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We reinvestigated non-dissociative chemisorption of O 2 on Ag(001) by the King and Wells reflectivity method and high-resolution electron energy loss spectroscopy over a wide range of impact energies of O 2 . In analogy to O 2 /Ag(110), we find that the sticking probability has a maximum at a total energy of 0.7 eV corresponding to the onset of direct inelastic scattering from the repulsive part of the chemisorption potential. Below it, normal energy scaling is approximately obeyed, whereas above it, a breakdown of such behaviour is observed. The decrease in sticking probability at high impact energy is more pronounced for O 2 /Ag(001) than for O 2 /Ag(110).

The coverage dependence of the vibrational modes of O2 chemisorbed on Ag(001) was studied by electron energy loss spectroscopy. We observe the dipole active modes associated to the intramolecular stretch and to the molecule–surface vibrations. Moreover two further features are present in the EEL spectra, due to the anharmonicity of the molecular potential: the combination-band of the stretching and of the molecule–surface vibration, and the first overtone of the O–O stretching mode. From the EELS intensity of the O–O stretching mode versus coverage the dynamical charge transfer e*=0.61±0.02 e− is determined which corresponds to a dynamical dipole moment μ=0.16±0.01 D. The O2–Ag vibrational frequency is found to decrease with coverage, suggesting that a weakening of the molecule–surface bond takes place. This observation is confirmed by thermal desorption spectroscopy. The anharmonicity parameter xe of the O–O potential increases significantly upon chemisorption, in analogy to O2–Ag(110) and O2–Pt(111). Th...

We show that in presence of subsurface oxygen, CO oxidation of oxygen precovered Ag~001! modifies the surface in such a way that CO adsorption is stabilized up to crystal temperatures of 160 K. When diffusion is activated new surface oxide phases form, characterized by oxygen both in surface and in subsurface sites and by a high density of states just below the Fermi level. Subsurface oxygen accumulation might therefore be relevant for the understanding of the origin of the pressure gap observed for ethylene epoxidation and other important catalytic reactions.

We have built an apparatus for the measurement of high resolution diffractive scattering of hydrogen atoms from crystal surfaces. The apparatus comprises a hydrogen atom beam source, a hexapolar magnetic field velocity selector, a variable temperature UHV crystal manipulator, and a rotatable bolometer detector. The diffraction pattern of a beam of hydrogen atoms scattered by a (001) LiF surface at 78 K has been obtained for different angles of incidence and different orientations of the crystal. The Debye–Waller factor has been measured leading to a surface Debye temperature ϑS=550±38 K. The corrugated‐hard‐wall‐with‐a‐well model of Garibaldi et al. [Surf. Sci. 48, 649 (1975)] has been used for the interpretation of the intensities of the diffracted peaks. By means of a best fit procedure we obtain a main ’’corrugation’’ parameter ξ0=0.095 A. By comparison of the data with the theory of Cabrera et al. [Surf. Sci. 19, 70 (1967] at the first order, the strength parameters of a periodic Morse potential have ...

We have investigated desorption and dissociation of O2 chemisorbed on Ag(001) induced by collision with hyperthermal Xe and Ar atoms provided by a supersonic molecular beam. We find that the cross section for both processes increases rapidly with impact energy and with angle of incidence of the inert gas atom. A model is presented which accounts for the observed energy threshold and for the trend in the cross section.

Using atomic force microscopy and spot-profile analyzing low energy electron diffraction, we have observed the existence of a striking faceting instability in Al(110) homoepitaxy, characterized by the formation of nanocrystals with well-defined facets. These hut-shaped nanocrystals are over tenfold higher than the total film coverage, and coexist in a bimodal growth mode with much shallower and more populous surface mounds. We further use density functional theory calculations to elucidate the microscopic origin of the faceting instability, induced by surprisingly low activation barriers for adatom ascent at step edges and island corners.

We report evidence for the occurrence of sub-surface oxygen incorporation during the dissociation process of admolecules. The process is observed for O 2 chemisorbed on Ag(001) at a surface temperature of 100 K, where thermally activated sub-surface diffusion is inactive. The phenomenon occurs both when dissociation is induced thermally or by electron bombardment. EELS peaks corresponding to the vibration of adatoms and of sub-surface oxygen are observed.

The time evolution of the morphology of the Cu(110) surface during ion sputtering has been studied in situ and in real time by x-ray-based techniques. The surface was bombarded with ${\mathrm{Ar}}^{+}$ ions at an energy of 1 keV in the temperature range 100\char21{}320 K; the ion incidence angle was set out of the x-ray scattering plane in order to induce asymmetric nanostructures. The results show that the ripples or mounds formed by the ion sputtering evolve in time, changing the spatial periodicity and the local slope of the facets. The latter is strongly dependent on temperature; moreover the facets along the 〈1-10〉 direction evolve in a different way from those along 〈001〉.

The evolution of the Cu(110) surface morphology during low temperature (180 K) ion sputtering was studied as a function of the incident ion beam angle $\ensuremath{\theta}$ by means of scanning tunneling microscopy. The morphology was dominated by a ripple structure with the wave vector parallel or perpendicular to the direction of the incident beam. The time evolution of the interface shows that the ripple wavelength increases in time following a scaling law $\ensuremath{\lambda}\ensuremath{\propto}{t}^{z}$, with $z\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.26\ifmmode\pm\else\textpm\fi{}0.02$. These results are ascribed to the effect of a Schwoebel barrier on the interlayer diffusion of the recoiling atoms produced during ion sputtering.

O 2 adsorption on Ag(001) at 100 K has been investigated by HREELS. Contrary to previous reports we could resolve 4 oxygen related peaks, 3 of which are due to the internal stretching vibration as confirmed by isotope labelling. The lowest frequency mode at 63 meV is assigned to molecular oxygen chemisorbed at defect sites. The further two features are very close to each other in frequency, at 79 and 84 meV. We assign them to molecules chemisorbed in a peroxidic state in two different adsorption sites. Initially both sites are filled simultaneously while at higher coverage adsorption in the higher frequency site is favoured.

The morphology of the Ag(110) surface after ${\mathrm{Ar}}^{+}$ ion sputtering has been studied by a variable temperature scanning tunneling microscope. For ion energies greater than 800 eV and in the temperature range $270l{T}_{S}l320\mathrm{K}$ (where ${T}_{S}$ is the sputtering temperature), a peculiar ripple structure well aligned along the $〈1\overline{1}0〉$ direction and independent of the ion beam angle has been observed. A simple model of this effect, based on the anisotropy and on the temperature hierarchy of the Ag adatom and vacancy surface diffusion, explains the observed features.

The morphology of C60 films deposited on a Ag(001) single crystal has been studied using an UHV-STM. The C60 molecules are arranged in a quasi-hexagonal c(6 x 4) structure, having the long axis along the (110) direction of the Ag surface. Due to the existence of two equivalent directions, the C60 film presents different domains, separated by dislocation lines. In each domain, peculiar structures of bright C60 molecules have been resolved. These structures are due not to geometric effects, but to nonhomogeneous charge distributions and different chemical bonding with the Ag substrate. (C) 1998 Elsevier Science B.V. All rights reserved.

We investigated O2 adsorption on Ag(001) in the presence of defects induced by Ne+ sputtering at different crystal temperatures, corresponding to different surface morphologies recently identified by scanning tunneling microscopy. The gas-phase molecules were dosed with a supersonic molecular beam. The total sticking coefficient and the total uptake were measured with the retarded reflector method, while the adsorption products were characterized by high resolution electron energy loss spectroscopy. We find that, for the sputtered surfaces, both sticking probability and total O2 uptake decrease. Molecular adsorption takes place also for heavily damaged surfaces but, contrary to the flat surface case, dissociation occurs already at a crystal temperature, T, of 105 K. The internal vibrational frequency of the O2 admolecules indicates that two out of the three O2− moieties present on the flat Ag(001) surface are destabilized by the presence of defects. The dissociation probability depends on surface morpholo...

In metals, the surface curvature dependence of the sputtering yield and the presence of an extra energy barrier whenever diffusing adatoms try to descend step edges, produce a similar surface instability, which builds up regular patterns. By tuning the competition between these two mechanisms, it is possible to create self-organized structures of the size of few nanometers. Height, lateral distance and order of the structures change with the deposition parameters like ion energy, dose, incident angle and substrate temperature. The paper offers an overview of the experiments carried out and foresees possible applications of these results in the area of material science.

We have investigated the homoepitaxial growth of Ag(110) in the multilayer regime. After deposition of 30 monolayers of Ag at a temperature of 210 K a ripplelike surface instability is produced and the ridges of the ripples, as well as the majority steps, are found to be parallel to11;0which is the thermodynamically favored orientation. As the deposition temperature is decreased to 130 K, an unexpected 90 degrees switch of the ripple orientation is observed. The ridges of the ripples and the steps are in this case parallel to100. In the intermediate temperature range a checkerboard of rectangular mounds results. We interpret our results in terms of the peculiar hierarchy of interlayer and intralayer diffusion barriers present on the anisotropic Ag(110) surface.

Surface morphologies of nanostructured carbon films, produced by supersonic cluster beam deposition, have been investigated by atomic force microscopy. From topographical images, we evaluated the surface roughness corresponding to different scan lengths and thicknesses, and deduced that the images examined are self-affine. Experimental scaling exponents and the hypothesis of non-destructive primeval cluster aggregation suggest that quenched noise effects may be important in the process of growth of the nanostructured surfaces.

The first stages of aggregation of low energy deposited carbon clusters on Ag(1 1 0) surface have been observed in situ by scanning tunnelling microscopy (STM). Sub-monolayer coverages of the substrate at room temperature have been obtained by using a supersonic neutral cluster beam with a mass distribution centered at an average of 900 atoms/cluster. We observe that clusters are able to diffuse on the surface and along steps, and organise themselves in compact islands principally located at step edges. Size of supported islands ranges from a few nm up to 40 nm. A statistical analysis based on several STM images shows that the island size follows a lognormal distribution law. Possible mechanisms involved in the diffusion and coalescence of carbon clusters on metallic surfaces are discussed.

In recent years there has been a rapid increase in nanotechnology applications to medicine in order to prevent and treat diseases in the human body. The established and future applications have the potential to dramatically change medical science. The present paper will give a few examples that could transform common medical procedures.

We investigated the dynamics of the adsorption of 02 on Ag(001) with the supersonic molecular beam technique combined with EEL spectroscopy, TDS and the reflection detector technique. The initial sticking coefficient S O was measured as a function of the impact energy and of the angle of incidence of the molecules at crystal temperatures of 100 and 300 K, corresponding to molecular and dissociative adsorption, respectively. Both regimes are characterized by the same distribution of energy barriers. This indicates that, in analogy to the case of our previous investigation of O2-Ag(110), the main reaction path involves activated adsorption into the molecular well, followed eventually by thermally induced dissociation for T~> 150K. Again in analogy to O2-Ag(II0), the physisorbed state plays no role in the chemisorption process.

Total differential collision cross sections for the scattering of H atoms from ethane (C/sub 2/H/sub 6/), propane (C/sub 2/H/sub 8/), acetylene (C/sub 2/H/sub 2/), and O/sub 2/ have been measured and information on the spherical and nonspherical parts of the interaction potential extracted by means of scattering calculations which make use of the infinite-order sudden (IOS) approximation. The interactions of three other systems, H + Ar, H + CH/sub 4/, and H + CO, previously measured in our laboratory have been calculated by means of hybrid Hartree-Fock plus damped dispersion (HFD) energy calculation and found to be in good agreement with the experimental potentials. A model is then constructed to derive the main features of the interaction of H atoms with the higher hydrocarbons from the knowledge of the H + CH/sub 4/ interaction. The predictions of this model are found to be in excellent agreement with the experimental parameters for both the spherical and nonspherical parts of the potentials. 4 figures, 9 tables.

We investigated the dynamics of the adsorption of O2 on Ag(110) with the molecular beam technique combined with EEL spectroscopy and with the method of King and Wells. The initial sticking coefficient S0 is reported for molecules impinging along both high symmetry azimuthal directions 〈001〉 and 〈110〉 as a function of total energy and angle of incidence of the molecules and crystal temperature. The initial sticking coefficient is anisotropic for low as well as for room temperature adsorption. The dependence of the sticking coefficient on the temperature of the sample, Ts, indicates that adsorption takes place directly into the molecular well. Dissociation is eventually induced thermally for Ts≳150 K. The physisorbed state plays no role in the chemisorption process in the investigated impact energy range.

A simple system for measuring electrical resistance R versus temperature T in the range 1–20 K is described. It is based on the use of a lock-in amplifier SR 510 (Stanford Research) interfaced with a personal computer IBM AT. Measurements in the range 10 MΩ-1 kΩ dissipating 1 nW-0.1 pW with an accuracy better than 1% can be carried out on eight samples at the same time.

A new detector of ballistic phonons is presented. It is a P‐doped Si bolometer realized on top of a (100)Si crystal by ion implantation. The open circuit responsivity of the bolometer is 8000 V/W and the response time less than 50 ns. Time‐resolved spectra of ballistic phonons propagating in the same Si crystal along the 〈100〉 direction are reported.

Measurements of H-atom diffraction from monolayer and bilayer xenon on graphite are presented. Less complete measurements of krypton, nitrogen and carbon dioxide layers are also given. Because the potentials are quite well known in physisorption, these systems are ideally suited for accurate calculations. The comparison of the diffraction data to close-coupling calculations yields excellent agreement. The procedures developed in the analysis, including a new convolution technique, will be useful in extending the measurements to systems where the potential is less well known. The analysis of the temperature dependence of the diffraction is also accurately modeled, and may serve as a reference system for this type of measurements.

He scattering data from the (001) surface of graphite reveal the dispersion of three acoustic surface phonon branches. They are clearly associated with the three bulk phonon bands, but appreciable deviations from the respective bulk and edges are observed. In particular the Rayleigh wave is close to the band edge only in the long-wave limit, as expected from the continuum theory, and displays an anomalous stiffening starting from about one third of the Brillouin zone in the 〈100〉 direction.

In scattering measurements of atoms and molecules from surfaces the intensities of the diffraction and specular peaks contain contributions from quasi-elastic scattering events, in addition to the coherent elastic scattering. These may occur because of inelastic collisions with very small changes in energy and momentum and because of elastic collisions with random defects of the surface. In the present article it is shown how to separate the quasi-elastic contribution from the elastic one by measuring both angular and energy distributions of the scattered particles. As an example the procedure is used to study the scattering of Ne from LiF in order to measure the Debye-Waller Factor (DWF) at low surface temperatures. The analysis of the DWF temperature dependence yields detailed information on the nature of the interaction, and shows good agreement with the Levi-Suhl model for the DWF.

The NaCl(001) surface is studied using an H atom beam at energies of 76.3 and 86.3 meV. Elastic diffractive scattering is investigated under different incident angles and crystal orientations. Selective adsorption resonances are measured, determining experimentally the energy levels of the H atom bound to the surface. The energy spectrum is reproduced reasonably by a Morse potential with a well depth D = 29.2 meV and a range parameter k = 0.04 nm −1 .

Total differential collision cross section measurements for H+CO have been carried out at the relative energy of 870 K. The results have been analyzed using a simple spherical Buckingham-Corner potential. Previously published potentials for these systems are tested against the experimental results. It is suggested that one can use the present or similar data as a constraint for theoretically derived nonspherical potentials in order to arrive at more realistic predictions for the collision cross sections into the inelastic channels.

EELS measurements of the dispersion curve of the Rayleigh wave on Ag(100) are reported along the Γ - X direction up to the Brillouin zone border. The comparison of the measured spectra with the theoretical predictions by Castiel et al. and Black et al. shows that the surface is not contracted in contrast with the behavior of the same face of Cu and Ni. The sign and magnitude of the change of the surface force constants are therefore shown to be related not only to the surface structure but also to the (sp)-d hybridization of the electronic states at the surface.