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In this work, the decay of secondary-electron emission (SEE) intensity and charging of hydrogenated and hydrogen-free diamond film surfaces subjected to incident electron irradiation at energies between 5 and 20 eV are investigated. Electron emission curves as a function of incident electron energy were measured. For the hydrogenated films, it was found that the SEE intensity decays in intensity under continuous electron irradiation, albeit maintains a nearly constant onset. The decay in time of the SEE intensity was measured for various incident electron energies. From these measurements, the SEE intensity decay rate from the hydrogenated diamond surface was calculated as a function of incident electron energy and found to display a broad peak at ∼9 eV. The decay of the SEE intensity is explained as due to electron trapping in the near-surface region of the hydrogenated diamond films resulting in the formation of a depletion layer and upward surface band bending while overall charge neutrality is maintai...

H desorption cross-section and kinetic energy distribution (KED) from hydrogenated diamond surfaces stimulated by electron irradiation in the 5-45 eV range is investigated. It is determined that the desorption cross-section displays two peaks at 9 and 22 eV overlapping a monotonic increase for electron energies higher than 14 eV. The KED of the emitted ions was measured for several incident electron energies. These measurements show that for incident electrons of up to ∼1 eV the most probable kinetic energy of H ions monotonically increases from about 1.5 to 3 eV. For higher incident electron energies the ion energy distribution peaks at about 1.5 eV and is nearly constant. Ion desorption at specific energy as function of incident electron energy is also measured. These results suggest two different mechanisms for the desorption process: the mechanism of H electron stimulated desorption from hydrogenated diamond surface displaying peaks at 9 and 22 eV is driven by dissociative electron attachment, the other mechanism involves dipolar dissociation processes with a threshold at ∼14 eV.

In this paper an experimental investigation of the survival rates of negative fluorine ions scattered against an Ag(110) surface is presented. A theoretical analysis shows that the charge transfer dynamics can be attributed to resonant tunneling. We show that the results can be well described using a time-dependent Anderson model. We discuss the role of many-electron interactions and show that they must be included for an accurate modeling of the charge transfer.

L'irradiation d'un film d'oxygene moleculaire par des electrons conduit a la production d'ozone. O 3 a ete mis en evidence par spectroscopie de perte d'energie d'electrons (HREELS) qui permet d'observer les modes d'excitation vibrationnelle des molecules. L'identification de O 3 est confirmee par l'etude de l'effet isotopique. Le seuil energetique de formation est de l'ordre de 3,5 eV. Il correspond a la dissociation de O 2 par attachement d'un electron : 0 2 +e→O( 3 P)+O-( 2 P), suivi de la reaction spontanee de recombinaison: O + O 2 +O 2 →+O 3 +P 2 . A des energies superieures a 5,1eV, l'excitation de O 2 vers des etats de Rydberg et du continuum de Schumann-Runge est la principale source d'oxygene atomique et donc d'ozone. Des mesures en fonction du temps d'irradiation montrent que la quantite d'ozone forme sature.

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 have probed the interface of a ferromagnetic/semiconductor (FM/SC) heterojunction by a combined high resolution photoemission spectroscopy and x-ray photoelectron diffraction study. Fe/ZnSe(001) is considered as an example of a very low reactivity interface system and it is expected to constitute large tunnel magnetoresistance devices. We focus on the interface atomic environment, on the microscopic processes of the interface formation and on the iron valence band. We show that the Fe contact with ZnSe induces a chemical conversion of the ZnSe outermost atomic layers. The main driving force that induces this rearrangement is the requirement for a stable Fe-Se bonding at the interface and a Se monolayer that floats at the Fe growth front. The released Zn atoms are incorporated in substitution in the Fe lattice position. This formation process is independent of the ZnSe surface termination (Zn or Se). The Fe valence-band evolution indicates that the d-states at the Fermi level show up even at submonolayer Fe coverage but that the Fe bulk character is only recovered above 10 monolayers. Indeed, the Fe Delta(1)-band states, theoretically predicted to dominate the tunneling conductance of Fe/ZnSe/Fe junctions, are strongly modified at the FM/SC interface.

In this paper, we report on the influence of surface temperature on low-energy ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ electron stimulated desorption occurring via dissociative electron attachment from hydrogenated diamond films. By measuring the ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ kinetic energy distribution (KED) induced by electron bombardment in the 7--18 eV range for surface temperatures ranging between 100 and 450 K, we investigate the dynamics of the desorption process. It is determined that the ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ion yield continuously decreases with increasing temperature and that the most probable ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ kinetic energy shifts to lower energies. It is proposed that the effect of temperature on the ${\mathrm{H}}^{\mathrm{\ensuremath{-}}},$ KED and consequently, on the reduction in ion yield is predominantly due to an increase in the energy relaxation of the anion resonance and energy losses of the outgoing ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ion through interactions with the solid's multiphonon background and collisions.

In this work we report, a study of the mechanism of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ electron stimulated desorption (ESD) from hydrogenated diamond films for incident electron energies in the 2\char21{}45 eV range. Two types of experiments were carried out in order to assess the nature of the ESD processes leading to desorption as a function of incident electron energy: (i) kinetic energy distribution (KED) of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ and (ii) ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ anions yield at fixed ion energy (FIE) measurements. The KED measurements show that for incident electrons of up to \ensuremath{\sim}11 eV the most probable kinetic energy of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ions monotonically increases from about 1.7 to 3.3 eV. For higher incident electron energies, the ion energy distribution peaks at about 1.5 eV and is nearly constant. From these measurements it is derived that the ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ESD cross section has a resonance behavior displaying two well-defined peaks at 9 and 22 eV and a monotonic increase with a threshold at \ensuremath{\sim}14 eV as a function of incident electron energy. From the KED and FIE spectra the 9- and 22-eV peaks are interpreted as due to dissociative electron attachment via a single Feshbach anion resonance state, albeit accessed directly and indirectly, respectively. A possible intermediate process involving a well-known electronic excitation of the hydrogenated diamond at 13 eV is suggested. For incident electron energies higher than \ensuremath{\sim}14 eV, ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ ESD proceeds also via dipolar dissociation processes.

We report the first experimental study of the adsorption of cinnamaldehyde on surfaces under ultra high vacuum (UHV) conditions. Cinnamaldehyde is an α,β-unsaturated aldehyde with important applications in the fine chemicals sector. High-resolution electron energy loss spectroscopy (HREELS) is employed to investigate the vibrational modes of cinnamaldehyde condensed on graphite at 100 K and absorbed on the surface of a Pd islands film (supported on graphite), also at 100 K. In the case of the Pd film, we find strong evidence for a parallel orientation of the phenyl ring consistent with theoretical calculations.

The results of a study of changes in electron capture and loss rates in the interaction of fluorine atoms and anions with oxygen-covered Al(111) and Ag(110) surfaces are reported for a wide range of coverages. A strong decrease in F− formation is observed. The magnitude of this decrease shows that the capture rates are much smaller than those expected on the basis of work-function changes. The observed decrease is assigned to changes in local electronic structure in the vicinity of the adsorbate site and modifications in positions and widths of the anion levels. It was found that for incident F− and F°, the final F− fractions are different, indicating that the memory of the initial charge state is not lost as in the clean metal case. This difference indicates that the electron-loss rate is smaller in the presence of oxygen. An incident-energy dependence of the loss process is observed.

We present the results of a study of collisions of 1 to 4 keV oxygen ions and atoms with Mg, Al and Ag surfaces. Formation of O- is in particular investigated. This is an interesting multichannel problem, since the ground state electronic configuration of oxygen 2p(4) corresponds to three states and electron capture processes involve three atom (P-3, D-1 and S-1)-metal continua. We report scattered ion fractions, measured in an angular range extending from 2 degrees to 40 degrees with respect to the surface plane. This allowed us to investigate the characteristics of the resonant charge transfer process for a large range of collision velocities normal to the surface, thus probing the charge transfer process in different atom-surface distance ranges. The ion fractions are found to increase with increasing angle and increasing energy. Similar fractions are obtained for Al and Ag, but significantly higher ones for Mg. Ionisation processes in hard collisions with surface atoms are observed. An electron spectroscopy study was performed and did not reveal any signs of autoionising state (O**2p(2)3s(2)) production.

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 collisions of 0.5–4 keV fluorine and chlorine negative ions and atoms with Mg, Al and Ag surfaces. Production of negative and positive ions is observed. An electron spectroscopy study aimed at identifying excitation of autoionizing states was performed. We report scattered ion fractions as a function of energy and exit angle with respect to the surface. For these high electron affinity systems, the negative ion fractions are very large. Their angular dependence indicates the existence of a loss process. At low energies the ion fractions can be described using existing calculations [P. Nordlander, N.D. Lang, Phys. Rev. B, 46 (1992) 2584] of widths of the negative ion states in front of a metal surface, once the multielectron nature of the problem, i.e. the fact that the halogen ions have six essentially equivalent electrons, is properly taken into account.

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.