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The field emission (FE) properties of carbon nanotube (CNT)-based cathodes have been investigated on nanostructured surfaces grown by plasma enhanced chemical vapor deposition. The FE angular properties and temporal stability of the emergent electron beam have been determined using a dedicated apparatus for cathodes of various architectures and geometries, characterized by scanning electron microscopy and I-V measurements. The angular electron beam divergence and time instability at the extraction stage, which are crucial parameters in order to obtain high brilliance of FE-based-cathode electron sources, have been measured for electrons emitted by several regular architectures of vertically aligned arrays and critically compared to conventional disordered cathodes. The measured divergences strongly depend on the grid mesh. For regular arrays of individual CNT, divergences from 2° to 5° have been obtained; in this specific case, measurements together with ray-tracing simulations suggest that the maximum emission angle is of the order of ±30° about the tube main axis. Larger divergences have been measured for electron beams emitted from honeycomb-structured cathodes (6°) and significantly broader angle distributions (10°) from disordered CNT surfaces. Emission current instabilities of the order of 1% for temporal stability studies conducted across a medium time scale (hours) have been noted for all cathodes consisting of a high number (10$^{4}$ and larger) of aligned CNTs, with the degree of stability being largely independent of the architecture.

Photoelectron emission spectra in a photon energy range between 7.5 and 21 eV are measured for in situ grown polycrystalline Yb films. By comparing bulk and surface core level shifted 4f components we give an estimation of the effective attenuation length (EAL) for low energy (6-20 eV) electrons in Yb, establishing a moderate increase of the EAL upon electron energy decrease. The experimental EAL data are found to be a factor of four smaller than those predicted from the so-called 'universal curve'.

The mechanism of secondary electron emission by impact of 100-eV electrons on an Al(100) surface has been investigated by measuring the secondary electron spectrum in coincidence with loss features in the spectrum of reflected electrons. Distinct peaks are observed at energies corresponding accurately to the surface and bulk plasmon energies minus the work function of the analyzer, demonstrating that plasmons excited by electron energy losses predominantly decay via creation of single-electron-hole pairs that act as a source for the secondary electron spectrum. These findings suggest a mechanism for emission of secondary electrons very similar to photoelectron (PE) emission, the difference being the step leading to electron liberation, i.e., plasmon decay in the present case versus photoionization in the case of PE.

This paper deals with the mechanism of grazing incidence (e,2e) events from surfaces. Two different approaches are considered. In both cases, elastic scattering with the crystal lattice assists the inelastic collision; these two steps are coupled either coherently or incoherently. Experimental evidence is given that the "coherent" approach reproduces better the cross section dependence on momentum transfer in the specific case of asymmetric kinematics at moderate electron energies. This model has allowed us to map out the band dispersion of the outermost valence states of highly oriented pyrolytic graphite and to measure the momentum distribution of pi-electron states without invoking the contribution of reciprocal lattice vectors in the momentum conservation. Agreement between theory and experiment is satisfactory, though the presence of events where crystal momentum is reconstructed cannot be ruled out. These results, obtained with a significant reduction of the experiment duration by an implemented apparatus, show that reflection (e,2e) can be used to build up a momentum spectroscopy with high surface sensitivity.

This paper deals with the mechanism of grazing incidence (e,2e) events from surfaces. Two different approaches are considered. In both cases, elastic scattering with the crystal lattice assists the inelastic collision; these two steps are coupled either coherently or incoherently. Experimental evidence is given that the ``coherent'' approach reproduces better the cross section dependence on momentum transfer in the specific case of asymmetric kinematics at moderate electron energies. This model has allowed us to map out the band dispersion of the outermost valence states of highly oriented pyrolytic graphite and to measure the momentum distribution of $\ensuremath{\pi}$-electron states without invoking the contribution of reciprocal lattice vectors in the momentum conservation. Agreement between theory and experiment is satisfactory, though the presence of events where crystal momentum is reconstructed cannot be ruled out. These results, obtained with a significant reduction of the experiment duration by an implemented apparatus, show that reflection (e,2e) can be used to build up a momentum spectroscopy with high surface sensitivity.

To study electron momentum densities in solids by grazing angle reflection kinematics has been shown to be feasible [S. lacobucci, S. Rioual, A. Ruocco, M. Mastropietro, G. Stefani, Surf. Sci. 454 (2000) 1026], but development of this spectroscopy has been hampered by long acquisition time; to fully exploit potentialities of this method is mandatory to reduce duration of the experiment within times comparable with clean surface lifetimes in ultra-high vacuum. This paper reports on recent developments of the reflection (e,2e) spectrometer that make a sizeable step forward in attaining this goal. It operates in asymmetric kinematics and at small grazing angle, thus allowing to enhance the surface sensitivity. A drastic reduction in acquisition time has been achieved by implementing parallel acquisition, both in energy and angle, of the detected electron pairs. To achieve parallel acquisition in energy and momentum, each of the two electron analysers is equipped with a two-dimensional position sensitive detector. A custom-made electronic hardware and software have been developed for the automatic control of the experiment and for acquisition and storage of the coincidence events. After discussing in some details the relevant features of the new spectrometer, few examples of valence band mapping and electron momentum densities measured in highly oriented pyrolitic graphite with energy and momentum resolutions of 1.3 eV and 0.15 angstrom(-1) are given. 0 2006 Elsevier B.V. All rights reserved.

Advances in materials science have produced a wide array of new solid-state systems with tunable properties and previously unattainable combinations of phenomena that hold the promise of entirely new approaches to technological applications. Invariably, these new materials are increasingly complex and include a large number of constituents in a variety of chemical states. Entirely new theoretical and experimental approaches are needed to gain the insights necessary for intelligent engineering of these materials. In the past 20 years, a steadily increasing number of electron-electron coincidence experiments on atoms and molecules have demonstrated the capability of investigating complicated systems with sensitivity and specificity well beyond the limits imposed by conventional electron spectroscopies. Over the past decade or so, Auger-photoelectron coincidence spectroscopy (APECS) has emerged as a powerful technique for obtaining detailed information about complex materials systems. Moreover, the recent advent of angle-resolved (AR)-APECS has introduced a new level of discrimination in studying the distribution of electrons photoemitted from complex systems. In this review, we describe the basic ideas behind APECS and discuss a study of the SiO(2) system as an example of the unique information this technique can provide. We then introduce the concept of AR-APECS, explain its novel state and angular momentum selectivity that can be used to disentangle information about complex systems that is hidden to conventional spectroscopies. Examples of AR-APECS measurements from Cu, Ge, and graphite that exemplify the capabilities of this technique are presented. (C) 2004 Elsevier B.V. All rights reserved.

Core hole creation and subsequent Auger decay processes are studied with unprecedented discrimination by Auger-Photoelectron Coincidence Spectroscopy (APECS). Early works in this field have already pointed out the intrinsic surface sensitivity of these experiments. However, it was not until recently that a model calculation was developed to quantitatively evaluate it. Here we present the first attempt to experimentally establish an effective target thickness for such experiments. The angular distribution of 3P(3/2) photoelectron with kinetic energy of 160 eV is measured in coincidence with the M3VV Auger electron with kinetic energy of 55 eV on a Cu (111) surface. Coincidence and non-coincidence photoelectron angular distributions display differences that, to large extent, are explained by confining the source of the coincident signal within the first two layers of Cu target, thus establishing an experimental upper limit for the effective target thickness of the APECS experiment. (C) 2004 Published by Elsevier B.V.

Core hole creation and subsequent Auger decay processes are studied with unprecedented discrimination by Auger-Photoelectron Coincidence Spectroscopy (APECS). Early works in this field have already pointed out the intrinsic surface sensitivity of these experiments. However, it was not until recently that a model calculation was developed to quantitatively evaluate it. Here we present the first attempt to experimentally establish an effective target thickness for such experiments. The angular distribution of 3P(3/2) photoelectron with kinetic energy of 160 eV is measured in coincidence with the M3VV Auger electron with kinetic energy of 55 eV on a Cu (111) surface. Coincidence and non-coincidence photoelectron angular distributions display differences that, to large extent, are explained by confining the source of the coincident signal within the first two layers of Cu target, thus establishing an experimental upper limit for the effective target thickness of the APECS experiment. (C) 2004 Published by Elsevier B.V.

Auger-photoelectron coincidence spectroscopy (APECS) has shown that singling out Auger spectra individual components on the basis of the related photoelectron, i.e. defining the parent core ionic state, increases energy discrimination capability of both Auger and photoelectron spectroscopy. Angle resolved APECS (AR-APECS) on Ge(100) surface has shown peculiar new features of the electron pair angular distribution upon Auger detection angle. It was speculated that these features were to be ascribed to the alignment of the core-hole state induced by selection in angle of the photoelectron. Recent results on Ge(100) and Cu(111) have drawn attention on the possibility of selecting specific magnetic sublevels of both Auger and photoelectron wavefunctions. Good use of this capability has been made in singling out otherwise overlapping components of both Ge L3VV and Cu M3VV multiplet split spectra, thus demonstrating that resolution in angle improves discrimination in the energy scale as well.

We have measured the angular distribution of Ge L(3)M(45)M(45) Auger electrons in coincidence with Ge 2p(3/2) core photoelectrons along the (001) azimuth of the Ge(100) surface. Intensity modulations arising from diffraction effects are suppressed in the coincidence Auger angular distribution and, when specific emission angles of the photoelectrons are considered, new features appear. We attribute the former effect to enhanced surface specificity of the coincidence technique and the latter to sensitivity of the coincidence measurement to alignment of the core hole state.

We have investigated the Auger emission in Fe for photon energies varying across the 2p edges. The probability of the L2L3V Coster-Kronig (CK) decay process has been measured by photoemission spectroscopy using synchrotron radiation. The branching ratio between the process L-2 --> L-3'VV (CK preceded) and the direct Auger decay of the L-2 hole has been determined as a function of the photon energy near the 2p(1/2)-edge (717-735 eV): this branching ratio is found to rapidly converge to an asymptotic value of about 0.83, which favorably compares with the Auger-photoemission coincidence spectroscopy results. (C) 2002 Elsevier Science B.V. All rights reserved.

In the past 20 years, a steadily increasing number of electron-electron coincidence experiments on atoms and molecules have contributed to a deeper understanding of electron-electron correlation effects. In more recent years this technique has been extended to the study of solid surfaces. This class of one photon IN two electrons OUT experiments will be discussed with an emphasis on grazing incidence geometry, that is expected to be particularly suited for studying surfaces. The crucial question of which is the dominant mechanism that leads to ejection of pairs of electron from the surface will be addressed. It will be shown that, depending on the kinematics chosen, the correlated behaviour of the pairs of electrons detected might be singled out from independent particle one. (C) 2002 Elsevier Science B.V. All rights reserved.

In this work we deal with the relaxation of the 2p core hole in transition metals via LVV Auger decay, a specific aspect in itself, though contributing a piece of information for a more general understanding of charge redistribution in the final states of both primary photoemission and following de-excitation process. In the past, the analysis of the process was performed mainly by high energy photoemission spectroscopy (PS) experiments, either standard PS [1,2] or coincidence spectroscopy [3,4]. We have carried out a study of the Coster Kronig (CK) decay in Fe, Ni and NiO by PS using synchrotron radiation. In particular, we have measured the branching ratio beta between the direct L2VV and L2L3V-->L-3'VVV CK transitions. P has been determined as a function of the photon energy in the proximity of the L, edges. For all samples 6 becomes independent upon the photon energy already at few eV above threshold. Nonetheless, the asymptotic value is found to be larger in the metals than in the insulator, this reflecting the different localization of the electronic states involved in the cascade process. Capability and limitations of the method used for the quantitative evaluation of 6 are also discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

This work presents the implementation of a position-sensitive detector in the Montelibretti surface (e,2e) apparatus that allows to fully define the direction of the ejected electron from the surface, and hence to improve the momentum resolution. The coincidence electronic chain and the acquisition software have been upgraded in order to achieve high data collection speed, storage of large dimension data files and fast visualisation of the results. Preliminary angle resolved (e,2e) data relative to the ionisation of the pi band of highly oriented pyrolitic graphite show that a momentum resolution better than 0.2 Angstrom(-1) has been achieved without degrading the overall speed of acquisition.

The feasibility of angle resolved APECS (Auger Photo-Electron Coincidence Spectroscopy) on solids is demonstrated with an experiment performed at the ALOISA (ELETTRA, Trieste) beamline on the L3M45M45 Auger transition of the Cu(111) surface. This beamline, with its multicoincidence detection system based on several individual electron analysers, was designed in order to perform such experiment with good efficiency. The correlation effects displayed by the measured angular distribution are not explained by the two step model, that conversely is commonly accepted for non coincidence experiments performed at the same energy and on the same orbital.

The probability of the L2L3V Coster-Kronig (CK) decay process in Ni following the: creation of a 2p(1/2) core hole has been measured by photoemission spectroscopy using synchrotron radiation. The branching ratio between CK and L2VV Auger decay has been determined as a function of the photon energy in the proximity of the L-2 edge (870-890 eV). It is found to become independent upon the photon energy range already a few eV above threshold. The direct low-energy (about 7 eV) CK emission spectrum has also been observed. [S0163-1829(99)05415-6].

The capability of grazing angle (e,2e) experiments to map momentum distribution is demonstrated.: This is an application of reflection (e,2e) experiments as a unique twofold spectroscopy, binding energy and momentum distribution, for solid surfaces. The reported experiments have been performed on highly oriented pyrolytic graphite at about 300-eV incident energy, grazing angle, and for ejected electron energies ranging from 3.7 to 30 eV. These experiments, interpreted in the framework of the plane-wave impulse approximation, yield the binding energy vs momentum dispersion curves of valence electron states as well as the individual pi-state momentum density. Both results compare favorably with calculated graphite band structure. [S0163-1829(98)06404-2].

We develop a theoretical model to describe a slow electron ejection from a crystal by electron impact at a moderate incident energy. The electron impact ionization is considered within the first Born approximation. The projectile is treated as a plane wave whereas the target electron initial and final states are described by the bulk one-electron wave functions in the momentum space representation. To allow the ionized electron to escape from the crystal the final state in the bulk of the crystal is matched in energy and a parallel component of momentum by a plane wave in the vacuum. This theoretical model is used to simulate the binding-energy spectra obtained by the grazing-angle reflection mode (e,2e) reaction on the surface of highly oriented pyrolytic graphite.

We develop a theoretical model to describe a slow electron ejection from a crystal by electron impact at a moderate incident energy. The electron impact ionization is considered within the first Born approximation. The projectile is treated as a plane wave whereas the target electron initial and final states are described by the bulk one-electron wave functions in the momentum space representation. To allow the ionized electron to escape from the crystal the final state in the bulk of the crystal is matched in energy and a parallel component of momentum by a plane wave in the vacuum. This theoretical model is used to simulate the binding-energy spectra obtained by the grazing-angle reflection mode (e,2e) reaction on the surface of highly oriented pyrolytic graphite.

We used the partial charge tight binding method to perform a full structure optimization to determine equilibrium adsorption geometries, energetics, and local charge redistribution for molecular adsorption and multilayer growth of pentacene on Cu(100). We found that single molecule adsorption induces only a localized perturbation of the metal lattice which is limited to the topmost layers. At saturation coverage four stable topologies (Brick, Wave, Lines and Zigzag) were identified, all based on pentacene molecules lying flat on the metal surface and with the central phenyl ring adsorbed in top position. Only two (Brick and Wave) out of the four structures are able to sustain multilayer growth. In both cases, assembling beyond the second layer corresponds to a transition from the flat to a tilted geometry, in which the pentacenes adopt a face-plane-face arrangement leading to a herringbone structure. The energetics of the different structure are reported as a function of the molecular number density of the pentacene multilayer by calculating cohesive, stress, and electrostatic energies. The dominant tilted molecular orientation in the pentacene multilayer is in agreement with the average tilt angle of 65\ifmmode^\circ\else\textdegree\fi{} between the molecular plane and the Cu surface derived by near edge x-ray absorption spectroscopy of a four monolayer pentacene film deposited on Cu(100).

The possibility of using the grazing angle (e,2e) technique as a binding energy and/or momentum spectroscopy of surface states rests on the accurate knowledge of the ionisation mechanism and on the capability of achieving a sufficiently good energy resolution. Two possible mechanisms are envisaged that can generate pairs of correlated electrons in the reflection geometry: a single inelastic collision at large momentum transfer or a double collision (elastic plus inelastic). In this paper are presented the results of new (e,2e) experiments that allow to elucidate the ionisation mechanism at intermediate energies (300 eV) and asymmetric kinematics. The measurements, performed on highly oriented pyrolitic graphite, also show that an overall energy resolution as good as 1.2 eV can be achieved.

Angular resolved electron energy loss measurements on highly oriented pyrolitic graphite have been performed in reflection kinematics and at grazing incidence angles. Because of the selected kinematics it has been possible to change, almost independently, either the momentum transfer parallel to the surface (q(parallel-to)) or the angle between the momentum transfer and the graphite c axis (q^c). The present measurements have proved to be very sensitive to the surface structure of the sample and have allowed the assignment of the prominent loss at about 6 eV to a pi --> pi* excitation mainly localized perpendicular to the c axis. Possible contributions from surface localized states have also been detected. This transition shows negligible dispersion in q(parallel-to) and its energy centroid appears to be shifted with respect to previous surface and bulk measurements. Furthermore, the measured cross-sections are brought to the same relative scale for each grazing angle, and the loss and scattering functions are derived from data taken in reflection kinematics. The relevance of the kinematic prefactor in deriving transition energies from loss spectra is discussed with an emphasis on the role of q(parallel-to) zeroes.

We have investigated the Auger emission in Fe for photon energies varying across the 2p edges. The probability of the L2L3V Coster-Kronig (CK) decay process has been measured by photoemission spectroscopy using synchrotron radiation. The branching ratio between the process L2-L'3VV (CK preceded) and the direct Auger decay of the L2 hole has been determined as a function of the photon energy near the 2p1/2 -edge (717-735 eV): this branching ratio is found to rapidly converge to an asymptotic value of about 0.83, which favorably compares with the Auger-photoemission coincidence spectroscopy results.