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V Encuentro Sud Americano de Colisiones Inelásticas en la Materia Energy-loss spectra measured for some crystal azimuthal directions are shown in Figs.2(a,b) and 3(a,b) for hydrogen and fluorine incident ions. In Figs.2(a,b), the Full-Width-at-Half-Maximum (FWHM) of scattered beam is considerably broad and seems to have a large energy loss tail. The most striking difference is observed for F ions scattering with an azimuthal angle of 70.5°, where a double peak structure is observed in Figs. 3(a,b) for 4 keV but disappears for 1 keV. Intensity relative intensity 150 100 50 0 0 200 400 600 800 1000 1200 1.1 1.0 0.9 (b) simulation 0.8 experiment 0.7 0.6 1 keV H + -Au(110) 0.5 0.4 azimuthal angle 0 0.3 0.2 0.1 0.0 -50 0 50 100 150 200 250 300 350 Fig. 2. (a) and (b) Energy loss spectra of 4 and 1 keV H + ions scattered off a Au (110) surface along the indicated azimuthal direction of 0 0 respectively intensity 200 150 100 50 (a) (a) energy Loss (eV) experiment selected total selected on top of the surface selected bellow the first layer 4 keV H + -Au(110) azimuthal angle 0 0 experiment selected on top of the surface selected bellow the first layer 4 keV F - -Au(110) azimuthal angle 70.5 0 electron capture again leads to F - formation. Here we introduce neutralization empirically, using theoretical treatments as a guideline. Electron transfer processes occur over the characteristic distances of around Z F =5.0 atomic units from the image plane for fluorine. A similar range of distances is assumed for Auger neutralization of hydrogen. Image charge effects are “switched off” for neutralized particles. The simulated [1, 2] energy loss spectra are also shown in Figs.2(a,b) and 3(a,b), which are in good agreement with the experiment. We furthermore observed a splitting of the energy loss spectrum into two components due to two different types of the trajectory, as shown in Figs.2(c) and 3(c). The trajectory a results from scattering from the most top layer of the surface and b from subsurface has a longer effective length. Energy-loss spectra of hydrogen and fluorine ions scattered off a Au (110) single-crystal surface in grazing scattering conditions were reported for various orientations of the surface. We presented simulations which take into account the corrugation of the electron density above the surface are in good agreement with the experimental data. relative intensity 1.0 0.8 0.6 0.4 0.2 0 0 50 100 150 200 250 300 350 (b) simulation experiment 1 keV F - -Au(110) azimuthal angle 70.5 0 0.0 -20 0 20 40 60 80 100 energy loss (eV) Fig. 3. (a) and (b) Energy loss spectrum of 4 and 1 keV F - scattering along the 70.5° direction for Au(110) respectively. In describing H + and F - scattering on Au one should correctly describe the effect of the image potential. This implies a correct description of electron transfer processesresonant and Auger neutralization on the incoming and outgoing path of the trajectory. We consider H + neutralization in the incoming path and in case of F - we consider that at large distances (Z> Z F ) when the affinity level is above the Fermi level electron loss occurs leading to F° formation, while at small distances, when the F - level lies below the Fermi level References [1] Valdés J E, Vargas P, Celedón C, Sanchez E, Guillemot L and Esaulov V A 2008 Phys. Rev. A 78 32902 [2] Valdés J E, Vargas P, Guillemot L and Esaulov V A 2007 Nucl. Inst. And Methods B 256 81 98 Valparaíso, Chile
V Encuentro Sud Americano de Colisiones Inelásticas en la Materia Inverse Photoemission Spectroscopy on Graphene V. del Campo 1 , P. Häberle 1 1 Depto. de Física, Universidad Técnica Federico Santa María, Casilla 110-V, Valparaíso, Chile email address corresponding author: valeria.delcampo@usm.cl Graphene is composed by sp 2 carbon atoms which are arranged in a twodimensional honeycomb lattice. It has been observed that carriers behave as massless Dirac fermions; this makes graphene a promising material for microelectronics. In this sense, plenty of research has been made on graphene electronic structure. It has been observed that graphene has a single state at the Fermi energy in which the π and π* bands cross at the K point of the Brillouin zone [1]. It was found that when graphene interacts with another graphene layer a band gap is opened [2]. It is also reported that when graphene grows on a metal surface like Ru(0001) there is a strong chemical bonding and graphene looses its 2D properties, but when a second layer is grown in that system, that layer shows similar properties to isolated graphene monolayer [3]. All these studies on graphene electronic structure have been performed with techniques which allow analysis of graphene valence band, no specific studies have been performed to characterize graphene conduction band. For graphene growth we heat a ruthenium cristal above 1000ºC in Ultra High Vacuum (~10 -10 Torr). We introduce ethylene (C 2 H 4 ) to the vacuum chamber, reaching a pressure of 1.5x10 -7 Torr. After a few minutes we cool the substrate down to 800ºC and monitor graphene growth with Low Energy Electron Diffraction (LEED). Once the graphene covers the substrate we cool down to room temperature. To study the unoccupied band (conduction band) structure of graphene, we use Inverse Photoemission Spectroscopy (IPS). This technique consists on impinging electrons on the sample. The inciding electrons decay from one unnocupied state to another (depending on their initial energy). From this trasitions photons are emmited from the sample. We perform isocromat IPS, detecting only the photons that leave the sample with 9.5 eV. Through the analysis of the photon intensity as a function of electrons initial energy we get an insight on the conduction band of the sample. Figure 1. Experimental set up for Inverse Photoemission Spectroscopy. Photons are emitted after the interaction between electrons and the sample, only photons with 9.5 eV are detected. References [1] S. Marchini, S. Günther, and J. Wintterlin, Phys. Rev. B 76, 075429 (2007). [2] T. Ohta, A. Bostwick, J. L. McChesney, T. Seyller, K. Horn and E. Rotenberg, PRL 98, 206802 (2007). [3] P. W. Sutter, J. I. Flege and E. A. Sutter, Nature Materials, 7, 406-411 (2008). 99 Valparaíso, Chile
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