Peer-reviewed Publications |
Bachellerie, D., Sizun, M., Aguillon, F., & Sidis, V. (2009). Effects of a Nonrigid Graphene Surface on the LH Associative Desorption of H Atoms and on the Deexcitation of Nascent H-2 Molecules Colliding with Model Walls of Carbonaceous Porous Material. J. Phys. Chem. A, 113(1), 108–117.
Résumé: A planar slab of 200 C atoms bound by the Brenner potential is used to study the Langmuir-Hinshelwood (LH) recombination of two physisorbed H atoms on a graphene sheet and to simulate afterward successive collisions of the nascent H-2 molecule with pore walls of a carbonaceous dust grain of the interstellar medium. The study is based on successive propagations of classical trajectories for the 200 C + 2 H atoms. The characteristics of H2 molecules formed by the LH reaction on the flexible surface are found to differ but negligibly from those formed on a rigid one. Collisions of those H,, molecules with graphitic pore walls are studied next. Reflection from and “trapping” onto the surface is observed and discussed. The most important energy transfer is from the molecule vibration to its rotation. This conversion mediates the transfer of the molecule internal energy to its translation or to surface heating. It is found that a single H-2-surface impact has little effect on the internal energy of the molecules. The grain absorbs on the average but a very weak energy. Several impacts are required to appreciably cool the molecule. The molecule cooling is accompanied by a significant increase of its translational energy. The swifter the molecules are or get, the larger the number of their impacts on the surface they undergo per unit time and the more efficiently cooled they get.
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Bachellerie, D., Sizun, M., Aguillon, F., Teillet-Billy, D., Rougeau, N., & Sidis, V. (2009). Unrestricted study of the Eley-Rideal formation of H-2 on graphene using a new multidimensional graphene-H-H potential: role of the substrate. Phys. Chem. Chem. Phys., 11(15), 2715–2729.
Résumé: The Brenner potential is adapted to handle chemical interactions and reactions of H atoms at a graphene surface. The adapted potential reproduces several important features of DFT computed data and reveals an extended puckering of the surface upon its adsorption of an H atom. This potential is used to investigate in a much more realistic way than has been done before, the Eley-Rideal abstraction reaction producing H-2 in H + H-graphene collisions at energies E-col <= 0.2 ev. The graphene surface is represented by a slab of 200 carbon atoms and the study is carried out using classical molecular dynamics for vertical incidences in a cylinder centered about the chemisorption axis. A highlight of the present study is that upon the arrival of the gas phase H atom, the adsorbent C atom is attracted and pulls out its surrounding surface atoms. The hillock thus formed remains puckered until the newly formed molecule is released. The range of impact parameters leading to reaction depends on the collision energy and is governed by the shape of the entrance channel potential; the reaction probability in this range is 100%. On average, in the studied E-col range, the available energy (3.92 eV + E-col) is shared as: 69 52% for the internal energy, 11-23% for the translation energy and 20 25% for the energy imparted the surface. Also, the average vibration and rotation energy levels of the nascent H-2 molecule are, respectively, < v > = 5-4 and < i > = 2-4.
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Baffou, G., Mayne, A. J., Comtet, G., Dujardin, G., Stauffer, L., & Sonnet, P. (2009). SiC(0001) 3 x 3 Heterochirality Revealed by Single-Molecule STM Imaging. J. Am. Chem. Soc., 131(9), 3210–3215.
Résumé: We report a description of the SiC(0001) 3 x 3 silicon carbide reconstruction based on single-molecule scanning tunneling microscopy (STM) observations and density functional theory calculations. We show that the SiC(0001) 3 x 3 reconstruction can be described as contiguous domains of right and left chirality distributed at the nanoscale, which breaks the to date supposed translational invariance of the surface. While this surface heterochirality remains invisible in STM topographies of clean surfaces, individual metal-free phthalocyanine molecules chemisorbed on the surface act as molecular lenses to reveal the surface chirality in the STM topographies. This original method exemplifies the ability of STM to probe atomic-scale structures in detail and provides a more complete vision of a frequently studied SiC reconstruction.
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Bellec, A., Ample, F., Riedel, D., Dujardin, G., & Joachim, C. (2009). Imaging Molecular Orbitals by Scanning Tunneling Microscopy on a Passivated Semiconductor. Nano Lett., 9(1), 144–147.
Résumé: Decoupling the electronic properties of a molecule from a substrate is of crucial importance for the development of single-molecule electronics. This is achieved here by adsorbing pentacene molecules at low temperature on a hydrogenated Si(100) surface (12 K). The low temperature (5 K) scanning tunneling microscope (STM) topography of the single pentacene molecule at the energy of the highest occupied molecular orbital (HOMO) tunnel resonance clearly resembles the native HOMO of the free molecule. The negligible electronic coupling between the molecule and the substrate is confirmed by theoretical STM topography and diffusion barrier energy calculations.
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Bellec, A., Riedel, D., Dujardin, G., Boudrioua, O., Chaput, L., Stauffer, L., & Sonnet, P. (2009). Electronic properties of the n-doped hydrogenated silicon (100) surface and dehydrogenated structures at 5 K. PHYSICAL REVIEW B, 80(24), 245434.
Résumé: We present a comparative study of the electronic properties of the clean Si(100) and the hydrogenated Si(100):H surfaces performed with a low-temperature (5 K) scanning tunneling microscope. Various surface structures such as single silicon dangling bonds and bare silicon dimers created by local desorption of hydrogen atoms from the Si(100):H surface are also investigated. The experimental scanning tunneling spectroscopy (STS) curves acquired locally on each of these structures are compared with STS measurements performed on the Si(100) and Si(100):H surfaces. First principle density-functional theory calculations of the projected local density of states, taking into account the influence of the dopant atoms (As), enable to assign the observed STS spectra.
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Bourguignon, B., Zheng, W., Carrez, S., Ouvrard, A., Fournier, F., & Dubost, H. (2009). Deriving the complete molecular conformation of self-assembled alkanethiol molecules from sum-frequency generation vibrational spectra. Phys. Rev. B, 79(12), 125433.
Résumé: The sum frequency generation (SFG) spectrum of CH(2) and CH(3) modes in the self-assembled monolayer of octadecanethiol is modeled in order to understand the reasons of the very low SFG intensity of CH(2) modes and to investigate quantitatively the two types of L-shaped conformations qualitatively suggested in the literature. CH(2) modes are assumed local in order to calculate easily the spectrum of any conformation, and ab initio molecular hyperpolarizabilities are used. It is found that the absence of CH(2) bands does not imply an all-trans conformation and vice versa. Several conformations are compatible with SFG, but only one of them can be arranged on Au(111) and completely agrees with x-ray diffraction. All conformational details of the L-shape molecules are obtained.
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Chiaravalloti, F., Riedel, D., Dujardin, G., Pinto, H. P., & Foster, A. S. (2009). STM topography and manipulation of single Au atoms on Si(100). Phys. Rev. B, 79(24), 245431.
Résumé: The low-temperature (12 K) adsorption of single Au atoms on Si(100) is studied by scanning tunneling microscopy (STM). Comparison between experimental and calculated STM topographies as well as density-functional-theory calculations of the adsorption energies enable us to identify two adsorption configurations of Au atoms between Si-dimer rows (BDRs) and on top of Si-dimer rows (TDRs). In both adsorption configurations, the Au atoms are covalently bound to two Si atoms through a partial electron transfer from Si to Au. STM manipulation confirms that the TDR adsorption configuration is metastable, whereas the BDR one is the most stable configuration.
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Cranney, M., Chalopin, Y., Mayne, A. J., & Dujardin, G. (2009). Adsorption and organisation of para-hexaphenyl molecules on Si(100). Appl. Phys. A, 94(4), 767–773.
Résumé: Para-hexaphenyl molecules (p-6P: C(36)H(26)) can be grown as nanofibres on various surfaces having optical properties of technological relevance. We report here the first observations of the initial stages of adsorption of individual p-6P molecules on Si(100)-(2x1) using room temperature Scanning Tunnelling Microscopy (STM). Depending on the substrate temperature, the hexaphenyl molecules adsorb in two configurations; indicating both weak and strong chemisorption. All six phenyl rings are clearly visible and the molecules are found to adsorb with characteristic angles between the long axis of the molecule and the silicon dimer rows. A statistical analysis of the spatial distribution of the molecules suggests that the clustering required for crystallographic nanofibre growth does not occur at the atomic scale under the experimental conditions used here.
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Diaz-Tendero, S., Borisov, A. G., & Gauyacq, J. - P. (2009). Extraordinary electron propagation length in a metallic double chain supported on a metal surface. Phys. Rev. Lett., 102(16), 166807.
Résumé: The present theoretical study shows that a double chain of Cu metal atoms adsorbed on a Cu(111) metal surface can guide an excited electron for distances exceeding 10 nm. The nanostructure appears to be quasi-decoupled from the substrate and thus to act as a nanowire. The origin of the above phenomenon is the interference between the decay of the quasistationary 1D sp-band states localized on each chain. This allows to approach the situation of the formation of bound states in the continuum first discussed in quantum systems by von Neumann and Wigner.
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Diaz-Tendero, S., Olsson, F. E., Borisov, A. G., & Gauyacq, J. - P. (2009). Excited electron dynamics in Cu nanowires supported on a Cu(111) surface. Phys. Rev. B, 79(11), 115438.
Résumé: We present a theoretical study of the excited electron dynamics in infinite Cu monoatomic chains (nanowires) supported on a Cu(111) surface. A joint approach based on the wave packet propagation and the density functional theory is used. The nanowire-induced potential obtained from ab initio density functional theory calculations serves as an input for the wave-packet propagation study of the excited electron dynamics. The energy dispersion and the lifetime of an unoccupied one-dimensional (1D) nanowire-localized electronic band with sp character are obtained. From the group velocity and lifetime of the 1D sp-band states, it follows that an excited electron can travel about four to five atomic sites along the nanowire before its escape into the bulk. We show that the surface projected band gap and the surface Brillouin zone backfolding of the substrate states play a fundamental role in the lifetime of the nanowire-localized states.
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Gauyacq, J. P., Diaz-Tendero, S., & Borisov, A. G. (2009). Mapping of the electron transmission through the wall of a quantum corral. Surf. Sci., 603(13), 2074–2081.
Résumé: We report on a theoretical study of the escape of confined surface states electrons from quantum corrals made of Cu adatoms on a Cu(1 1 1) surface. This study maps electron transmission through the corral wall and provides an extension of our earlier work focused on confinement in Cu corrals [S. Diaz-Tendero, F.E. Olsson, A.G. Borisov, J.P. Gauyacq, Phys. Rev. B 77 (2008) 205403]. The existence of two decay modes for the confined surface state is stressed: (i) non-resonant tunnelling through the corral wall concentrated on the Cu adatoms and (ii) a resonant-induced decay involving the transient formation of a resonant state localized on top of the corral wall. The present mapping of the electron transmission reveals how the interference between the two decay modes works: there exist regions where the electron leaves the corral, balanced by regions where it enters the corral, though the global behaviour of the quasi-stationary states is electron escape from the corral. (C) 2009 Elsevier B.V. All rights reserved.
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Khemliche, H., Rousseau, P., Roncin, P., Etgens, V. H., & Finocchi, F. (2009). Grazing incidence fast atom diffraction: An innovative approach to surface structure analysis. Appl. Phys. Lett., 95(15), 151901.
Résumé: An alternative diffraction technique, based on grazing incidence scattering of high energy atoms, is applied to surface structure determination of crystalline surfaces. This technique, named GIFAD for grazing incidence fast atom diffraction, uses the same geometry as reflection high energy electron diffraction but is less invasive, more surface sensitive, and readily interpretable quantitatively. We present here a demonstration of this approach on a prototypical II-VI compound, ZnSe(001). Besides providing lattice parameter with high accuracy, we show that GIFAD gives straightforward access to the surface valence electron density profile, allowing clear identification of an electron transfer from Zn to Se. (C) 2009 American Institute of Physics. [doi:10.1063/1.3246162]
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Lorente, N., & Gauyacq, J. - P. (2009). Efficient Spin Transitions in Inelastic Electron Tunneling Spectroscopy. Phys. Rev. Lett., 103(17), 176601.
Résumé: The excitation of the spin degrees of freedom of an adsorbed atom by tunneling electrons is computed using strong coupling theory. Recent measurements [Heinrich et al., Science 306, 466 ( 2004)] reveal that electron currents in a magnetic system efficiently excite its magnetic moments. Our theory shows that the incoming electron spin strongly couples with that of the adsorbate so that memory of the initial spin state is lost, leading to large excitation efficiencies. First-principles transmissions are evaluated in quantitative agreement with the experiment.
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M.Ziegler, J. K., R.Berndt, A.G.Borisov et J.P.Gauyacq. (2009). Linewidth of a cesium adatom resonance on Ag(111). Phys. Rev. B, 79, 075401.
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Riedel, D., Bocquet, M. L., Lesnard, H., Lastapis, M., Lorente, N., Sonnet, P., & Dujardin, G. (2009). Selective Scanning Tunnelling Microscope Electron-induced Reactions of Single Biphenyl Molecules on a Si(100) Surface. J. Am. Chem. Soc., 131(21), 7344–7352.
Résumé: Selective electron-induced reactions of individual biphenyl molecules adsorbed in their weakly chemisorbed configuration on a Si(100) surface are investigated by using the tip of a low-temperature (5 K) scanning tunnelling microscope (STM) as an atomic size source of electrons. Selected types of molecular reactions are produced, depending on the polarity of the surface voltage during STM excitation. At negative surface voltages, the biphenyl molecule diffuses across the surface in its weakly chemisorbed configuration. At positive surface voltages, different types of molecular reactions are activated, which involve the change of adsorption configuration from the weakly chemisorbed to the strongly chemisorbed bistable and quadristable configurations. Calculated reaction pathways of the molecular reactions on the silicon surface, using the nudge elastic band method, provide evidence that the observed selectivity as a function of the surface voltage polarity cannot be ascribed to different activation energies. These results, together with the measured threshold surface voltages and the calculated molecular electronic structures via density functional theory, suggest that the electron-induced molecular reactions are driven by selective electron detachment (oxidation) or attachment (reduction) processes.
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Riedel, D., Cranney, M., Martin, M., Guillory, R., Dujardin, G., Dubois, M., & Sonnet, P. (2009). Surface-Isomerization Dynamics of trans-Stilbene Molecules Adsorbed on Si(100)-2x1. J. Am. Chem. Soc., 131(15), 5414–5423.
Résumé: Photoinduced trans-cis isomerization studies of stilbene molecules in the gas phase have led to a precise understanding of the corresponding molecular dynamics. Yet, when such molecules are adsorbed on surfaces, these reactions are expected to be strongly modified as compared to what is know in the gas phase. In this work, a low temperature (5 K) scanning tunneling microscope (STM) is used to image the trans-stilbene molecules deposited on a Si(100)-2x1 surface at 12 K. trans-Stilbene undergoes conformational changes during the adsorption process such that four different stilbene conformers are observed: trans-stilbene (TS), cis-stilbene (CS), and two new conformers I(1) and I(2). Furthermore, electronic excitation of individual stilbene molecules, by means of tunnel electrons, is shown to activate specific reversible molecular surface isomerization (TS <-> I(1) and CS <-> I(2)) combined with diffusion across the surface. Calculated STM topographies, using the tight binding method, indicate that the CS and TS molecules are physisorbed. The molecular conformations of the surface isomers I(1) and I(2) are suggested to be analogous to transient states conformations of the stilbene molecule when stabilized by the silicon surface. The measurements of the molecular surface isomerization and diffusion reaction yields are used to build a qualitative potential energy surface of the various stilbene reactions. The molecular surface-isomerization dynamics is shown to be influenced by the type of dopant (n or p). This is related to surface charging, which reveals modifications in the stilbene ionization potential.
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Riedel, D., Delacour, C., Mayne, A. J., & Dujardin, G. (2009). Very low thermally induced tip expansion by vacuum ultraviolet irradiation in a scanning tunneling microscope junction. Phys. Rev. B, 80(15), 155451.
Résumé: The thermal and photoelectronic processes induced when a vacuum ultraviolet (VUV) laser irradiates the junction of a scanning tunneling microscope (STM) are studied. This is performed by synchronizing the VUV laser shots with the STM scan signal. Compared to other wavelengths, the photoinduced thermal STM-tip expansion is not observed when the VUV radiation is freed from spurious emissions. Furthermore, we demonstrate that the purified VUV photoinduced transient signal detected in the tunnel current is entirely due to photoelectronic emission and not combined with thermionic processes. The ensuing photoelectron emission is shown to be independent of the tip-surface distance while varying linearly with the pure VUV laser intensity. These results illustrate a strong decoupling between phonons and photoelectrons which allows a very weak STM-tip expansion.
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Teperik, T. V., & Borisov, A. G. (2009). Optical resonances in the scattering of light from a nanostructured metal surface: A three-dimensional numerical study. Phys. Rev. B, 79(24), 245409.
Résumé: We present the full three-dimensional numerical study of the scattering of light by the gold substrate composed of square periodic array of inverted pyramidal pits. The time-dependent wave-packet-propagation approach was used to extract the complete scattering matrix as well as the near fields. The role of the pit-localized and surface-supported plasmonic modes in resonant reflection spectra and field enhancement is revealed. We show that the resonances in the specular reflection arise because of the excitation of the pit-localized plasmons while resonant absorption is linked with excitation of the surface-plasmon polaritons. For certain structure parameters absorption can reach 100%. Our theoretical results are in a good agreement with recently published experimental data [N. M. B. Perney , Opt. Express 14, 847 (2006); Phys. Rev. B 76, 035426 (2007)]. We also show that present structure allows one to obtain zero specular reflection where all scattered intensity is redirected into the grazing beams.</p>.
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Tranvouez, E., Boer-Duchemin, E., Mayne, A. J., Vanderbruggen, T., Scheele, M., Cartwright, R., Comtet, G., Dujardin, G., Schneegans, O., Chretien, P., & Houze, F. (2009). Influence of morphology on the conductance of single-crystal diamond surfaces measured by atomic force microscopy. J. Appl. Phys., 106(5), 054301.
Résumé: A detailed atomic force microscopy (AFM) study of the conductance of hydrogenated single-crystal diamond (100) surfaces over submicron length scales is presented. The hydrogenation process was found to influence the surface morphology by reducing surface roughness, with the subsequent appearance of small triangular structures (with side lengths of 200 nm). Conducting AFM measurements revealed variations in the current and resistance that were related to the topography. Possible explanations include surface inhomogeneity of the electronic structure or variations in the contact resistance due to the surface roughness.
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Tranvouez, E., Orieux, A., Boer-Duchemin, E., Devillers, C. H., Huc, V., Comtet, G., & Dujardin, G. (2009). Manipulation of cadmium selenide nanorods with an atomic force microscope. Nanotechnology, 20(16), 165304.
Résumé: We have used an atomic force microscope (AFM) to manipulate and study ligand-capped cadmium selenide nanorods deposited on highly oriented pyrolitic graphite (HOPG). The AFM tip was used to manipulate (i.e., translate and rotate) the nanorods by applying a force perpendicular to the nanorod axis. The manipulation result was shown to depend on the point of impact of the AFM tip with the nanorod and whether the nanorod had been manipulated previously. Forces applied parallel to the nanorod axis, however, did not give rise to manipulation. These results are interpreted by considering the atomic-scale interactions of the HOPG substrate with the organic ligands surrounding the nanorods. The vertical deflection of the cantilever was recorded during manipulation and was combined with a model in order to estimate the value of the horizontal force between the tip and nanorod during manipulation. This horizontal force is estimated to be on the order of a few tens of nN.
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Zugarramurdi, A., Zabala, N., Silkin, V. M., Borisov, A. G., & Chulkov, E. V. (2009). Lifetimes of quantum well states and resonances in Pb overlayers on Cu(111). Phys. Rev. B, 80(11), 115425.
Résumé: We present results of calculations of the lifetimes of excited electrons (holes) in quantum well states and quantum well resonances in Pb overlayers supported on Cu(111). Many-body decay via inelastic energy relaxation and one-electron decay via energy-conserving one-electron transfer into the substrate are considered. One-electron energies and wave functions have been computed for different coverages of the Pb overlayer (from 1 to 18 monolayers) by using a one-dimensional pseudopotential for the entire overlayer-substrate system in the framework of density functional theory within the local density approximation. The elastic (energy-conserving resonant electron transfer) contribution to the total lifetime broadening of quantum well resonances has been calculated within the wave packet propagation method. The inelastic electron-electron (many-body) contribution to the lifetime broadening of both occupied and unoccupied quantum well states has been evaluated using GW approximation. The decay mechanisms of both quantum well states and quantum well resonances in thick overlayers are discussed.
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Actes de Conférences |
Bundaleski, N., Khemliche, H., Rousseau, P., Cassimi, A., Maunoury, L., & Roncin, P. (2009). Discharging dynamics of insulator surfaces irradiated by highly charged ions. In 14TH INTERNATIONAL CONFERENCE ON THE PHYSICS OF HIGHLY CHARGED IONS (HCI 2008) (Vol. 163, 012091).
Résumé: Guiding and focusing of keV to MeV ions by insulator micro-capillaries offer exciting perspectives for the production of low divergence micro-sized beams and the spatial control over the irradiated zone. These effects result from the local charging of the capillary inner wall and depend strongly on the charging/discharging dynamics of the insulating material. This dynamics has been studied on various glass insulator surfaces (borosilicate, fused silica and quartz) by grazing incidence highly charged ion beams. We propose simple experimental methods to derive the relevant time constant and study the influence of temperature and different material properties to the charging process.
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Cassimi, A., Maunoury, L., Muranaka, T., Huber, B., Dey, K. R., Lebius, H., Lelievre, D., Ramillon, J. M., Been, T., Ikeda, T., Kanai, Y., Kojima, T. M., Iwai, Y., Yamazaki, Y., Khemliche, H., Bundaleski, N., & Roncin, P. (2009). Imaging dynamics of charge-auto-organisation in glass capillaries. In NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS (Vol. 267, pp. 674–677).
Résumé: Multiply charged ion beam transmission through insulating capillaries is today a very active field of research. Thanks to the work of several groups during the last five years, several features of this unexpected process have been evidenced. The open challenge is to understand and control the self-organized charging-up of the capillary walls, which leads finally to the ion transmission. Up to now, the specific charge distribution on the inner surface, as well as the dynamics of the build-up, are still to be understood. While capillaries usually studied are microscopic pore networks etched in different materials, our concern is in macroscopic single capillaries made of glass. With a length of several centimeters and a diameter of a few micrometers at the exit, these capillaries have nevertheless the same aspect ratio as the etched pores (length/diameter approximate to 100). One of the leading goals of this research on single capillaries is to produce multi-charged ion beams with diameters smaller than a micrometer (nano-beams). These glass capillaries offer the opportunity to be used as an ion funnel due to their amazing proper-ties of guiding and focusing highly charged ion beams without altering neither their initial charge state nor the beam emittance (<10(-3) pi mm mrad). However, the understanding of the underlying process is not complete and relies on models assuming charge patches distributed along the capillary and which still need to be tested. We present the first observation imaging the dynamics of the charging-up process in single glass capillaries. During the build-up of the self-organized charge deposition on the capillary walls, the 230 keV Xe(23+) transmitted beam is deflected back and forth several times as the outgoing current increases. This is in agreement with the picture of charge patches created sequentially along the capillary and thus deflecting the beam until a stationary state is reached. (C) 2008 Elsevier B.V. All rights reserved.
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