface of Copper. Our results confirmed the recent data obtained in Gottingen and show that at large momentum transfer in the [Formula: see text] direction the intensity of the longitudinal resonance is larger than that of the Rayleigh wave. We interpret these results by evaluating the reflection coefficient in the Distorted Wave Born Approximation. By taking the He surface interaction potential as a sum of nonspherical pairwise potential we are able to explain quantitatively the behavior of the experimental spectra.

The electronic properties of a C70 single layer chemisorbed on Cu(1 1 1) surface, in which the charge state has been modified by Na doping, were studied via synchrotron radiation photoemission and absorption spectroscopy. A two-dimensional metallic phase is observed in the whole range of charge states investigated (from 1 up to 3 electrons/C70). Moreover, all the results suggest that the orientation of the molecules, with the C5v axis perpendicular to the surface, is not affected by Na doping.

The C 60 molecules are known to bind covalently to the Pt(1 1 1) surface. We performed a structural study of one C 60 layer on Pt(1 1 1) by using X-ray photoelectron diffraction (XPD). Our results show that the molecules are bonded to the metal surface with their six-carbon rings facing the Pt(1 1 1) surface and with the CC bonds perpendicularly oriented with respect to the direction of the close-packed atom rows. Unlike to what previously found for Cu(1 1 1) and Al(1 1 1), the measured diffraction pattern shows a three-fold symmetry indicating that only one orientation is taken by the chemisorbed molecules. This can be explained by a stronger interaction of the C 60 molecules with the second layer Pt atoms.

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.

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.

Here we demonstrate that a molecular dye film composed by a single layer Of C-70 and a single layer of Zn-tetraphenyl-porphyrin (ZnTPP) adsorbed atop can be grown on large substrates using molecular beam epitaxy. The electrostatic interactions between the constituent molecules drive the formation of this ordered donor-acceptor layer. The overlapping between the pi* orbitals of ZnTPP and C-70 allows the delocalisation of the excited electrons from the excited molecule on a timescale of similar to 6 fs. Generally, the ZnTPP/C-70 film, acting as a p-n junction, delocalise more efficiently the excited electrons as compared to pure ZnTPP films on a timescale that competes effectively against loss processes, suggesting its application for a more efficient solar energy conversion in organic-based devices.

The electronic properties and the molecular orientation of Zn-tetraphenyl-porphyrin films deposited on Si(111) have been investigated using synchrotron radiation. For the first time we have revealed and assigned the fine structures in the electronic spectra related to the HOMOs and LUMOs states. This is particularly important in order to understand the orbital interactions, the bond formation and the evolution of the electronic properties with oxidation or reduction of the porphyrins in supramolecular donor-acceptor complexes used in photovoltaic devices., text 11 pages, 4 figures submitted for publication

Photoelectron spectroscopy, low energy electron diffraction, and reflection high energy electron diffraction have been employed to probe the electronic and geometric structure of the TiO2(1 0 0)/K interface. Data have been obtained from two (1 0 0) oriented single crystal samples, which differ in their extent of bulk reduction. It is demonstrated that this parameter is important for both the electronic and geometric structure of ordered TiO2(1 0 0)/K surfaces, formed by annealing to ∼1000 K. Photoelectron spectra, recorded following this preparation, indicate that reduced Ti states are present for the less stoichiometric sample, which exhibits a 1 −1 2 2 surface unit cell. In contrast, the other sample displays a c(2×2) overlayer, with no Ti reduction evidenced.

Resonant photoemission at the Mn 2p --> 3d absorption threshold is used to probe the distribution of the Mn spectral weight in the valence band of Mn thin layers grown onto the CdTe (1 1 0) surface. It is shown that at the early stages of the evaporation process, a Cd-Mn-Te surface alloy is formed, even at room temperature. The Mn 3d spectral weight in the valence band, which is enhanced across the resonance, is quite similar to that observed for Cd1-xMnx Te single crystals. At larger Mn coverages, the growth of a metallic Mn phase on top of the Mn-Cd-Te surface alloy is detected both at resonance and by collecting Mn 2p core level photoemission data. (C) 2004 Elsevier B.V. All rights reserved.

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.

Photoelectron spectroscopy has been employed to probe the electronic structure of TiO 2 (1 0 0)c(2 × 2)K. In sharp contrast to earlier work, no features indicating the presence of Ti 3+ cations are found in the photoelectron spectra. This unexpected result is rationalised on the basis of ab initio periodic slab calculations. These calculations demonstrate that in this system K is fully ionised (K → K + ), and that the majority of the charge transferred across the TiO 2 /K interface resides on Ti cations, reducing them formally from Ti 4+ to Ti 3+ . However, the excess charge is not restricted to near surface Ti ions, but rather can occupy any Ti site throughout the bulk, and so is unobservable by photoelectron spectroscopy, due to the negligible concentration of Ti 3+ cations in the surface region.

NiO ultrathin films have been grown on Ag(001) by Ni deposition in an O-2 atmosphere. The thickness range 5-50 NIL has been investigated. X-ray photoelectron spectroscopy has been used to study the stoichiometric composition and chemical purity of the oxide films. We found completely oxidized stoichiometric NiO films. Their contamination has been found to be limited to the topmost layers. Photoelectron diffraction has given information concerning the local crystal structure of the films. The film atomic geometry has been found to be the same independent of thickness in the 0-50 ML range. The films have the expected (001) rock-salt structure with the same in plane orientation as the Ag(001) substrate. Specular X-ray reflectivity has allowed a very accurate thickness evaluation and has given information on the width of the density gradients at the film-substrate and vacuum-film interfaces, found to be of the order of a few atomic layers.

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 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.