Recent Publications:

Gahl, C., Azima, A., Beye, M., Deppe, M., Döbrich, K., Hasslinger, U., Hennies, F., Melnikov, A., Nagasono, M., Pietzsch, A., Wolf, M., Wurth, W.,Föhlisch, A.

A femtosecond X-ray/optical cross-correlator

(2008) Nature Photonics.http://dx.doi.org/10.1038/nphoton.2007.298

Abstract
For a fundamental understanding of ultrafast dynamics in chemistry, biology and materials science it has been a longstanding dream to record a molecular movie in which both the atomic trajectories and the chemical states of every atom in matter are followed in real time1. Free-electron lasers provide this perspective as they deliver brilliant femtosecond X-ray pulses spanning a wide photon energy range, which is necessary to gather element-specific and chemical-stateselective information with femtosecond time resolution. The key challenge lies in synchronizing the free-electron lasers with separate optical lasers. We exploit the peak brilliance of the free-electron laser in Hamburg (FLASH) and establish X-raypulse-induced transient changes of the optical reflectivity in
GaAs as a powerful tool for X-ray/optical cross-correlation. This constitutes a breakthrough in the path towards recording a molecular movie andâ€”equally importantlyâ€”opens up the field of femtosecond X-ray-induced dynamics, only accessible with high-brilliance X-ray free-electron lasers.

http://dx.doi.org/10.1038/nphoton.2007.298

Föhlisch, A., Feulner, P., Hennies, F., Fink, A., Menzel, D., Sanchez-Portal, D., Echenique, P.M., Wurth, W.

Direct observation of electron dynamics in the attosecond domain

(2005) Nature, 436 (7049), Pages 373-376.

Abstract
Dynamical processes are commonly investigated using laser pump-probe experiments, with a pump pulse exciting the system of interest and a second probe pulse tracking its temporal evolution as a function of the delay between the pulses. Because the time resolution attainable in such experiments depends on the temporal definition of the laser pulses, pulse compression to 200 attoseconds (1 as = 10^-18 s) is a promising recent development. These ultrafast pulses have been fully characterized, and used to directly measure light waves and electronic relaxation in free atoms. But attosecond pulses can only be realized in the extreme ultraviolet and X-ray regime; in contrast, the optical laser pulses typically used for experiments on complex systems last several femtoseconds (1 fs = 10-15 s). Here we monitor the dynamics of ultrafast electron transfer-a process important in photo- and electrochemistry and used in solid-state solar cells, molecular electronics and single-electron devices-on attosecond timescales using core-hole spectroscopy. We push the method, which uses the lifetime of a core electron hole as an internal reference clock for following dynamic processes, into the attosecond regime by focusing on short-lived holes with initial and final states in the same electronic shell. This allows us to show that electron transfer from an adsorbed sulphur atom to a ruthenium surface proceeds in about 320 as.

Feulner, P., Ecker, M., Jakob, P., Romberg, R., Weimar, R., Menzel, D., Föhlisch, A., (...), Tyuliev, G.

Search for a local effect in multiatom resonant core excitation in a surface species: Photoemission and photon-stimulated desorption from N ₂ on Ni(111)

(2005) Physical Review B - Condensed Matter and Materials Physics, 71 (12), Pages 1-8.

Abstract
We have investigated photoabsorption, N 1s photoelectron emission, and photon-stimulated desorption sPSDd of N⁺ ions, from N₂ molecules perpendicularly chemisorbed on Ni(111) surfaces, in the energy range of the Ni 2p_3/2,_1/2 excitations. For this system, N 1s photoemission monitors single-core-hole production, whereas PSD of N⁺ is mainly due to excitation of multiply excited N 1s core-hole states. The amplitude variations of these two signals and the kinetic energy distributions (KED's) of the N⁺ ions were recorded as functions of the photon energy. In addition, we measured the amplitude variations of PE and PSD as a function of the photon incidence angle, which was varied from grazing (7 with respect to the surface) to steep! er angles (43 and 50 with respect to the surface). For grazing incidence, strong variations of both the photoelectron and the N⁺ signals with photon energy and angle of incidence were found in the Ni 2p region which are compatible with x-ray optical sdielectricd effects, one manifestation of multiatom resonant photoemission. The N⁺ KED's, which are known to depend strongly on the nature of the electronic excitation responsible, did not change across the Ni 2p _3/2 threshold, which excludes any type of state selectivity in the interatomic core-coupling effects observed. For N 1s photoemission, a first analysis of our data suggests a variation of the N 1s signal at the Ni 2p edges also for steeper angles of light incidence, of comparable magnitude to that at grazing incidence. However, more careful x-ray photoelectron spectroscopy experiments and the investigation of electronically stimulated desorption of neutral N₂ molecules and N! atoms reveal that these effects are due to a strong increase of beam damage when passing the Ni 2p edge; these effects could be reduced by rapidly scanning the sample under the beam. We thus conclude that for high angles of incidence most of the Ni 2p-related changes in our N 1s photoemission signal are due to beam damage. 2005 The American Physical Society.

Nietubyc, R., Föhlisch, A., Glaser, L., Lau, J.T., Martins, M., Reif, M., Wurth, W.

L-edge x-ray absorption fine structure study of growth and morphology of ultrathin nickel films deposited on copper

(2004) Physical Review B - Condensed Matter and Materials Physics, 70 (23), Pages 1-8.

Abstract
We have studied the Ni L edge x-ray absorption fine structure for thin Ni films evaporated on a Cu(001) substrate. The measurements have been carried out for films having coverages ranging from 0.07 monolayers (ML) up to 3.1 ML. The coverage has been calibrated using the ratio of the Ni L₃ to Cu L₃ edge jump heights and independently verified with titration experiments. We have found a clear evolution of the x-ray absorption fine structure at the Ni L₃ edge with coverage. To interpret the measured spectra first-principles calculations have been performed modeling a two-dimensional growth. The calculations reproduced all features observed experimentally. From the comparison between experiment and theory we can conclude that submonolayer films contain a large n! umber of small islands. Deposition of an amount of nickel corresponding to a single layer results in the formation of an almost perfect flat layer. Our studies show that L edge x-ray absorption spectroscopy can provide useful information on thin-film growth and morphology.

Föhlisch, A., Wurth, W., Stichler, M., Keller, C., Nilsson, A.

X-ray emission spectroscopy of (2?32?3)R30CO/ Ru(0001): Comparison to c(22)CO/Ni(100) and c(22)CO/Cu(100)

(2004) Journal of Chemical Physics, 121 (10), Pages 4848-4852.

Abstract
The resonantly excited x-ray emission spectroscopy (XES) was investigated combined with ab initio cluster calculations the electronic structure of adsorbed CO on the Ni and Cu (100) surfaces. It was found that the general features of the local adsorbate electronic structure was similar to the situation of CO adsorbed on the fcc metals Ni(100) and Cu(100). The interpretation of the local valence electronic structure gave full agreement with the C-O stretch frequency and the heat of adsorption for the zero-coverage limit on Ru(0001), Ni(100) and Cu(100). It was concluded that repulsive adsorbate-adsorbate interactions in the dense adsorbate sites on the Ru(0001) surface give rise to additional nonlocal contributions influencing the heat of adsorption.

Pietzsch, A., Hennies, F., Föhlisch, A., Wurth, W., Nagasono, M., Witkowski, N., Piancastelli, M.N.

Adsorption geometry of C₂H₂ on the single-domain Si(0 0 1)-(2 1) surface: Fully polarization resolved NEXAFS

(2004) Surface Science, 562 (1-3), Pages 65-72.

Abstract
The adsorption of acetylene on single-domain Si(0 0 1)-(2 1) surfaces has been investigated at room temperature using fully polarization resolved near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Two coexisting adsorption species have been observed, which we ascribe to the dimerized and end-bridge structures. In both cases, the Si dimers remain intact and the acetylene carbon atoms rehybridize to sp². 2004 Elsevier B.V. All rights reserved.

Föhlisch, A., Hennies, F., Wurth, W., Witkowski, N., Nagasono, M., Piancastelli, M.N., Moskaleva, L.V., (...), Rösch, N.

Electronic structure and screening dynamics of ethene on single-domain Si(001) from resonant inelastic x-ray scattering

(2004) Physical Review B - Condensed Matter and Materials Physics, 69 (15), Pages 153408-1.

Abstract
We present a resonant inelastic x-ray scattering (RIXS) study of a strongly bound adsorbate on a semiconductor surface, C₂H ₄/Si(001). The valence electronic structure as well as the photon energy dependence in RIXS can be studied without the dominating effect of dynamic metallic screening. We demonstrate that for this strongly coupled system the RIXS spectrum resulting from a selective excitation into the unoccupied ?*_CSi resonance can be interpreted with the help of density-functional calculations. In addition, we show how excitation into different resonances leads to a significant photon energy dependence of the RIXS spectral features, not seen in strongly coupled adsorbate systems on metals.

Keller, C., Stichler, M., Fink, A., Feulner, P., Menzel, D., Föhlisch, A., Hennies, F., Wurth, W.

Electronic transfer processes studied at different time scales by selective resonant core hole excitation of adsorbed molecules

(2004) Applied Physics A: Materials Science and Processing, 78 (2), Pages 125-129.

Abstract
Core hole spectroscopy has been shown to provide information on ultrafast charge transfer processes on a time scale of the core hole lifetime. In this paper we present high-resolution autoionization studies of SF₆ and thiophene molecules adsorbed on a Ru(001) surface to further illustrate the potential of the so-called core hole clock method. Resonance states with very different core hole lifetimes can be excited with these sulfur-containing molecules. The selection of a specific resonant core hole excitation enables a variation of the time scale for probing electronic as well as nuclear dynamics.

Lau, J.T., Wurth, W., Ehrke, H.-U., Achleitner, A.

Soft landing of size-selected clusters in rare gas matrices

(2003) Low Temperature Physics, 29 (3), Pages 223-227.

Abstract
Soft landing of mass-selected clusters in rare gas matrices is a technique used to preserve mass selection in cluster deposition. To prevent fragmentation upon deposition, the substrate is covered with rare gas matrices to dissipate the cluster kinetic energy upon impact. Theoretical and experimental studies demonstrate the power of this technique. Besides STM, optical absorption, excitation, and fluorescence experiments, x-ray absorption at core levels can be used as a tool to study soft landing conditions, as will be shown here. X-ray absorption spectroscopy is also well suited to follow diffusion and agglomeration of clusters on surfaces via energy shifts in core level absorption. 2003 American Institute of Physics.

Witkowski, N., Hennies, F., Pietzsch, A., Mattsson, S., Föhlisch, A., Wurth, W., Nagasono, M., Piancastelli, M.N.

Polarization and angle-resolved NEXAFS of benzene adsorbed on oriented single-domain Si(001)-2 1 surfaces

(2003) Physical Review B - Condensed Matter and Materials Physics, 68 (11), Pages 1154081-1154085.

Abstract
We have investigated the adsorption of benzene on oriented single-domain Si(001)-(2 1) surfaces at room temperature by means of fully polarization-resolved near-edge x-ray-absorption fine-structure (NEXAFS) experiments. The present study reveals that benzene chemisorbs in a stable cyclohexadienelike configuration, labeled as a "butterfly" in the literature, which is di-? bonded to silicon atoms.

Hennies, F., Föhlisch, A., Wurth, W., Witkowski, N., Nagasono, M., Piancastelli, M.N.

Fully polarization resolved X-ray absorption spectroscopy of C₂H₄ on single-domain Si(0 0 1)-(2x1)

(2003) Surface Science, 529 (1-2), Pages 144-150.

Abstract
We present a polarization resolved near edge X-ray absorption fine structure (NEXAFS) investigation of ethylene (C₂H₄) adsorbed on the oriented single-domain Si(001)-(2 1) surface. From the detected resonances and their polarization dependences C₂H₄ is found to be strongly bound to the silicon dimers with the carbon atoms in a sp³-hybridized state. The molecular axis is rotated around the surface normal with respect to the dimer axis. 2003 Elsevier Science B.V. All rights reserved.

Föhlisch, A., Menzel, D., Feulner, P., Ecker, M., Weimar, R., Kostov, K.L., Tyuliev, G., (...), Wurth, W.

Energy dependence of resonant charge transfer from adsorbates to metal substrates

(2003) Chemical Physics, 289 (1), Pages 107-115.

Abstract
We have investigated the resonant charge transfer from an adsorbed Ar atom in an electronically excited 2p⁵_3/24s¹ state into different metal substrates, i.e., Ag(111), Ni(111) and Cu(111). Using the so-called core hole clock method we have been able to determine the resonant charge transfer times for the three systems. They differ roughly by a factor of two with the fastest transfer time being observed for Ni(111) and the slowest for Ag(111). Furthermore we observe strong energy dependences of the charge transfer times across the resonances which are opposite to the expectations from a simple tunneling model, or from a detuning induced shortening of the interaction time. Possible reasons for the observed behavior are discussed. 2002 Elsevier Science B.V. Al! l rights reserved.

Lau, J.T., Föhlisch, A., Martins, M., Nietubyc?, R., Reif, M., Wurth, W.

Spin and orbital magnetic moments of deposited small iron clusters studied by x-ray magnetic circular dichroism spectroscopy

(2002) New Journal of Physics, 4.

Abstract
The size-dependent magnetic properties of small iron clusters deposited on ultrathin Ni/Cu(100) films have been studied with circularly polarized synchrotron radiation. For x-ray magnetic circular dichroism studies, the magnetic moments of size-selected clusters were aligned perpendicular to the sample surface. Exchange coupling of the clusters to the ultrathin Ni/Cu(100) film determines the orientation of their magnetic moments. All clusters are coupled ferromagnetically to the underlayer. With the use of sum rules, orbital and spin magnetic moments as well as their ratios have been extracted from x-ray magnetic circular dichroism spectra. The ratio of orbital to spin magnetic moments varies considerably as a function of cluster size, reflecting the dependence of magnetic properties on clus! ter size and geometry. These variations can be explained in terms of a strongly size-dependent orbital moment. Both orbital and spin magnetic moments are significantly enhanced in small clusters as compared to bulk iron, although this effect is more pronounced for the orbital moment. Magnetic properties of deposited clusters are governed by the interplay of cluster-specific properties on the one hand and cluster-substrate interactions on the other hand. Size dependent variations of magnetic moments are modified upon contact with the substrate.

Lau, L.T., Föhlisch, A., Nietubyc?, R., Reif, M., Wurth, W.

Size-dependent magnetism of deposited small iron clusters studied by x-ray magnetic circular dichroism

(2002) Physical Review Letters, 89 (5).

Abstract
Orbital and spin magnetic moments were measured as a function of cluster size for small size selected iron clusters deposited onto Ni/Cu(001). The ratios of m_l to m_s were generally larger than those observed for bulk ion, iron ultrathin films, and iron nanocluster films. There was a strong, nonmonotonous variation particularly in m_l, with strongly enhanced orbital magnetism for the clusters.

Lau, J.T., Achleitner, A., Wurth, W.

Electronic and geometric structure of chromium clusters deposited onto Ru(001) studied by soft X-ray absorption spectroscopy

(2000) Surface Science, 467 (1-3).

Abstract
Small, size-selected Cr_n clusters deposited onto Ru(001) have been studied by soft X-ray absorption spectroscopy. Broad, bulk-like L₃ absorption line shapes are observed for all clusters. Sizeable shifts in X-ray absorption energy after flash heating are indicative of diffusion or surface alloying. Pure Cr_n clusters can be easily oxidized, yielding Cr₂O₃-like X-ray absorption signals. The results are discussed in comparison to Cr_n clusters deposited onto (11)O/Ru(001).

Lau, J.T., Achleitner, A., Wurth, W.

X-ray absorption spectroscopy of size-selected, deposited small chromium clusters

(2000) Chemical Physics Letters, 317 (3-5), Pages 269-275.

Abstract
Small, mass-selected Cr_n clusters soft landed onto a (11)O/Ru(001) substrate were investigated by near-edge X-ray absorption spectroscopy. Cr_n X-ray absorption spectra exhibit size-dependent shifts of L₃ absorption edges. Furthermore, strong cluster-substrate interaction of Cr_n clusters deposited onto (11)O/Ru(001) results in Cr₂O₃-like absorption signatures and effectively inhibits diffusion and island formation even at elevated temperatures. In contrast, chromium atoms on Ru(001) have a high mobility, showing indications of coalescence and island formation; the resulting larger clusters can easily be oxidized.

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Letzte Aktualisierung: 5. Feb. 2008 N.N.