1,721,058 research outputs found
Evolution of Auger peaks in GaP(110) with hydrogen chemisorption
No full textAuger spectroscopy flanked by energy loss spectroscopy was used to investigate the evolution of GaP(110) surface electronic structure upon hydrogen exposure. The paper is focussed on the modification of PL(2,3)VV Auger line shape as a function of hydrogen exposure in the exposure range between zero and Ga3d surface exciton quenching. The discussion is concentrated on the Auger emission region about 9 eV below the emission from valence band top. The line shape change can be explained on the basis of present knowledge of electronic structure of H:GaP(110) surface and ascribed to the birth of a hydrogen-induced state (H-4) at similar to 4.4 eV below valence band top. Moreover the surface stoichiometry was monitored by Auger spectroscopy through the PL(2,3)VV to GaM(2,3)M(4,5)M(4,5) peak-to-peak ratio which was found to be extremely affected at high hydrogen exposures
Mechanism of the low-ejection-energy (e,2e) reaction on a graphite surface
No full textWe 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
Two-step-wise interpretation of highly asymmetric, grazing angle (e,2e) on solids: A real momentum spectroscopy for surfaces and overlayers
No full textThis 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
Plasmon resonant (e, 2e) spectroscopy on Be(0001)
No full textWe investigated the mechanisms of secondary electron (SE) emission from Be(0001) by impact of 100 and 150 eV electrons. We made use of (e, 2e) spectroscopy to disentangle the different SE production mechanisms. We observed a large increase in the SE yield when the energy loss of the primary electron equals the characteristic energy of volume and surface plasmons. The line shape of the SE spectrum associated with plasmon excitation reveals that one relevant emission mechanism corresponds to direct single-particle excitation in which the plasmon energy and momentum are transferred to a valence band electron of the solid. The contributions to the SE yield associated with surface and volume plasmon excitation are comparable in the case of specular geometry, where the projectile momentum is mainly transferred perpendicular to the surface. On the contrary, the emission of SEs associated with surface plasmon excitation is significantly enhanced when the exchanged momentum lies close to the surface plane and electrons are emitted from Be surface state. This reflects the increased sensitivity to surface modes of the latter geometry. Finally, the coupling between the direct ionization channel and the plasmon-assisted one results in a resonant increase of the secondary emission
Evidence of charge transfer at the Cu-phthalocyanine/Al(100) lnterface
No full textElectronic and structural properties of the CuPc/Al(100) organic-inorganic interface were investigated by means of a multitechnique experimental approach based on synchrotron radiation. The chemical selectivity of X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure of copper-phthalocyanine (CuPc) as a function of the molecular thickness ranging from the submonolayer to 40 angstrom. Photoemission from core levels shows a dramatic alteration of the electronic structure of molecules localized at the interface. At the lowest CuPc coverages, the complete reduction of the oxidation state of copper was observed, while C 1s and N 1s shake-up satellites were no longer visible. Both findings are explained with a sizable charge transfer from the substrate to the molecule involving the b(1g) (Cu 3d-derived) and the LUMO hybridized with the substrate conduction band. The linear polarization of the synchrotron light was employed in X-ray absorption near-edge spectroscopy (XANES) to determine the orientation of CuPc molecules. Molecular planes oriented almost perpendicular with respect to the metal surface were observed from the second layer on
Valence band mapping by reflection (e,2e) spectroscopy in grazing angle mode: the HOPG case
No full textMedium electron energy (e,2e) spectroscopy in grazing incidencegeometry is able to reconstruct the band structure (bindingenergy vs electron momentum q) [1] and the electronmomentum density .(q) [2] of the valence electronic states ofsurfaces. (e,2e) experiments reveal in time-coincidence, withdefined energy and angle, the two final electrons generated bythe impact of a primary electron on the surface. By the newlyimplemented spectrometer for surface (e,2e) spectroscopy newexperiments were performed on HOPG in fully-multichanneldetection mode. A model available in literature [3,4] for themeasured cross-section in reflection mode has been specializedfor the specific asymmetric kinematics used in our experiment(10-350eV). This model describes the (e,2e) event taking intoaccount those processes in which the projectile electron undergoesa single scattering from the crystal potential before orafter the interaction with the bound electron: the cross-sectionresults to be factorized in two components, a kinematics factortimes the momentum density of bounded electron states. Tocheck the validity of the model an (e,2e) experiment has beenperformed choosing a condition where the electron momentumdensity was constant. The calculated kinematics factor reproducesthe angular distribution of the fast scattered electronmeasured while keeping fixed the directions of primary andslow electrons and all three electron energies. The enhancedluminosity of the apparatus by energy-multichannel detectionallowed us to map the valence band structure of . states reducingthe acquisition time by a factor of 8 with respect tosingle-channel detection mode without degradation of the energyand the momentum resolutions (1 eV and 0.2 °A €1). Takinginto account the inelastic multiple scattering contributionthe measured intensities correspond to the electron momentumdensities projected into the direction parallel to the surface ina momentum range from the first Brillouin zone to the secondone. The observed minimum near the zone boundary can bean indication that real momentum of . states were measured[5]. We have measured a value of Fermi momentum (1.7 °A €1)which is in agreement with band calculations. The observedmaximum of electron momentum density is in agreement withthe . states, additionally we observe minor structures correspondingto .2 states. The observation of these electron statesinvolves processes assisted by reciprocal lattice vectors, hencefor these states the reconstructed momentum should be thecrystal momentum
Scattering mechanism of electrons interacting with surfaces in specular reflection geometry: Graphite
No full textWe have studied the scattering mechanism of electron-energy-loss process in specular reflection geometry highlighting the presence of an elastic collision that always accompanies the inelastic one. It implies that two independent channels contribute to the inelastic cross section depending on whether the inelastic event precedes or follows the elastic one. Our results indicate that neither one of the channels is favored by propensity rules. Nevertheless, suitable experimental conditions permit to enhance contribution to the cross section of one channel with respect to the other. The possibility to single out the contribution of a given channel allows to determine without ambiguity the momentum exchanged in the inelastic collision. This is of fundamental relevance for several electron impact spectroscopies, such as electron-energy-loss spectroscopy and (e,2e), in specular reflection geometry. These results are derived from measuring the current of elastically and inelastically specularly reflected electrons as a function of the primary electron beam kinetic energy (IV curve). The incident beam energy was varied between 150 and 450 eV, the target was an highly oriented pyrolitic graphite and the range of losses investigated was 6-35 eV. A simple kinematics model that accounts for refractive effects due to the surface potential barrier, gives good agreement with the observed diffraction pattern of the elastically reflected electrons. [S0163-1829(99)01519-2]
Electron-electron coincidence spectroscopies at surfaces
No full textIn 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
A high efficiency spectrometer for reflection (e,2e) experiments at surfaces
No full textTo 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
Fully resolved kinematics of grazing-incidence (e,2e) experiments
No full textThe capability of grazing-incidence (e,2e) spetroscopy to map the spectral momentum density rho(epsilon, q) of the valence electronic states in surfaces was demonstrated by a pioneering experiment in which both the band structure (binding energy, epsilon, versus momentum, q) and the momentum density, rho, were obtained [S, Rioual et al., Phys. Rev. B 57 (1998) 2545]. (e,2e) experiments reveal, in coincidence, after energy and angle selection, the two final electrons generated by the impact of a primary electron on the surface. The momentum resolution achieved by the first experiment was limited because of the integration over the wide solid angle accepted by the ejected-electrons analyzer. This limitation has been overcome in the present work by upgrading the ejected-electrons analyzer with a position-sensitive detector (PSD). The PSD allows us to measure the azimuth angle of the ejected electrons, and hence to collect in a parallel mode (e,2e) spectra that are fully differential in momentum. Preliminary data relative to the ionization of the pi-band of highly oriented pyrolitic graphite (HOPG) show that a momentum resolution better than 0.2 Angstrom(-1) has been achieved without degrading the overall spectrometer luminosity. (C) 2000 Elsevier Science B.V. All rights reserved
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