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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 (1 1 1) 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. © 2004 Published by Elsevier B.V.

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.

A set of electron-correlation energies as large as 10 eV have been measured for a magnetic 2ML Fefilm deposited on Ag(001). By exploiting the spin selectivity in angle-resolved Auger-photoelectroncoincidence spectroscopy and the Cini-Sawatzky theory, the core-valence-valence Auger spectrumof a spin-polarized system have been resolved: correlation energies have been determined for eachindividual combination of the two holes created in the four sub-bands involved in the decay: majorityand minority spin, as well asegandt2g. The energy difference between final states with paralleland antiparallel spin of the two emitted electrons is ascribed to the spin-flip energy for the final ionstate, thus disentangling the contributions of Coulomb and exchange interactions., Comment: 5 pages, 2 figures, 1 table

The authors have calculated the lateral distribution of field emitted electrons from a carbon nanofiber (CNF) array - a quantity of importance in designing field emission displays - by calculating the electron distribution from an individual CNF and subsequently summing the contribution from all individual CNFs. The authors have not obtained the absolute value of the current but only its relative distribution in space. The full width at half maximum of the lateral distribution has been examined with respect to the following parameters: 1) the CNF tip radius, 2) the anode to cathode distance, and 3) the cathode to anode potential difference. Reasonable agreement with experimental results is obtained. © 2014 American Vacuum Society.

""Spin selectivity in angle-resolved Auger photoelectron coincidence spectroscopy (AR-APECS) is used to. probe electronic structure in antiferromagnetic thin films. In particular, exploiting the AR-APECS capability to. discriminate Auger electron emission events characterized by a different spin of the ion in its final state, a sharp. multiplet structure in the Ni MVV Auger line shape of NiO\\\/Ag(001) thin films is measured below the critical. Néel temperature. The assignment of multiplet terms follows from a close comparison of the experimental. AR-APECS line shapes with the predictions based on semiempirical calculations on a cluster model and an. open-band extension of the Cini-Sawatzky approach. In analogy to CoO, also in NiO, above the Néel temperature. a more featureless Auger spectrum appears and AR-APECS does not disentangle anymore high-spin and low-spin. contributions to the total Auger intensity. Such a behavior, which seems to be a general result for metal oxide. antiferromagnetic systems, is discussed.""

""\\"Low-energy photoemission spectroscopy in the photon energy range between 5.5 and 21 eV is used to investigate the valence electronic structure of Yb polycrystalline films. With the lowering of the photon energy below 11 eV, additional spectral features, nearly absent at higher photon energy, become predominant, in particular a peak located right at the Fermi level. With the help of an ab initio theoretical calculation, we interpret the peak as due to the joint influence of filled and empty densities of states, identifying a p-band contribution close to the Fermi level, whose intensity is strongly enhanced at certain photon energies corresponding to modulation of the empty d band.\\"""

Secondary electron spectra emitted from polycrystalline Al have been measured in coincidence with 500 eV electrons reflected after undergoing a given energy loss. Contributions from bulk- and surface-plasmon decay have been observed, as well as contributions from LVV Auger electrons and electron-electron scattering.

The H:GaP(110) system was studied by EELS and AES. H induced losses were found in the 8, 12 and 18 eV region respectively. The PL 2.3 Auger lineshape was recorded as a function of emitted electron take-off angle. This method was proven to be able to identify structures in the electronic density of states at the surface both in the clean and H exposed surface. Two H induced states. located at about 2 and 4.5 eV below the valence band respectively, have been singled out.

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.

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.

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.

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 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 L 3 M 45 M 45 Auger transition of the Cu(111) surface. This beam-line, 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.

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.