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The development of electronic devices based on Silicon Carbide (SiC) has been strongly limited by the difficulties in growing high quality crystalline bulk materials and films. We have recently elaborated a new technique for the synthesis of SiC on clean Si substrates by means of supersonic beams of C60: the electronic and structural properties of the film can be controlled by monitoring the beam parameters, i.e. flux and particles energy and aggregation state. SiC films were grown in Ultra High Vacuum on Si(111)-7×7, at substrates temperatures of 800 ° C, using two different supersonic beams of C60: He and H2 have been used as seeding gases, leading to particles energy of 5 eV and 20 eV, respectively. Surface characterisation was done in situ by Auger and X-Ray photoelectron spectroscopy, as well as by low energy electron diffraction and ex situ by atomic force microscopy technique. SiC films exhibited good structural and electronic properties, with presence of defects different from the typical triangular voids.

We have developed a novel approach to the synthesis of SiC on clean Si substrates in ultra high vacuum (UHV), taking advantage of the features offered by a supersonic beam of C60. The kinetic energy, ranging from 0.5 up to 60 eV, is much higher than that of standard evaporation techniques (about 0.05 eV). We show that kinetic activation allows the formation of Si–C bonds even at substrate’s temperatures lower than 800 °C. Films have been grown on the Si (1 1 1)-7×7 surface, at two different kinetic energies of the C60 supersonic beam (5 and 20 eV) and at a substrate temperature as low as 750 °C. They have been characterized by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and UPS techniques. Electronic and structural properties of films can be controlled by the beam parameters. SiC films synthesized with precursors at 20 eV of kinetic energy are ordered and give rise to good LEED diffraction patterns.

We performed an extended X-ray absorption fine structure (EXAFS) investigation of ultrathin Fe films grown in situ. on Ni(1 0 0) surface. From the first shell analysis of the EXAFS oscillations we obtained values of the bond length, of the effective coordination number and of the Debye-Waller factor that are compatible with a model where there is coexistence of face centred cubic phase (with tetragonal distortion) and of body centred cubic (bcc) phase in the films. We also performed theoretical simulations of the EXAFS data including signals from higher order shells with multiple scattering effects. From these simulations we obtained that the experimental data can be well reproduced with a linear combination of spectra, one reproducing the bulk bcc Fe structure and one obtained by a face centred tetragonal (fct) lattice structure with an interlayer distance d = 1.9 A. In the model the relative weights of the two spectra change with an increasing value for the weight corresponding to the bcc phase. At 20 ML, the weight for the fct phase (30-40%) is still considerable. (C) 2001 Elsevier Science B.V. All rights reserved.

The interplay between atomic geometry and magnetic properties of Fe atoms at the Fe–Ni(1 1 1) interface was studied. To this end dichroic optical absorption and reflectivity (both in energy and θ–2θ angular scan mode) were carried out in the L 2 , 3 edge region of Fe in (Fe 6 A /Ni 24 A (1 1 1)) 10 multilayer while the atomic geometry of the Fe site at the interface was studied for 1 atomic layer of Fe on Ni(1 1 1) by photo-electron diffraction (in angular scan mode and back-scattering regime). The Fe atom was found to be chemisorbed in an fcc hollow site and to stack pseudomorphically to Ni(1 1 1). Its magnetic moment was found to be in the range 2.9±0.9–1.93±0.6 μ B . Possibility of surface anisotropy is discussed.

A new electron bombardment evaporation source for ultrahigh vacuum (UHV) thin film deposition is presented. It is based on an original electrodes configuration (modeled on that of a vacuum triode) featuring, besides anode and filament, a biased grid acting as a power modulator. The presence of a polarized grid helps to minimize space charge problems, to improve electron focusing and to fine control the dissipation on the evaporating material, governing the sublimation rate. The electrodes geometry was optimized by studying electron trajectories and electrical field distribution in the electrodes zone with ray-tracing analysis. Mechanical solutions and electrodes geometry were designed to ensure low outgassing and UHV compatibility, as well as to facilitate maintenance and source cleaning. The evaporator is water cooled and a mechanically operated shutter is present. The working principles and the technical details are presented together with operating data and evaporation performances.

We investigated the evolution during film growth of the electronic properties of Fe/Ni(111) ultrathin films with electron energy loss spectroscopy from the valence band and from the Fe L 2,3 ionization edges, relating them with structural modifications monitored with low energy electron diffraction. At ca. 4 monolayers of Fe a fcc to bcc phase transition was observed with the major changes in the electronic structure occurring in the 3d band region.

In this paper, we report on the fabrication of N,N'-1H,1H-perfluorobutil dicyanoperylenediimide (PDIF-CN2) organic thin-film transistors by Supersonic Molecular Beam Deposition. The devices exhibit mobility up to 0.2 cm(2)/V s even if the substrate is kept at room temperature during the organic film growth, exceeding by three orders of magnitude the electrical performance of those grown at the same temperature by conventional Organic Molecular Beam Deposition. The possibility to get high-mobility n-type transistors avoiding thermal treatments during or after the deposition could significantly extend the number of substrates suitable to the fabrication of flexible high-performance complementary circuits by using this compound. (C) 2014 AIP Publishing LLC.

Photodynamic Therapy (PDT) is a therapeutical approach in the cancer treatment, which consists in the activation of a photosensitizer (phorphyrins) with visible light in order to produce singlet oxygen, to exert a cytotoxic activity towards cancer cells. The use of visible light limits the PDT application only to shallow deseases, for this reason we propose a new "X­Ray induced PDT" approach, using nanohybrid systems as photosensitizer, consisting in Silicon Carbide (SiC) nanowires (NWs) functionalized with organic molecules (fluorinated porphyrins). Modification of the inorganic semiconductor surface with organic or bio­molecules represents the route to activate processes at the interface and can be achieved by functionalizing the SiC NWs with porphyrins, that showing a good match between the organic absorption Q band and the 3C­SiC near­band­edge­optical emission. In fact SiC NWs have interesting light emission properties [1] so it is promising the idea to couple this light emission with an organic absorber showing strong fluorescence properties, a viable route to increase the optical emission efficiency as well as to promote the anchoring of biological groups. We demonstrated the functionalization of SiOx/SiC core shell NWs, grown by a carbothermal method, showing enhanced fluorescence, with fluorinated porphyrins H2TPP(F) by supersonic molecular beam deposition (SuMBD), an approach that can promote and activate chemical/physical processes at the interface by means of the organic precursor kinetics properties [2]. The H2TPP(F)/SiC­NWs system has been deeply investigated in­situ with surface photoelectron spectroscopy and ex­situ by Cathodoluminescence (CL) to clarify the growth kinetics at low coverages and the interface processes. Results concerning the core level (C1s, Si2p, N1s, F1s) analysis at different growth steps on planar oxidized surface and SiOx/SiC core shell NWs will be presented. The role of morphology of inorganic surfaces together with kinetic activation of H2TPP(F) molecules in molecular beams will be discussed.

A study of electron capture and loss processes during the scattering of H, O, and F atoms and anions on a MgO(100) surface is described. Large fractions of anions in the scattering of atoms are observed, indicating the existence of an efficient electron capture process. This is ascribed to a nonresonant, localized charge exchange mechanism between an atom and a MgO lattice oxygen anion. This charge transfer becomes possible because of anion level shifts in the Madelung field. The existence of an electron loss channel is demonstrated using incident anions and is ascribed to loss to the conduction band or Mg cations. {copyright} {ital 1997} {ital The American Physical Society}

Changes in electron capture rates during the oxidation of metal surfaces are investigated on the example of F− formation in fluorine ion/atom scattering on a Mg surface exposed to O2 and also for MgO(100). In the low-coverage, chemisorption range, F− formation decreases in spite of a decrease in the surface work function. This is assigned to a local effect due to specifics of the electronic structure at the adsorbate site, which is akin to surface poisoning effects. At high exposures corresponding to oxide formation electron capture is very efficient and can be understood in terms of a quasi-resonant localised charge exchange mechanism between a F atom and a MgO lattice O anion.

We studied the Auger electron emission induced by energetic (600 and 900 eV) atoms bombardment in oblique incidence on Si films from submonolayer to 4 monolayers (ML) thickness. Atomic and bulk contributions to the Si Auger emission were separately determined, and show a different trend versus film thickness. We ascribe these differences to the interplay between processes (different mechanisms of sputter ejection, and the Auger decay) that occur over different timescales. Atomic emission originating in prompt, projectile-target collisions is maximum at low coverage. Bulk emission intensity increases with coverage, as the cascade regime (target-target collisions) fully develops. Surface sensitivity has been found to be significantly larger for particle-induced Auger electron emission than for the conventional electron-induced Auger emission.

We studied the Auger electron emission induced by Si atom bombardment in grazing incidence near the ionization threshold (800 eV) in Si films on a Ge substrate, from submonolayer to 4 monolayers (ML) coverage. Atomic-like and bulk-like contributions to the particle induced Si Auger emission were separately determined. We found that the atomic-to-bulk yield ratio is strongly dependent on the coverage in the 0–2 ML range. At low coverage, atomic emission originating in prompt, projectile-target collisions largely prevails. At increasing coverage, the cascade collisional regime (target-target collisions) leads to a predominance of bulk emission. A very large fraction (≥ 60%) of this bulk emission has been found to originate in the outermost Si layer.

We studied in detail the electronic properties of C44H10F20N4 (tetrakis(pentafluorophenyl)porphyrin, hereafter H2TPP(F)) via a combined study by photoelectron spectroscopy (PES) and density functional (DF) calculations, shedding new light on the role of the halide in this very interesting molecule for organic electronics. Valence and core levels have been investigated by means of PES on H2TPP(F) thin films deposited on the SiO2/Si(100) native oxide surface by supersonic molecular beam deposition (SuMBD). These experiments have been carefully interpreted on the basis of DF results pertaining to the isolated H2TPP(F). Non-relativistic calculations have been run to investigate valence states, whereas a two component relativistic approach within the zeroth-order regular approximation has been adopted to study core levels. The present results, in conjunction with those obtained previously on the H2TPP parent compound [M. Nardi, R. Verucchi, C. Corradi, M. Pola, M. Casarin, A. Vittadini and S. Iannotta, Phys. Chem. Chem. Phys., 2010, 12, 871], pave the way towards designing fully organic p–n junctions by using these macrocycles.

In this work, we investigate the processes leading to the room-temperature growth of silicon carbide thin films by supersonic molecular beam epitaxy technique. We present experimental data showing that the collision of fullerene on a silicon surface induces strong chemical-physical perturbations and, for sufficient velocity, disruption of molecular bonds, and cage breaking with formation of nanostructures with different stoichiometric character. We show that in these out-of-equilibrium conditions, it is necessary to go beyond the standard implementations of density functional theory, as ab initio methods based on the Born-Oppenheimer approximation fail to capture the excited-state dynamics. In particular, we analyse the Si-C-60 collision within the non-adiabatic nuclear dynamics framework, where stochastic hops occur between adiabatic surfaces calculated with time-dependent density functional theory. This theoretical description of the C-60 impact on the Si surface is in good agreement with our experimental findings.

Silicon carbide (SiC) has unique chemical, physical, and mechanical properties. A factor strongly limiting SiC-based technologies is the high-temperature synthesis. In this work, we provide unprecedented experimental and theoretical evidence of 3C-SiC epitaxy on silicon at room temperature by using a buckminsterfullerene (C(60)) supersonic beam. Chemical processes, such as C(60) rupture, are activated at a precursor kinetic energy of 30-35 eV, far from thermodynamic equilibrium. This result paves the way for SiC synthesis on polymers or plastics that cannot withstand high temperatures.

In this paper, we discuss the surface doping in sexithiophene (T6) organic field-effect transistors by PDI-8CN2. We show that an accumulation heterojunction is formed at the interface between the organic semiconductors and that the consequent band bending in T6 caused by PDI-8CN2 deposition can be addressed as the cause of the surface doping in T6 transistors. Several evidences of this phenomenon have been furnished both by electrical transport and photoemission measurements, namely the increase in the conductivity, the shift of the threshold voltage and the shift of the T6 HOMO peak towards higher binding energies., 5 pages, 5 figures

Ni-silicides surface and subsurface altered layer induced by 1–14 keV Ar + bardment with incidence angle between 0° and 78° has been studied by Auger electron spectroscopy. Elemental standard method was used for quantification, and low energy sputter-profiles provided in-depth analysis of the high energy processed surfaces. Different incidence of the ion beam results in a weak but detectable change in the surface composition, and also in a different dependence of the surface composition on the ion energy. The subsurface Si depletion is larger as the ion beam approaches the normal incidence, while the altered layer thickness is independent of the impact geometry.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Porphyrins and their metal complexes are particularly well suitable for applications in photoelectronics, sensing, energy production, because of their chemical, electronic and optical properties. The understanding of the electronic properties of the pristine molecule is of great relevance for the study and application of the wide class of these compounds. This is notably important for the recently achieved in-vacuo synthesis of organo-metallic thin films directly from the pure free base organic-inorganic precursors in the vapor phase, and its interpretation by means of surface electron spectroscopies. We report on a combined experimental and theoretical study of the physical/chemical properties of tetraphenylporphyrin, H(2)TPP, deposited on the SiO(2)/Si(100) native oxide surface by supersonic molecular beam deposition (SuMBD). Valence states and 1s core level emissions of carbon and nitrogen have been investigated with surface photoelectron spectroscopies by using synchrotron radiation light. The interpretation of the spectra has been guided by density functional numerical experiments on the gas-phase molecule. Non-relativistic calculations were carried out for the valence states, whereas a two component relativistic approach in the zeroth-order regular approximation was used to investigate the core levels. The good agreement between theoretical and experimental analysis results in a comprehensive overview of the chemical properties of the H(2)TPP molecule, highly improving reliability in the interpretation of experimental photoemission spectra.

Hybrid Er-Tetraphenylporphyrin (Er-TPP) thin films have been synthesized on a SiO{sub 2}/Si(100) substrate in a co-deposition approach that combines an H{sub 2}TPP seeded supersonic molecular beam and an effusive molecular beam of the metal. As confirmed by detailed surface electron spectroscopies, our experiments in ultrahigh vacuum demonstrate the ability to produce and control the chemical reactivity between the two species on the surface of the growing film. This is achieved by fully exploiting the unique features of the supersonic molecular-beam deposition approach in a co-deposition scheme that we show to be ideally suited to induce and control chemical reactivity. We believe that the ability to synthesize organolanthanides films, showing a thickness that is suitable for applications in optoelectronics, could pave the way to a class of devices based on active materials chemically engineered via this solid-state/vapor phase approach in vacuum.

We performed a detailed, angle resolved study of the Ar + -induced Si LMM Auger electron emission from silicon single crystals in the 1–15 keV ion energy range. Attention has been focused on the Doppler shift of the atomic Unes, which arises from the motion of the decaying atoms with respect to the analyzer. It has been found to be a powerful tool to investigate the spatial and energy distribution of sputter-ejected atoms. The Doppler shifted contributions have been up to now associated with an anisotropic “spike” of fast atoms which originates from projectile-target collisions, superimposed on the nearly isotropic distribution of slow atoms which originate from collisions between target particles in the cascade. We report on new features in the shifted spectrum, whose behaviour vs ion energy suggests a more complex picture of the surface collisional processes and of the consequent fast particle ejection. The total yield, the relative weight of the atomic and bulk contributions to the SiL 23 derived Auger spectra and the behaviour of the asymmetric projectile-target collisions have been investigated as a function of the ion energy at different incident angles.

Hybrid organic-inorganic gas sensing devices have been developed by means of supersonic beam co-deposition of titania clusters and copper phthalocyanine. This technique allows controlling the cluster size and structure as well as the kinetic properties of organic precursors, thus enabling both the room-temperature growth of nanocrystalline Ti02 and its functionalization. Particularly the high kinetic energy transferred to the organic molecules is exploited to enhance reactivity at the interface between the two counterparts, resulting in the synthesis of a new material with interesting gas sensing properties. The hybrid devices show improved performances as to stability and sensitivity towards oxidizing and even nonoxidizing gases, which IS a remarkable result for metal-phthalocyanine based sensors. The role of the organic-inorganic interface in the development of such characteristics is briefly discussed.

Supersonic beams of organic molecules seeded in light carrier gases are a viable method to control the deposition of highly ordered thin films, to activate kinetically chemical reactions to functionalize surfaces or to synthesize hybrid compounds. The advantage of supersonic molecular beam deposition (SuMBE) with respect to the more standard OMBE/OMBD method is the control of the energetic parameters of the precursors that we have shown to give an unprecedented control on structural, morphological and hence functional properties in the deposition of organic thin films. The basic mechanism is envisaged to be a sort of local annealing activated by the kinetic energy of the molecules that collides with the surface. Depending on the molecular precursor used and on the kinetic energy achievable this same method can be exploited to activate reactivity at the surface and hence to synthesize different structures. Here we report on a few examples including the deposition of highly ordered organic thin films of pentacene, the synthesis of SiC at moderate temperatures and the functionalization of inorganic nanostructured materials such as TiO2.

We have developed a new technique for SiC heteroepitaxy on Si by means of supersonic free jets of C-60. The carbide synthesis can be achieved by the kinetic activation of the process. The electronic and structural properties of the film can be controlled by monitoring the beam parameters (flux and particle energy). SiC films were grown in Ultra High Vacuum on Si(l 11)7x7, at substrates temperatures of 800degreesC and 750degreesC, using two supersonic beams Of C-60 characterized by a particle energy of 5eV and 20eV. Surface electronic and structural characterisations were done both in-situ and the structural properties improved.

The growth of SiO x nanowires (NWs) with intense white emission is reported. Due to carbon monoxide gas being used as a dopant precursor, carbon-doped under-stoichiometric silicon dioxide NWs are obtained. The doping of the NWs is studied by means of x-ray photoelectron spectroscopy, which allows to assess the presence of carbon atoms in the silicon oxide amorphous structure. The light emission properties are studied by means of cathodoluminescence spectroscopy, which shows three main emission bands set at 2.7 eV (blue), 2.3 eV (green) and 1.9 eV (red), resulting in the white emission.

Fullerene (C60) was deposited on Si(1 0 0) 2 � 1 double domain reconstructed substrate. The interface has been treated with annealing procedure in order to fragment the C60 precursor and to induce covalent Si-C bond formation and to obtain carbidization. In this way SiC(1 0 0) films of thousands of Ahave been grown. Depending on the growth procedure different surface structures, 1 � 1o r 2� 1, have been obtained. The different stages of growth has been checked by in situ low energy electron diffraction (LEED), and Auger spectroscopy. Ex situ we verified surface order by means of LEED technique, observing either a 1 � 1o r a2 � 1 double domain reconstruction. We characterized electronic properties collecting valence band and core level spectra employing synchrotron radiation source. Valence band spectra showed evident electronic surface states similar to those revealed on SiC(1 0 0) surfaces grown by different techniques. # 2001 Published by Elsevier Science B.V.

Unprecedented room temperature excitonic emissions are achieved from TiO2 nanocrystals synthesized at 300 K by supersonic cluster beams. Transmission electron microscopy studies show the crystalline nature of the nanoparticles (NPs) with a diameter ranging from 5 to 30 nm. All the samples show mixed rutile and anatase phases as confirmed by Raman spectroscopy. XPS core level analyses evidence an O/Ti ratio of the as-grown nanoparticles of 2.30 +/- 0.04. Two room temperature cathodoluminescence excitonic peaks observed at 3.16 and 3.25 eV are ascribed to the coexistence of rutile and anatase crystallographic phases respectively. Subsequent thermal treatments at 450 degrees C cause the complete quenching of the UV excitonic emissions and result in a more conventional broad visible band centered at 2.5 eV. HRTEM and XPS studies reveal that, after annealing, the NPs remain single crystals in nature with an O/Ti ratio of 2.20 +/- 0.04. These results suggest a correlation between the emission properties and the oxygen concentration of our NPs. The achieved ability to tune the optical properties of TiO2 nanoparticles is very promising for sensing and energy applications.

We report the influence of the native amorphous SiO(2) shell on the cathodoluminescence emission of 3C-SiC/SiO(2) core/shell nanowires. A shell-induced enhancement of the SiC near-band-edge emission is observed and studied as a function of the silicon dioxide thickness. Since the diameter of the investigated SiC cores rules out any direct bandgap optical transitions due to confinement effects, this enhancement is ascribed to a carrier diffusion from the shell to the core, promoted by the alignment of the SiO(2) and SiC bands in a type I quantum well. An accurate correlation between the optical emission and structural and SiO(2)-SiC interface properties is also reported.

WSe2 is a layered ambipolar semiconductor enabling hole and electron transport, which renders it a suitable active component for logic circuitry. However, solid-state devices based on single- and bilayer WSe2 typically exhibit unipolar transport and poor electrical performance when conventional SiO2 dielectric and Au electrodes are used. Here, we show that silane-containing functional molecules form ordered monolayers on the top of the WSe2 surface, thereby boosting its electrical performance in single- and bilayer field-effect transistors. In particular, by employing SiO2 dielectric substrates and top Au electrodes, we measure unipolar mobility as high as μh = 150 cm2 V-1 s-1 and μe = 17.9 cm2 V-1 s-1 in WSe2 single-layer devices when ad hoc molecular monolayers are chosen. Additionally, by asymmetric double-side functionalization with two different molecules, we provide opposite polarity to the top and bottom layer of bilayer WSe2, demonstrating nearly balanced ambipolarity at the bilayer limit. Our results indicate that the controlled functionalization of the two sides of the WSe2 mono- and bilayer flakes with highly ordered molecular monolayers offers the possibility to simultaneously achieve energy level engineering and defect functionalization, representing a path toward deterministic control over charge transport in 2D materials., We acknowledge funding from the European Commission through the Graphene Flagship Core 2 project (GA-785219), the Marie Sklodowska-Curie IEF SUPER2D (GA-748971) and IEF-GALACTIC (GA-2014-628563), the Agence Nationale de la Recherche through the Labex projects CSC (ANR-10-LABX-0026 CSC) and NIE (ANR-11-LABX-0058 NIE) within the Investissement d’Avenir program (ANR-10-120 IDEX-0002-02), the International Center for Frontier Research in Chemistry (icFRC), as well as the Chinese Scholarship Council. M.V.N. and M.T. gratefully acknowledge the support by the CARITRO Foundation (project MILA, grant no. 2017.0369), Trento (Italy). E.O. is supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through an individual Discovery Grant and by Fonds de recherche du Quebec - Nature et Technologies (FRQNT).

Electron capture and loss processes on MgO (1 0 0) and oxygen covered Mg and Al surfaces are studied on the example of formation and destruction of H − , O − and F − in atom/anion scattering. Electron capture is demonstrated to occur very efficiently on MgO (1 0 0) and can be understood in terms of a localised non-resonant capture mechanism between the incident atom and a lattice O anion. Electron loss from incident negative ions is observed and is attributed to loss to the conduction band. Changes in capture rates on oxygen covered surfaces are studied from the limit corresponding to chemisorption to oxide film formation. In the chemisorption range capture is attenuated because of changes in the local density of states (LDOS) and in the positions and widths of the anion level near the adsorbate.

Results of a study of the modification in electron capture processes during the oxidation of metal surfaces are presented by the example of Oδ- formation in oxygen ion/atom scattering. Measurements of scattered-ion fractions were made for a clean Mg surface, which was then exposed to O2 and also MgO(100). It was found that a strong increase in capture occurs with increasing oxygen coverages from the submonolayer range to very high coverages corresponding to oxide formation. At low coverages these changes are interpreted as resulting from competing local effects at oxygen chemisorption sites and non-local effects corresponding to work function changes. The results for high exposures are similar to what was observed for the MgO(100) crystal. Here scattering on an ionic solid is discussed and non-resonant electron capture on the Oδ− site has to be considered after inclusion of level shifts in the Madelung field. Here a sequence of collisions, or sufficiently close approaches to Oδ− sites, resulting in non-resonant charge transfer, can lead to a build up the O− population even if the capture probability at a given site is low.

We present the results of a study of the modification in electron capture rates on metal surfaces with an electronegative impurity: oxygen. Electron capture is investigated on the example of O − and F − formation in oxygen and fluorine ion/atom scattering. O − and F − ion fraction measurements were made for Mg and Al surfaces exposed to O 2 . Changes in electron capture rates were continuously followed from the limit corresponding to a metal with submonolayer chemisorbed oxygen to those on an oxide film. A comparative measurement was made for an MgO(100) surface. It is shown that in the low coverage, chemisorption range local effects due to specifics of the electronic structure at the adsorbate site can strongly attenuate electron capture. This effect, akin to surface poisoning, may be attributed to changes in the local density of states and modification in the positions and widths of the anion level near the adsorbate. At high exposures corresponding to oxide formation electron capture is very efficient and can be understood in terms of a gas-phase-like, localized non-resonant charge exchange mechanism between the incident atom and, for example, an MgO lattice O anion. At intermediate coverages, the behaviour of the ion fractions is affected by appearance of oxide islands.