Abstracts of the invited contributions by alphabetic order:

Photoinduced processes at nanoparticulate systems

Katharina Al−Shamery

Carl v. Ossietzky Universität Oldenburg, Physikalische Chemie, Postfach 2503, 26111Oldenburg, Germany

Nano−sized transition metal clusters exhibit unique strongly size dependent properties withrespect to their electronic structure as well as morphology. This can be further influenced by theinteraction with the solid support. It will be shown that control over the branching ratio ofreaction channels of photoinduced processes at nanoparticulate systems can be obtained bytuning the particle size. Experiments will be presented about the UV−laser inducedphotochemistry of methane, NO and CO adsorbed on Pd aggregates deposited on an epitaxialfilm of Al2O3 on NiAl(110) at 3.5 eV and 6.4 eV. Aggregate diameters range between 5 Å up tomore than 80 Å. Within this size regime properties change from more molecular to metallicbehaviour which is furthermore depending on the adsorbate coverage. Aggregates ofintermediate size have diameters in the size regime of the mean free path of the electrons. Thestrong cluster size dependence of the ratio of photodesorption to photodissociation of methanewill be discussed. Photodesorption of NO is more complex and depends on the adsorptionspecies which differ from bulk adsorption species. CO does not desorb even from smallaggregates. However, interesting adsorption state changes are observed after laser excitationinverse to thermally induced processes. This behaviour is only observed in a certain coverageregime and is related to the occurrence of solitons.

Probing Electron Dynamics with STM

Richard Berndt

Institut für Experimentelle und Angewandte Physik Christian−Albrechts−Universität zu KielOlshausenstr. 40 D−24098 Kiel Germany

Two−dimensional (2D) electronic states give rise to distinct structure in scanning tunnelingspectroscopy (STS) data. In favorable cases, line shape analyses can be performed and electroniclifetimes can be determined. We use STS of pristine surfaces (noble metals, Na films) and ofartificial nanoscale arrays of adatoms to determine electronic lifetimes of 2D states and theirvariation with the binding energy.

Surface dynamics studied with femtosecond vibrational spectroscopy

M. Bonn1, Ch. Hess2, M. Wolf 3, and G. Ertl2

1Leiden Institute of Chemistry, P.O. Box 9502, 2300 RA Leiden, The Netherlands2Fritz−Haber−Institut der MPG, Faradayweg 4−6, 14195 Berlin, Germany

3Freie Universität Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin, Germany

We use femtosecond IR sum−frequency generation (SFG) to study various dynamical processesfor the model system of CO adsorbed on Ru(0001). We investigate the dynamics of vibrationalexcitations, such as energy (de−)localization and relaxation, which are believed to be offundamental importance in surface chemistry. We find that, at low coverage (below 0.02 ML), we can excite a significant fraction of the COmolecules to their first and second vibrationally excited state. This is evidenced by SFG lightemitted from higher−lying vibrational states, and allows for a direct determination of thevibrational anharmonicity [1]. With increasing coverage, the appearance of intermolecularcoupling leads to the disappearance of the discrete vibrational transitions due to delocalization ofthe vibrational energy within the adlayer. Applying a lineshape model, we deduce resonantvibrational energy transfer times within the adlayer in the ps range [2].

In a pump−SFG−probe experiment, where about 50% of the CO coverage is desorbed by astrong 800 nm fs−pump pulse, vibrational snapshots of the highly excited CO−adlayer during thedesorption process show a pronounced transient redshift of the center frequency and broadeningof the linewidth [3]. This is attributed to strong anharmonic coupling of the CO stretch vibrationto the frustrated rotation of CO on Ru(0001). These time−resolved SFG results demonstrate theperspective for real−time probing of transient species and transitions states in femtosecond laser−induced surface reactions [4].

[1] Ch. Hess, M. Bonn, S. Funk and M. Wolf, Chem. Phys. Lett. 325, 139 (2000).[2] Ch. Hess, M. Wolf and M. Bonn, Phys. Rev. Lett. 85, 4341 (2000).[3] M. Bonn, Ch. Hess, S. Funk, J. Miners, B. N. J. Persson, M. Wolf and G.Ertl,

Phys. Rev. Lett. 84, 4653 (2000).[4] S. Roke, A.W. Kleyn and M. Bonn, J. Phys. Chem. A (Letter) 105, 1683 (2001).

Optical Properties of nanostructures: a first−principleapproach

A. Castro(1,2), M.A.L. Marques(1), G.F. Bertsch(3) and A. Rubio(1)

(1) Material Physics Department, University of the Basque Country, Donostia, SpainCentro Mixto UPV/EHU−CSIC and Donostia International Physics Center (DIPC)

(alberto.castro@tddft.org)(2) Departamento de Física Teórica, Universidad de Valladolid.

(3)Physics Department and Institute for Nuclear Theory, University of Washington, Seattle WA98195 USA

A computational approach to the investigation of optical properties of nanostructures ispresented. Stress is made on the interaction of these systems with intense, femtosecond−longlaser radiation. The underlying theoretical framework is time dependent density−functionaltheory. The resulting code may be utilized for determining the linear optical response (someorganic, inorganic and biological examples are presented), or to study highly non−linearfemtosecond dynamics. Tbe latter possibility enables simulation of photo−reactions(isomerization, dissociation), observation of high−harmonic generation, etc.

Momentum resolved electron and phonon contribution to thequasiparticle decay at metal surfaces

Eugene Chulkov

Material Physics Department, University of the Basque Country, Donostia, Spain Centro MixtoFisica de Materiales UPV/EHU−CSIC and Donostia In ternational Physics Center (DIPC)

Screening and lifetimes of quasiparticles (electrons and holes) are of paramount importance forunderstanding many phenomena in condensed matter science. Here the calculation results forboth electron−electron and electron−phonon contribution are presented for simple and noblemetal surfaces. Electron−electron contribution as a function of energy and momentum isevaluated by using the self−energy formalism of many−body theory within the GWapproximation. Electron−phonon contribution is calculated for a hole in surface states onCu(111) and Ag(111) surface. Key ingredients (peculiar character of band stricture and surfacescreening) in the quasiparticle dynamics in surface states are discussed. Comparison with recentphotoemission and scanning tunneling spectroscopy measurements is made.

Atomic−scale control of electronic and dynamical processes onsemiconductor and insulator surfaces

G. Dujardin

Laboratoire de Photophysique Moléculaire, Orsay, France

Manipulation of individual atoms and molecules using the scanning tunneling microscope (STM)has opened fascinating areas of research. New concepts have recently emerged where atoms andmolecules on surfaces are considered not only as elementary building blocks of matter butmoreover as nano−objects or nano−machines in themselves. Using a single atom or molecule as afunctionalized nano−machine will require being able to control not only the position but alsonumerous electronic and dynamical processes at the atomic−scale. More specifically thisinvolves :

− the controlled connexion of functionalized organic molecules on surfaces− the control of both the elastic and inelastic electronic channels through a single atom or

molecule− the control of the dynamical processes accompanying the inelastic electronic processes− the search for atomic−scale controlled semiconductor or insulator surfaces having a band

gap wide enough so that the nano−machines can be operated through their electronicchannels located withtin the surface band gap.

Probing nanomagnetism on the femtosecond time scale

Hermann A. Dürr

BESSY, Albert−Einstein−Str. 15, 12489 Berlin, Germany

Magnetism is a collective phenomenon involving correlated electrons and is often dependent onsystem dimensions. The relevant interactions in magnetic solids such as exchange, spin−orbit andelectron−phonon coupling are of various strength and lead to different characteristic time scalesfor energy transfer between orbital, spin and lattice degrees of freedom. In this talk an overviewwill be given how pump−probe experiments using optical fs−lasers offer unique possibilities toinvestigate the ultra fast spin dynamics following a fs excitation of the electronic system. Spin−polarized photoelectron spectroscopy is used to achieve magnetic sensitivity while photoemissionelectron microscopy allows nm lateral resolution. Such studies are of direct relevance forestablishing the ultimate time scale for magnetic switching in future data storage devices.

Fs Spectroscopy at the planned BESSY SASE FEL in BerlinAdlershof

Wolfgang Eberhardt

BESSY GmbH Albert Einstein Strasse 15 12489 Berlin Germany

BESSY is planning to build a SASE FEL facility covering the photon energy range from 20 eVto 1 keV at the site next to the existing BESSY II storage ring. This new facility will offer laserlike photon beams with fully coherent, high power (mJ) pulses of ?20 fs duration, enabling awhole set of novel experiments dedicated to understand dynamical processes in matter or for theinvestigations of very dilute systems. The traditional BESSY II photon energy range, incombination with the fs pulse structure, is especially suited for the investigation of electrondynamics in atoms, molecules, clusters, and solids, as well as time−resolved stroboscopicmicroscopy on soft−matter and biological samples. Accordingly, the scientific scope of theBESSY FEL is complementary to the science envisioned for the planned TESLA X−FEL facilityat DESY.

Following the presentation of the parameters and the layout of the proposed facility, theScientific Case will be presented as it was developed by the prospective user community in thecourse of several scientific workshops and in discussions.

Phonon contribution to the decay rate of surface states

A. Eiguren

Departmento de Física de Materiales and Centro Mixto CSIC−UPV/EHU, Facultad deCiencias Químicas, Universidad del Pais Vasco/Euskal Herriko Unibertsitatea, Adpo. 1072,

20018 San Sebastián/Donostia, Basque Country, Spain

Near the Fermi level, a proper physical description of the lifetime of surface states must take intoaccount the interaction with bulk and surface phonon modes. The Rayleigh surface modecontribution alone determines at T=0, about ~85 % of the total broadening of noble metal (111)surface states within an energy window less than the maximum energy of this mode. This modeenergy range is about a factor of three smaller than the Debye energy, thus its importance for the$\lambda$ parameter (broadening slope with temperature) is even stronger.

Hot electron lifetimes from BEEM data and LAPW−GWcalculations

F.Flores* , P.Fernández de Pablos* , F.J.García−Vidal* , P.deAndrés** , F.Ladstadter+ ,

P.Puschning+ , C.Ambrosh−Draxl+ and U.Hohenester+

*Departamento de Física Teórica de la Materia Condensada , U.A.M., Madrid , Spain ** Instituto de Materiales Cantoblanco , Madrid , Spain ,

+ Institute for Theoretical Physics ,University of Graz , Austria

Hot electron lifetimes have been traditionally determined experimentally using timeresolved two−photon photoemission (2PPE) data . In this communication we show how ballisticelectron emission microscopy (BEEM) can be used to determine accurately those hot electronlifetimes . The main problem for an appropriate interpretation of BEEM−experiments is two−fold : (a) firstly , it is ne− cessary to have a correct description of the electron propagation alongthe metal base ; (b) on the other hand , tunneling between the tip and the metal surface should beappropriately described . We show how these two processes can be taken into account by meansof a Keldish−Green function formalism . Our ana− lysis allows us to obtain accurate lifetimes forAu and Pd . In a second step , we also present theoretical estimations of the hot electronlifetimes of Au and Pd using a LAPW−GW method combined with an RPA−dielectric responsecalculation . Finally , we compare both our BEEM and LAPW results with 2PPE and othertheoretical hot electron lifetimes .

Effect of adsorbates on image states at metal surfaces

A.G. Borisova, A.K. Kazanskyb and J.P. Gauyacqa

a Laboratoire des Collisions Atomiques et Moléculaires,Unité Mixte de Recherche CNRS−Université Paris Sud UMR 8625,Bât 351, Université Paris−Sud, 91405 Orsay Cedex, France

b Fock Institute of Physics, St Petersburg University, 198504 St Petersburg, Russia

On a metallic surface, in the case when a surface projected band gap exists, electronic imagestates appear, in which the electrons are trapped in the long range image potential of the surface.They are localised in front of the surface and decay by transfer to the bulk via inelastic electron−electron interaction1. Due to their localisation, the image states are very sensitive probes of thesurface, in particular of the local modifications introduced by defects or adsorbates. Indeed, theelectrons in an image state can be viewed as a 2−dimension electron moving quasi−freely parallelto the surface and any defect or adsorbate on the surface can scatter it, either inside the imagestate continuum (change of direction of the electron motion) or into other continua (energyconserving inter−band transitions). The first experimental observation of such scattering was byKevan in 1986 [2]. We report on the theoretical study of the scattering of image state electrons by alkali adsorbateson Cu(111) and Cu(100) surfaces. Inter−band transitions induced by scattering are shown to leadto a very efficient decay process of the image state, even for trace concentrations of the alkali onthe Cu surface. The dependence of the transitions on the kinetic energy of the electron parallel tothe surface exhibits many structures due to the various energy thresholds and to quasi−stationarystates induced by the presence of the alkali on the surface. In addition to the long−lived quasi−stationary state, which has been experimentally observed by Time−resolved−2−Photon−Photo−Emission [3,4], we found quasi−stationary states with an energy just below the image statecontinuum. They are interpreted as due to localisation of the 2D image state continuum by theattractive electron−adsorbate interaction potential.

1. P.M.Echenique and J.B.Pendry Prog.Surf.Sci. 32 (1990) 1112. S.D.Kevan Phys.Rev.B 33, R4364 (1986)3. M.Bauer, S.Pawlik and M.Aeschlimann Phys. Rev. B 55 (1997) 100404. S.Ogawa, H.Nagano and H.Petek Phys.Rev.Lett. 82 (1999) 1931

The core level clock in free molecules and in adsorbates

Z.W. GortelPhysics Department, University of Alberta, Edmonton, Alberta, Canada,

D. MenzelPhysik−Dept. E20, Techn. University Muenchen, Garching, Germany.

When atoms or molecules are resonantly core excited with radiation band widths below thelifetime width of the excited state, and the decay electron spectra are measured with sufficientresolution, the entire process has to be treated as a one−step process, as has been shown by manyauthors. This leads to a number of interesting effects often summarized as Auger resonant Ramaneffect and due to competition between core hole decay and nuclear motion. Wave packetcalculations are well suited to lead to a better conceptual understanding of the underlyingphysical processes. We will demonstrate the main effects in free molecules for bound as well asfor repulsive intermediate states which can all be understood in terms of energy conservation, theoccurring interference effects, and the competition between nuclear motion and core hole decay.The coupling of an atom or molecule to the surface of a solid destroys the energy and phaseconservation for a fraction of the events which depends on the coupling strength. This can beused as basis to study this coupling by measuring extremely fast charge transfer processes (seetalk by W. Wurth at this conference), using the core hole life time as clock. An analysis within ageneral framework suitable for small systems coupled to a dissipative continuum justifies asimple rate approach to the interpretation of such experiments.

Surface state on noble metal (111) surfaces

S. Huefner

Fachbereich Physik, Universitaet des Saarlandes D−66041 Saarbruecken

The lifetime width of the surface states on the (111) surfaces of the noble metals is determinedby very high ultraviolet photoemission spectroscopy. The dispersion curves are measured fromwhich the effective masses are determined. For the case of gold a spin orbit splitting is clearlydetected. The temperature dependence of the width and the position of the surface states aremeasured and the comparison with a sophisticated calculation shows good agreement.Experiments with surfaces covered by Xe show large shifts in the position of the surface states.

Embedding methods for surface and interface calculations

J.E. Inglesfield

Department of Physics and Astrophysics, Cardiff University,PO Box 913, Cardiff, CF24 3YB

The embedding method enables the electronic Schroedinger equation for a region of interest −here the surface or interface − to be solved, an embedding potential replacing the substrates andensuring that the wavefunction matching conditions are satisfied. There have been recentadvances in the embedding method, which enable embedding potentials to be found moreaccurately than before, and these shall be described. I shall discuss applications of embedding tointerfaces, confined systems, and solving Maxwell’s equations by the same technique.

Ultrafast control of bond breaking in photodesorption from oxide surfaces

C. Rakete, W. Drachsel, N. Nilius, N. Ernst, T. Klüner, and H.−J. Freund

Fritz−Haber−Institut der Max−Planck−Gesellschaft, Department of ChemicalPhysicsFaradayweg 4−6, D−14195 Berlin, Germany

Quantum state resolved photodesorption studies from clean oxide surfaces as well as metalparticles on oxide surfaces are reported. Experiments where pump probe techniques are coupledwith REMPI detection of desorbing molecules will be described and interpreted on the basis ofab initio calculation of potential energy surfaces. Surface polariton excitations in depositedcoinage metal clusters are studied in detail using photon−STM techniques. Two−photon−photoemission involving such excitations is feasible and may be used with respect to ultrafastcontrol of photochemical processes.

Five−wave mixing investigation of electron dynamics at siliconsurfaces

U. Höfer, C. Voelkmann, M. Reichelt, T. Meier, and S. W. Koch

Philipps−Universität, Fachbereich Physik, D−35032 Marburg, GermanyE−mail: hoefer@physik.uni−marburg.de

Ultrafast energy relaxation, dephasing, and diffusion of excited carriers in semiconductors arewidely studied using transient grating experiments. We employed a synthesis of degenerate four−wave mixing (DFWM) and second−harmonic generation (SHG) to study the coherent dynamicsof dangling−bond states at silicon surfaces with fs time−resolution. The resulting five−wavemixing process is characterized by a χ(4)−tensor; like the χ(2)−processes SHG or SFG it is dipole−forbidden in the bulk of centrosymmetric materials. The thereby surface−sensitive probe can bedescribed by diffraction of probe photons at a transient population grating with theirupconversion to the second harmonic.

The experiments were performed with a cavity−dumped Ti:sapphire laser that produced 13−fspulses centered at a wavelength of 790 nm with pulse energies up to 50 nJ and repetition rates ashigh as 2 MHz. The short laser pulses excited the dangling bond states of Si(001) and Si(111).For both surfaces we find population relaxation times (T1) on the order of 200 fs anddecoherence times (T2) around 5 fs. In the case of Si(001) the experiments show an unexpectedslow rise (~ 50 fs) of the signal intensity as a function of the probe pulse delay. Using opticalBloch equations this response can be described by postulating a mechanism in which thephotoexcited carriers are rapidly scattered to other states which then dominantly contribute to thesubsequent generated second−harmonic signal.

Selective bond breaking in adsorbates by core excitations

Peter Feulner and Dietrich Menzel

Physik−Department E20. Techn. Universitaet MuenchenGarching, Germany

Selective bond breaking is a highly desirable procedure in chemistry, and photo−excitations are asuitable tool. One possibility to localize the necessary energy on a specific atom is to start with acore excitation which by nature is localized. For resonant excitations, use of narrowbandsynchrotron radiation allows to distinguish even between like atoms in different surroundings.Some success has been obtained in this way in molecules, although the fact that the dissociativeaction is usually that of the decay states of the core excitation, which are more spread out, tendsto limit the selectivity. We have recently looked into the corresponding processes in adsorbates,starting with the simple model systems of diatomic molecules (CO, N2) on transition metalsurfaces, where selectiveness can be checked by comparing the ratio of breaking the internal andthe adsorbate bond. In desorbing ions, strong selectivity is indeed observed, but due to very fastcharge transfer the overall yields are extremely small, except for complex multiple excitations.Higher yields are found for desorbing neutrals, and selectivity has been found first for N2 onRu(001), albeit in a counterintuitive way (resonant excitation of the outer N breaks the adsorbatebond preferentially, that of the inner N the intramolecular bond). This was originally interpretedas being mainly due to the different predominant decay states reached via these atom−specificexcitations. However, comparison of data for N2 on Ru and Ni, and for N2 and CO, show thatthis − while not negligible − is not a sufficient explanation. Selective deexcitation processes tothe substrate are of equal importance and can even supersede the influence of decay stateselectivity. In any case, selective bond breaking at surfaces does take place and is governed bycomplex interactions both in the adsorbed molecules and between adsorbate and substrate.

Electron−induced manipulation of water on surfaces: Fromhexamer formation to dissociation

Karina Morgenstern, Karl−Heinz Rieder

Institut für Experimentalphysik, FU Berlin, D−14195 Berlin, Germany(corresponding author: K. Morgenstern, e−mail: karina.morgenstern@physik.fu−berlin.de)

Starting from water monomers adsorbed on Cu(111), we have both formed hydrogen−bondedclusters and dissociated the water molecule via electrons tunnelling inelastically from the tip of aSTM. We have identified the different processes due to inelastic interaction of tunnellingelectrons with adsorbed water molecules for voltages between 100 mV and 4.5 V. After depositing water in submonolayer coverage at 16 K, i.e., below the temperature ofmonomer diffusion, we have created small ice clusters by excitation of vibrational modes of themolecule. Electrons may couple to a vibrational mode of the water molecules. The energylocalised in the nuclear vibrations subsequent to electronic relaxation may be sufficient tosurmount the diffusion barrier. The experimentally determined threshold energy of 220 mVcorresponds to the energy of the scissoring mode of 202 and 204 meV, another peak at (430 ? 20)mV to the O−H stretching frequency of 405 and 417 meV on Cu(111). Via this excitation,enough energy was transferred into the system to create regular clusters and hydrogen bonds.This includes the important basic unit of ice and water, the cyclic hexamer. Dimers, cyclictrimers, and cyclic hexamers show all systematic deviations from high symmetry structures. Thehexamer sacrifices on−top adsorption to take on the bilayer lattice distance. For higher energies, we find a threshold region at UD = (1.6 ? 0.5) eV. Below this thresholdamorphous ice clusters on Cu(111) are crystallised to 2D clusters. The restructuring of theclusters from three−dimensional to two − dimensional can be compared to the water filmsdeposited at T < 110 K forming amorphous solid water, while those grown at T > 140 K formpolycrystalline ice. We can therefore speculate that the electron bombardment is equivalent toheating the cluster to 140 K. Above the threshold, the water dissociates. The dissociation probability increases linearly in thethreshold region from 1.1 V to 2.1 V. The clear threshold in the bias indicates a resonance (anti−bonding orbital) near the molecule’s energy level to which the electron attaches. The dependenceof the exact position of this anti−bonding orbital on the tip condition can be explained by electricfield shifts of the molecular levels reminiscent of hydrogen dissociation on silicon.

Ultrafast transport phenomena in metal−insulator−metal contacts

Walter Pfeiffer

Physikalisches Institut der Universität Würzburg Am Humbland ,D−97074Würzburg, Germany

Transport of excited carriers through interfaces in heterogeneous systems is of fundamentalinterest in a broad range of different fields, such as optoelectronics, biophotonics and surfacechemistry. Excitation of carriers with ultrashort laser pulses allows to investigate dynamicprocesses like for example charge transfer at the interface in the time domain. Here we applytime−resolved photocurrent spectroscopy to investigate the transport of excited carriers in metal−insulator−metal contacts. Three different transport mechanisms can be identified: (a) internalphotoemission (b) tunneling of excited electrons and (c) photoassisted tunneling. Furthermore thetime−resolved signal reveals the existence of rather longlived interface states (lifetime > 40 fs).In addition to these experiments the broader applicability of time−resolved photocurrentspectroscopy and the prospect of coherent control of charge transfer at interfaces are discussed.

Surface knowledge from ultra−high vacuum to technically−relevantconditions: the example of catalytic CO oxidation

Karsten Reuter and Matthias Scheffler

Fritz−Haber−Institut, Faradayweg 4−6, D−14195 Berlin

Present knowledge of surface properties and the function of materials is largely based on studies(experimental and theoretical) that are performed at low temperatures and ultra−low pressures.However, the majority of everyday applications, like e.g. catalysis, operate at atmosphericpressures and at temperatures often higher than 300K. Analyzing this gap we employ acombination of density−functional theory and thermodynamics [1] to construct a phase diagramof the most stable surface structures in the whole experimentally accessible (T,p)−space fromultra−high vacuum to technically−relevant pressures and temperatures. We emphasize the crucialimportance of such phase diagrams: Only when gas phase parameters are changed withoutcrossing from one phase in (T,p)−space to another one, the "pressure gap" can be bridged andcorresponding low−pressure studies may be used to model high−pressure applications. We alsostress that phase coexistence, i.e. regions in the phase diagram close to a boundary betweendifferent stable structures, that imply an enhanced dynamics at the surface, might be particularlyimportant for catalytic applications. To illustrate these concepts we present such an ab initiocalculated phase diagram for a catalyst surface in contact with the reactive environment presentduring CO oxidation. [1] K. Reuter and M. Scheffler, Phys. Rev. B 65, 035406 (2002).

The scanning tunneling microscope as operative tool: Physics andchemistry with single atoms and molecules

Karl−Heinz Rieder

Free University of Berlin, Department of Physics, Arnimalle 14, D−14195 Berlin

The scanning tunneling microscope (STM) has been developed in recent years from aninstrument capable of imaging surfaces with atomic resolution to an operative tool with whichsurfaces can be modified on nanometer and even atomic scale. The STM−provides severalparameters by which atoms and molecules at (and in) surfaces can be influenced: The tip/sampleforces, the tunneling current and the field between tip and surface [1]. For the technique of "soft"lateral manipulation, in which particles can be moved in a very precise manner on the surface,only the forces have to be applied. Different manipulation modes like pushing, pulling andsliding can be experimentally distinguished by tracing tip−height curves during movement.Uponmovement of native substrate atoms exchange processes appear to be important [2].Forcontrolled vertical manipulation, in which a particle is transferred from the surface to the tip andvice versa, also field and current effects play a role. We show that deliberate transfer ofmolecules to the tip can lead to chemical contrast in imaging and improved resolution [1,3].Examples of the buildup of nanostructures fabricated from single atoms and small molecules andmeasurements of physical properties like surface electron phase coherence lengths and longrange lateral interactions are presented [3,4]. We show also that parts of larger molecules can beinfluenced mechanically in a precise way leading to extreme differences in electrical behaviourthus opening up the possibility of a mechanical switch based on single molecules [5]. Effects dueto the tunneling current are decisive for the dissociation and association of molecules: Theartificial induction of all steps of a complex chemical reaction employing single molecules isdemonstrated with the technologically important Ullman reaction: (i) Preparation of thereactands (phenyl) by dissociation of the parent molecules (iodobezene), (ii) bringing togethertwo reactands by lateral manipulation and (iii) welding the reactands together to the final product(biphenyl) by electron bombardment [6]. Prospects to extend the STM−manipulation techniquesto thin layers of insulating materials on metal substrates are discussed [7]. [1] G. Meyer et al., Single Molecules 1, 25 (2000)[2] J. Schulz et al., Phys. Rev. Lett. 84, 4597 (2000)[3] K.F. Braun, Doctoral thesis, Free University Berlin (2001)[4] J. Repp et al., Phys. Rev. Lett. 85, 2981 (2000)[5] F. Moresco et al., Phys. Rev. Lett. 86, 672 (2001)[6] S.W. Hla et al., Phys. Rev. Lett. 85, 2777 (2000)[7] J. Repp et al., Phys. Rev. Lett. 86, 252 (2001)

Scanning tunneling spectroscopy and microscopy of ultrathindielectric films

Wolf−Dieter Schneider

Institut de Physique de la Matiere Condensee, Universite de Lausanne, CH−1015 Lausanne, Switzerland

Metal oxides play a crucial role as insulators im many electronic and magnetic devices. As thesedevices become ever smaller, it becomes more and more important to understand the behavior ofultrathin insulating layers. As a model system, we have investigated the electronic structureand morphology of ultrathin MgO films epitaxially grown on Ag(001) using low−temperaturescanning tunneling spectroscopy and scanning tunneling microscopy [1]. Layer−resolveddifferential conductance (dI/dU) measurements reveal that a bandgap of about 6 eVcorresponding to that of a MgO(001) single−crystal surface had formed by the time the filmthickness reached just three monolayers. This finding is confirmed by layer−resolved calculationsof the local density of states based on density functional theory. These results do not justconstrain the minimum usable thickness for layers of insulating MgO (and, by inference, otherwide bandgap materials). They also suggest that, by carefully controlling the number of dielectriclayers deposited on a metal substrate, the electronic, magnetic and chemical properties of theresulting surface could be tuned in a controlled manner. [1] S. Schintke, S. Messerli, M. Pivetta, F. Patthey, L. Libioulle, M. Stengel, A. De Vita, and W.−D.Schneider, Phys. Rev. Lett. 87, 276801 (2001).

Laser induced charge − transfer processes at adsorbate/metalinterfaces

Angel González Ureña

Instituto Pluridisciplinar, Universidad Complutense de Madrid, Juan XXIII−1, 28040 Madrid,Spain

Line tunable IR radiation is used to induce charge transfer processes at metal/adsorbateinterfaces. Both electron and molecular ion yields are monitored as a function of laser power,wavelength and surface coverage. Experimental results concerning the onset of vibrationalselectivity in surface reactions as well as the dynamics underlying the metal−adsorbateinteraction will be presented at the Workshop.

Femtochemistry and ultrafast electron dynamics at adsorbate/metalinterfaces

Martin Wolf

Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany

Surface femtochemistry is driven by charge and energy transfer between molecular adsorbatesand the under lying substrate. The dynamics of these processes is of fundamental importance fora microscopic understanding of gas surface interactions including chemical reactions at surfaces,e.g. in heterogenous catalysis. We have studied various femtosecond laser−induced surfacereactions, which are driven by the hot electron transient after excitation with intense fs−laserpulses. In this mechanism electron transfer into unoccupied antibonding electronic states of theadsorbate may open reaction pathways which are thermally not accessible, as has been shown inthe case of the CO oxidation reaction on a Ru(001) surface. We also present recent results on fs−laser−induced associative recombination of atomic to molecular hydrogen (Had+Had −> H2) aswell as on the dynamics of localization and solvation of photoinjcted electrons into thin layers ofice on Ru(001).

Ultrafast charge transfer processes at adsorbates investigated usingthe core level clock

W.Wurth 1 , P. Feulner 2 , A. Föhlisch 1 , and D. Menzel 2

1 Institut für Experimentalphysik, Universität Hamburg Luruper Chaussee 149, 22761 Hamburg,Germany

2 Physik Department E20, Technische Universität München James Franck Str, 85747 Garching,Germany

The physics of electron transfer processes at surfaces is of major importance for theunderstanding of surface photochemical reactions, molecular electronics as well as the formationof surface chemical bonds. For adsorbates on metal surfaces is has been established that thesecharge transfer processes typically occur on (sub)−femtosecond timescales. Ultrafast laserspectroscopy has recently been very successfully applied to the electron dynamics in imagepotential states in front of metal surfaces. However, resonant electron transfer processes atadsorbates especially in the strong coupling case require a time resolution which is at the extremelimit of current laser experiments. An alternative experimental approach based on the use ofresonant core electron spectroscopy has shown the potential to determine electron transfer timesin the femtosecond regime. The technique relies on the comparison of electron transfer timeswith known lifetimes of core excited states and hence is often termed core level clock method.Examples for the successful use of this experimental approach to determine charge transfer timesof a few femtoseconds for adsorbed argon atoms on different metal substrates as well as to gaininsight in the energy dependence of resonant electron transfer processes will be given.Furthermore new results on strongly coupled adsorbates will be presented which indicate that thetechnique might even be capable of studying electron dynamics on a sub−femtosecond timescale.This work has been supported by the Deutsche Forschungsgemeinschaft in the framework of theSchwerpunktprogramm Dynamik von Elektronentransferprozessen an Grenzflächen undergrant WU 207/1−1.

Femtosecond Processes in Primary Photosynthesis: ReactionsOptimized for Highest Efficiency

W. Zinth

BioMolecular Optics Group, Sektion Physik, Ludwig−Maximilians−Universität München,Oettingenstr. 67, 80538 München, Germany

zinth@physik.uni−muenchen.de

Femtosecond spectroscopy reveals the molecular mechanisms of the primary electron transfer.Data on native and mutated reaction centers show, that primary electron transfer is an ultrafaststepwise reaction. The electron is transferred via a chain of pigments: In a first reaction step theelectron is transported from the special pair to the accessory bacteriochlorophyll with a timeconstant of 3 ps. A second, faster reaction carries the electron with 0.65 ps to abacteriopheophytin before much slower reaction steps lead to long−distance charge separation.Experiments on mutated reaction centers with strongly modified reaction times yield additionalinformation on energetics, reorganization energies and electronic coupling of the reaction center.A consistent theoretical treatment of the data shows that standard non−adiabatic theory describesonly qualitatively the primary electron transfer process. It shows that the realized reactionparameters optimize the reaction centers for highest possible quantum yield.

However a quantitative understanding is still not completely reached: Picosecond electrontransfer (ET) reactions in photosynthetic reaction centers (RC) are strongly accelerated at lowtemperatures and only part of the observed acceleration can be predicted by current electrontransfer theories. New experimental data show that a very strong acceleration is evident from thefastest (femtosecond) ET reactions. These results call for a new description of ET in proteins orprotein−like media. We propose a model, which relies on the well−established temperaturedependence of the dynamic properties of proteins and the special properties of ET reactions: athigh temperatures, protein motions cause a local disorder, which results in high entropy and inlarge fluctuations of the donor/acceptor geometry leading to relatively weak electronic coupling.At low temperatures the protein is "frozen". Here, entropy is low and attractive interactionsamong donor/acceptor pairs favor the few geometries with strong electronic coupling. Thisapproach allows quantitative prediction of the temperature dependence of the various ET−processes in photosynthetic reaction centers over the entire experimental range and hasinteresting consequences concerning the mechanism of charge transfer in protein−likesurroundings under physiological conditions. In addition it gives valuable information on theoptimization of the photosynthetic energy conversion.

POSTER SESSION

Hot electron injection in thin metal films and adsorbates

S. Dantscher, D. Diesing*, S. Schramm, A. Thon and W. Pfeiffer

Physikalisches Institut, Universitaet Wuerzburg, 97074 Wuerzburg, Germany*Institut für Schichten und Grenzflaechen, Forschungszentrum Juelich, 52425 Juelich, Germany

After optical excitation of a heterogeneous system, charge transfer plays an important role. Theabsorption of a photon in one component (e.g. the substrate) is followed by the transfer of theexcited electron to other components (e.g. an adsorbate). In this transfer often tunnelingprocesses are involved. As a model system for tunneling of excited electrons through a barrier weuse macroscopic tunneling contacts (2 mm2) with thin Al and Ag films as electrodes. Theinsulating material is a 2 nm thick Al oxide layer, the effective barrier height for electrons at thefermi level is 3.9 eV. This barrier is lowered for electrons ecxited by light. With increasingphoton energy the probability for tunneling increases strongly. The minimal barrer height forelectrons which absorb two photons of 1.5eV is 0.9eV. Time resolved photocurrent spectroscopyshows that three−photon induced internal photoemission and two−photon induced tunneling ofexcited electrons are the dominant transport mechanisms. In addition, electronic states at the top−metal oxide interface, characterized by a lifetime of about 50 fs, are involved.

Two−photon photoemission at silver nano−particles on graphite:collective phenomena and single particle excitation

C. Kennerknecht, M. Merschdorf, S. Schramm, and W. Pfeiffer

Physikalisches Institut, Universität Wuerzburg, Am Hubland, 97074,Wuerzburg, Germany

The optical characteristics of silver nano−particles are dominated by the strong surface plasmonresonance. A multiple excitation of this collective mode with ultrashort laser pulses (390 nm, 40fs) leads to multi−photon photoemission. Polarization−resolved measurements show that thedominant path for the two−photon−photoemission occurs via a sequential excitation of singleparticle states. Accordingly, single particle intermediate states are involved and their lifetime isdetermined in time−resolved measurements. Both the polarization− and the time−resolvedexperiments are described in a 3−level−density−matrix formalism and the collective excitation,i.e. the field associated with the surface plasmon, is taken into account as a local field correction.

Femtosecond laser induced desorption : nonlinear regime using anopen system model

Doinita Bejan

Faculty of Physics, University of Bucharest, P.O. Box, M.G.−11, Magurele, Romania

Georges Raseev

Laboratoire de Photophysique Moléculaire du CNRS, bât. 210, 91405, Orsay, France

The desorption yield of molecular adsorbates from metallic surfaces, using subpico/femto secondlasers, display a strong non−linearity with the laser fluence, non−linearity never completelyelucidated using quantum mechanical models. Two and three electronic states quantum−mechanical one−dimensional diabatic models are presented in order to explain this non−linearity.These models include explicitely the laser electric field in the dipolar approximation and thecoupling near the curve crossing point both calculated to be dependent on the desorptioncoordinate. The wave packet propagation of the coupled set of Schrodinger equations isperformed and the desorption probability extracted asymptotically. Both models were applied toCO on Cu(111). The first model includes only a ground and an ionic state of the adsorbate−metal system andimply the direct excitation of the negative molecular ion resonance. Similar two state modelswere widely used in the literature but only seldomly the curve crossing between the states wasallowed. The highest dependence of the desorption yield on the laser fluence is F2 for anoscillating electric field of the photon and F4 for a static field. This conservative model provesthat the neglected interaction between the adsorbate and the metal bath would influencessubstantially the desorption yield. The second non−conservative model is the three−state model taking into account in an effectiveway the continuum of states in the metal. The excitation of the ion state is now direct andindirect, mediated by the metal bath. The system is open and all the complicated interactions thattake place in the metal are actually reduced to a dephasing and an optical potential thatcorrespond physically to electron−phonon, electron−adsorbate and electron−electron collisionsrespectively. This point of view is highly justified by the experimental findings in two−photonphotoemission experiments. It withdraws the need to use an arbitrary defined electronitemperature. We obtained a strong yield dependence on the fluence that scales to the power 6.9of the laser fluence, power that is function of the values of the parameters used.

TIME−AUTOCORRELATION IN SCANNING TUNNELINGMICROSCOPE−INDUCED PHOTON EMISSION FROM

METALLIC SURFACE

Fabien Silly 1,2 and Fabrice Charra 1

1CEA−Saclay, SPCSI, F−91191 Gif−sur−Yvette Cedex, France 2 Present address : Université de Lausanne, IPMC, CH−1015 Lausanne, Suisse

Contrarily to extended luminescent systems, nanoscale light sources may involve few quantumemitters. This makes possible the existence of strong time correlations between successivelyemitted single photons. Such correlations provide information on the mechanisms and thedynamics of luminescent processes. We have applied time−correlated Time−autocorrelated two−photon counting measurements for studying STM−induced photon emission, with timeresolutions down to the nanosecond time scale. In the case of a gold surface in air, we show thatthe light emission exhibits a strong bunching behaviour at the nanosecond scale, strongly biasdependent. The analysis of time−correlations in STM−luminescence could be expected to substantiallyimprove understanding of dynamic processes at a nanometer scale. This could be of particularinterest, for example in light−emitting molecular, or Coulomb blockade systems where anti−bunching phenomena could appear, opening new prospects for the realization of light sourceswith specific statistical properties. F. Silly and F. Charra, Appl. Phys. Lett. 77, 3648, (2000)

Hot electron dynamics probed with 2PPE at In−rich and P−rich reconstructed InP(100)

L. Toeben, R. Eichberger, K. Moeller, R. Ernstorfer, L. Gundlach, T. Hannappel, and F. Willig

Hahn−Meitner−Institut, Glienicker Str. 100, 14109 Berlin, Germany

The dynamics of hot electrons at the surface of a semiconductor can be intimately related to thespecific atomic structure that is formed on this surface. One useful approach towardsunderstanding this relationship appears to be the following: First, two distinctly different atomicsurface reconstructions are prepared for the same bulk semiconductor and characterized byapplying appropriate tools of surface science (LEED, RDS, UPS, XPS, STM). Second, two−photon−photoemission (2PPE) spectra are measured for the two different surfacereconstructions. Third, hot carrier dynamics are time−resolved addressing relevant intermediatestates, in particular surface resonances. We report here on corresponding measurements for theordered P−rich and the ordered In−rich surfaces, InP(100), prepared with appropriate growthparameters via MOCVD. The specific surface reconstruction was identified in the MOCVDreactor by a characteristic in−situ reflectance difference spectrum (RDS) that was correlated aftercontamination−free sample transfer with specific surface sensitive signals[1]. The 2PPEspectrum of the ordered In−rich surface revealed two surface resonances, at 0.25eV and 0.8eVabove the conduction band minimum, respectively, and occupied surface states located 0.1eVbelow the valence band maximum. These experimental findings are in agreement with theoreticalpredictions for the ordered In−rich surface reconstruction[2]. In contrast, the 2PPE spectrum forthe ordered P−rich surface reconstruction did not show these peaks. Decay curves were measuredin about 100 meV wide energy windows probing the occupancy of hot carriers in relevantintermediate states. A pronounced slow−down was found in the decay behavior when comparingintermediate states about 2eV up in the conduction band with intermediate states close to theconduction band minimum. A characteristic double peak structure was found in the decay curvesin the energy range where hot electrons are scattered from the Gamma−valley to the L−valleyand back. A characteristic difference was found in the decay curves for the In−rich and the P−rich surfaces in the energy range where the 2PPE spectrum showed the surface resonance on theIn−rich surface.

1. T. Hannappel, L. Töben,.S. Visbeck, H.−J. Crawack, C. Pettenkofer, and F. Willig,Surf. Sci. 470, L1 (2000) 2. W.G. Schmidt, F. Bechstedt, N. Esser, M. Pristovsek, C. Schultz, W. Richter,Phys. Rev. B 57, 14596 (1998); W.G. Schmidt, private communication.