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E. Magerl,
S. Neppl,
A. L. Cavalieri,
E. M. Bothschafter,
M. Stanislawski,
Th. Uphues,
M. Hofstetter,
U. Kleineberg,
J. V. Barth,
D. Menzel,
F. Krausz, R. Ernstorfer,
R. Kienberger,
P. Feulner
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ABSTRACT: We describe an apparatus for attosecond photoelectron spectroscopy of solids and surfaces, which combines the generation of isolated attosecond extreme-ultraviolet (XUV) laser pulses by high harmonic generation in gases with time-resolved photoelectron detection and surface science techniques in an ultrahigh vacuum environment. This versatile setup provides isolated attosecond pulses with photon energies of up to 140 eV and few-cycle near infrared pulses for studying ultrafast electron dynamics in a large variety of surfaces and interfaces. The samples can be prepared and characterized on an atomic scale in a dedicated flexible surface science end station. The extensive possibilities offered by this apparatus are demonstrated by applying attosecond XUV pulses with a central photon energy of ∼125 eV in an attosecond streaking experiment of a xenon multilayer grown on a Re(0001) substrate.
Review of Scientific Instruments 06/2011; 82(6):063104-063104-6. · 1.37 Impact Factor
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M Schultze,
M Fiess,
N Karpowicz,
J Gagnon,
M Korbman,
M Hofstetter,
S Neppl,
A L Cavalieri,
Y Komninos,
Th Mercouris, [......],
S Nagele,
J Feist,
J Burgdörfer,
A M Azzeer, R Ernstorfer,
R Kienberger,
U Kleineberg,
E Goulielmakis,
F Krausz,
V S Yakovlev
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ABSTRACT: Photoemission from atoms is assumed to occur instantly in response to incident radiation and provides the basis for setting the zero of time in clocking atomic-scale electron motion. We used attosecond metrology to reveal a delay of 21 +/- 5 attoseconds in the emission of electrons liberated from the 2p orbitals of neon atoms with respect to those released from the 2s orbital by the same 100-electron volt light pulse. Small differences in the timing of photoemission from different quantum states provide a probe for modeling many-electron dynamics. Theoretical models refined with the help of attosecond timing metrology may provide insight into electron correlations and allow the setting of the zero of time in atomic-scale chronoscopy with a precision of a few attoseconds.
Science 06/2010; 328(5986):1658-62. · 31.20 Impact Factor
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M. Schultze,
M. Fiess,
N. Karpowicz,
J. Gagnon,
M. Korbman,
M. Hofstetter,
S. Neppl,
A. L. Cavalieri,
Y. Komninos,
Th. Mercouris, [......],
S. Nagele,
J. Feist,
J. Burgdorfer,
A. M. Azzeer, R. Ernstorfer,
R. Kienberger,
U. Kleineberg,
E. Goulielmakis,
F. Krausz,
V. S. Yakovlev
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ABSTRACT: Photoemission from atoms is assumed to occur instantly in response to incident radiation and provides the basis for setting the zero of time in clocking atomic-scale electron motion. We used attosecond metrology to reveal a delay of [IMG]f1.gif" ALT="Formula" BORDER="0"\textgreater} attoseconds in the emission of electrons liberated from the 2p orbitals of neon atoms with respect to those released from the 2s orbital by the same 100-electron volt light pulse. Small differences in the timing of photoemission from different quantum states provide a probe for modeling many-electron dynamics. Theoretical models refined with the help of attosecond timing metrology may provide insight into electron correlations and allow the setting of the zero of time in atomic-scale chronoscopy with a precision of a few attoseconds.
Science. 06/2010; 328:1658--1662.
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ABSTRACT: We demonstrate the collinear generation of few-femtosecond ultraviolet and attosecond extreme ultraviolet pulses via a combination of third-harmonic and high harmonic generation in noble gases. The ultrashort coherent light bursts are produced by focusing a sub-1.5-cycle near-infrared/visible laser pulse in two subsequent quasi-static noble gas targets. This approach provides an inherently synchronized pair of UV and XUV pulses, where the UV radiation has a photon energy of approximately 5 eV and a pulse energy of up to 1 microJ and the XUV radiation contains up to 3.5 10(6) XUV photons per shot with a photon energy exceeding 100 eV. This source represents a novel tool for future UV pump/XUV probe experiments with unprecedented time-resolution.
Optics Express 04/2010; 18(9):9173-80. · 3.59 Impact Factor
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ABSTRACT: A broad distribution of time constants was found for photoinduced heterogeneous electron transfer (PHET) from the excited-state of a perylene chromophore when the latter was attached via long rigid bridge/anchor groups to the inner walls of nanometer-size cavities formed in a colloidal anatase TiO2 layer. In contrast, in the same environment PHET was dominated by only one short time constant when the perylene chromophore was attached via a short anchor/bridge group. The same results were obtained irrespective of the specific chemical composition of the short or long rigid anchor/bridge groups. To verify that the set of different time constants was caused by different microscopic environments in the nanometer-cavities, PHET was also measured for the same perylene compounds on the (110) surface of TiO2 rutile single crystals, employing here the more sensitive femtosecond two-photon photoemission technique in place of transient absorption. On the surface of the single crystals only one long time constant was measured for PHET also in the case of the long rigid bridge/anchor groups. Thus, the broad distribution of time constants observed in the nanometer-size cavities for the long rigid bridge/anchor groups can be attributed to different microscopic environments giving rise to different distances between the chromophore and the nearest TiO2 wall. Consequences of this pathway dependent PHET are discussed for the design of dye molecules and electrodes in dye-sensitized solar cells.
08/2007;
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ABSTRACT: A complete picture of different interfacial electron transfer dynamics has been obtained from transient absorption and two-photon
photoemission data when inserting different anchor/bridge groups between the excited organic donor and the electrode surface.
12/2006: pages 270-272;
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ABSTRACT: We demonstrate the operation of a 100 kHz noncollinear optical parametric amplifier that is pumped by just a few microjoules of 800 nm pulses with 50 fs duration. The device delivers sub-20 fs pulses tunable from 460 nm to beyond 1 microm and pulse energies up to 750 nJ when it is pumped with 7 microJ of energy. The design of the single-stage amplifier has been carefully optimized, and the design considerations are discussed.
Optics Letters 06/2006; 31(9):1289-91. · 3.40 Impact Factor
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L Töben,
L Gundlach, R Ernstorfer,
R Eichberger,
T Hannappel,
F Willig,
A Zeiser,
J Förstner,
A Knorr,
P H Hahn,
W G Schmidt
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ABSTRACT: Time-dependent two-photon photoemission spectra are used to resolve the femtosecond dynamics of hot electrons at the energetically lowest surface resonance of reconstructed InP(100). Two different cases are studied, where electrons either are lifted into the surface resonance via a direct optical transition or are captured from bulk states. These data are the first of this kind recorded with a time resolution below 70 fs. The microscopic analysis shows that electron-phonon scattering is a major mechanism for electron transfer between surface and bulk states.
Physical Review Letters 03/2005; 94(6):067601. · 7.37 Impact Factor
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L. Töben,
L. Gundlach, R. Ernstorfer,
R. Eichberger,
T. Hannappel,
F. Willig,
A. Zeiser,
J. Förstner,
A. Knorr,
P. H. Hahn,
W. G. Schmidt
Phys. Rev. Lett. 02/2005; 94:067601.
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ABSTRACT: MOCVD-preparation of ordered [100] surfaces of InP and GaP was monitored in-situ with reflectance difference/anisotropy spectroscopy (RDS/RAS). RDS was measured at 20 K after contamination-free transfer of the sample to UHV. Specific RD spectra with the highest peaks and fine structure were correlated with different ordered surface reconstructions. The ordered In-rich InP[100] surface was investigated with femtosecond 2PPE. Surface states and surface resonances near the Γ-point showed up as peaks in the 2PPE spectrum in agreement with theoretical predictions.
Indium Phosphide and Related Materials Conference, 2002. IPRM. 14th; 02/2002
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ABSTRACT: Ultrafast heterogeneous electron transfer (HET) from the excited singlet state of the large organic chromophore perylene into the inorganic semiconductor rutile TiO2 was investigated with femtosecond time-resolved two-photon photoemission (TR-2PPE). The strength of the electronic interaction between the chromophore and the semiconductor was varied by inserting different anchor/bridge groups that functioned either as electronic wire or electronic tunnelling barrier. Both anchor groups, i.e. carboxylic and phosphonic acid, formed strong chemical bonds at the TiO2 surface. The perylene chromophore with the different anchor/bridge groups was adsorbed from solution in a dedicated ultra-high-vacuum (UHV) chamber. The adsorption geometry of the chromophore perylene was determined from angle and polarization dependent two-photon photoemission (2PPE) signals and was found to be very different for the two different anchor/bridge groups. The measured adsorption geometries are compatible with recent DFT (density functional theory) calculations by P. Persson and co-workers [M. Nilsing, S. Lunell, P. Persson, L. Ojamäe, Phosphonic acid adsorption at the TiO2 anatase (1 0 1) surface investigated by periodic hybrid HF-DFT computations, Surf. Sci. 582 (2005) 49–60]. Two different processes contributed to the TR-2PPE transients, firstly electron transfer from the chromophore to the electronic acceptor states on the surface and secondly escape of the electrons from the surface into the bulk of the semiconductor. The latter escape process was measured separately by making the interfacial electron injection process instantaneous when the chromophore catechol was employed in place of the perylene compounds. The thus measured electron escape behavior was governed by the same time constants that have recently been predicted by Prezhdo and coworkers from time dependent DFT calculations [W.R. Duncan, W.M. Stier, O.V. Prezhdo, Ab initio nonadiabatic molecular dynamics of the ultrafast electron injection across the Alizarin-TiO2 interface, J. Am. Chem. Soc. 127 (2005) 7941–7951]. The HET times derived from the 2PPE transients showed very good agreement with HET times measured via transient absorption (TA) on anatase TiO2 layers. The measured energy distribution of the 2PPE signals for the injected electrons suggests that a high density of electronic acceptor states is operative in both systems and is spread over an at least 1 eV wide energy range. The acceptor states are tentatively identified with surface states created through the formation of chemical bonds between the anchor groups of the organic molecules and surface atoms of the semiconductor.
Progress in Surface Science.
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ABSTRACT: Femtosecond electron injection from optically populated donor states into the conduction band of semiconductors
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ABSTRACT: We demonstrate the operation of a 100 kHz noncollinear optical parametric amplifier that is pumped by just a few J of 800 nm pulses with 50 fs duration. The device delivers sub 20 fs pulses tunable from 460 nm to beyond 1 m and pulse energies up to 750 nJ when pumped with 7 J. The design of the single stage amplifier has been carefully optimized and the design considerations are discussed
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ABSTRACT: Ultrafast electron escape dynamics following excitation of the interfacial charge transfer complex of catechol prepared on the rutile TiO2 110 surface was investigated with femtosecond two photon photoemission 2PPE . Laser pulses were generated with two non collinear optical parametric ampli amp; 64257;ers operated simultaneously at a repetition rate of 150 kHz delivering a crosscorrelation function with 35 fs FWHM. Catechol was absorbed from solution. The experimental data were not depending on the choice between three di amp; 64256;erent solvents. Photo induced interfacial charge transfer was instantaneous and thus the rise of the signal was controlled by the crosscorrelation function. The energy distribution of the hot electrons generated at the surface was measured as 2PPE spectrum. The decay of the 2PPE signal was non exponential with a amp; 64257;rst time constant below 10 fs, a dip in the 50 fs to 100 fs range, and a tail lasting for picoseconds. It was attributed to the release of the electrons from the surface and escape into the bulk of the semiconductor
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ABSTRACT: Heterogeneous electron transfer from the excited state of a molecular chromophore to empty electronic states of a solid is an important process in a variety of fields ranging from photo chemistry [1] to molecular electronics [2]. In the last years numerous experimental and theoretical studies have addressed the dynamics of such processes. Light induced heterogeneous electron transfer has been extensively investigated mainly with transient absorption in polycrystalline or nano crystalline samples with anchored chromophores [3]. Transient two photon photoemission TR 2PPE is capable of detecting the signals of sub monolayer chromophore coverage on single crystal surfaces and gives information about the electron transfer dynamics, the position of the electronic levels at the surface, and the adsorption geometry of the molecules on the surface. Comparing the different 2PPE transients and the corresponding transient absorption measurements for identical systems revealed the origin of unexpected short time constants in the transient absorption measurements
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ABSTRACT: Transport of electrons through molecular units is an important topic in the field of molecular electronics 1 and future device modeling. Most of the research 2 has focused on saturated aliphatic chains or on ring structures with saturated C C bonds, and only few theoretical papers and experiments have dealt with conjugated molecular bridges. All the theoretical work has addressed molecular bridges in ultra high vacuum UHV , whereas all the previous measurements have been performed in a solvent environment. We report here on transient absorption experiments in UHV that probed heterogeneous electron transfer from the molecular chromophore perylen via different molecular anchor bridge groups into nm structured anatase TiO2 layers
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ABSTRACT: Electronic properties of dye sensitized semiconductor nanocrystals, consisting of perylene Pe chromophores attached to 2 nm TiO2 nanocrystals via different anchor cum spacer groups, have been studied theoretically using density functional theory DFT cluster calculations. Approximate effective electronic coupling strengths for the heterogeneous electron transfer interaction have been extracted from the calculated electronic structures and are used to estimate femtosecond electron transfer times theoretically. Results are presented for perylenes attached to the TiO2 via formic acid Pe COOH , propionic acid Pe CH2 CH2 COOH , and acrylic acid Pe CHdCH COOH . The calculated electron transfer times are between 5 and 10 fs with the formic acid and the conjugated acrylic acid bridges and about 35 fs with the saturated propionic acid bridge. The calculated electron injection times are of the same order of magnitude as the corresponding experimental values and qualitatively follow the experimental trend with respect to the influence of the different substitutions on the injection times
