Fritz-Haber-Institut der Max-Planck-Gesellschaft  Department of Physical Chemistry
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Recent Developments in the Department of Physical Chemistry (2014)

Director: Martin Wolf

1. General
1.1 Research of the Department
1.2 Selected Research Highlights 
2. Progress Report
2.1 Ultrafast Dynamics in Solids and at Interfaces
2.1.1 Transient Electronic Structure of Correlated Materials
2.1.2   Ultrafast Carrier and Exciton Dynamics in Inorganic/Organic Hybrid Systems 
2.1.3   Electronic Structure of Surfaces and Interfaces
2.1.4   Terahertz Physics: Low-energy Excitations and Control by THz pulses
2.1.5   Ultrafast Spin Dynamics in Epitaxial Metallic Multilayers 
2.2 Molecular Processes at Interfaces
2.2.1 Nanoscience with Functional Molecules
2.2.2 Molecular Manipulation and Spectroscopy at the Nanoscale
2.2.3   Real-time Observation of Photoinduced Surface Reactions 
2.2.4   Interfacial Molecular Spectroscopy 
2.2.5   Computational Dynamics of Protein Machines 
2.2.6   Electrochemical Dynamics 

 
 
Research:
 

Overview

 
Methods


Activity reports

2014 · 2011 · 2009

2007 · 2005 · 2003
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1. General

Since the last meeting of the Fachbeirat, the Department of Physical Chemistry has developed its experimental and laboratory infrastructure to become fully operational. Currently the research groups of the department are still distributed over several locations on the FHI campus and in rented external lab space. However, the construction of the new building for the department is now on its way and will be completed in early 2016.

Several changes have occurred among the group leaders and senior postdocs of the department:

·  Dr. Zefeng Ren received an offer for an associate professorship at Peking University and started there in January 2012.
·  Prof. Karsten Horn (formerly Department of Molecular Physics) joined the department in January 2012. He provides expertise in angle-resolved photoemission spectroscopy and continues his research on electronic structure of grapheme-based materials.
·  Dr. Simon Wall received an offer for an assistant professorship at ICFO (The Institut of Photonic Science) in Barcelona and left the department in August 2012 to set up an independent research group.
·  Also in August 2012 Dr. Patrick Kirchmann left for a staff scientist position at SLAC National Accelerator Laboratory in Stanford.
·  Dr. Leonhard Grill accepted an offer for a full professorship at the University of Graz and started in this position in August 2013. He is still affiliated with the department in order to complete an EU funded project (AtMol) until end of 2014.
·  Since May 2014 Dr. Takashi Kumagai is heading a new research group to study elementary molecular processes at surfaces employing low-temperature atomic force and scanning tunneling microscopy (AFM/STM) as well as tip-enhanced Raman spectroscopy (TERS).
·  Starting in January 2014 Dr. Alexander Paarmann will set up a new group to use the new infrared free electron laser (FHI-FEL) for time-resolved optical spectroscopy of ultrafast dynamics of solids, in particular phonon dynamics. He will also use femtosecond table-top sources.

Furthermore, several postdoc associates and graduate students moved on to new positions in industry or administration

Two large service groups of the institute, the Electronics Lab and (since 2013) the Mechanical Workshops, are associated with the department. After the retirement of Horst Schwäricke in April 2013, Petrik Bischoff, formerly with the Department of Molecular Physics, was appointed as the new head of the Mechanical Workshops. Currently the services of the Mechanical Workshops are expanded and the machine infrastructure is modernized, a process which will continue over the next few years. The Electronics Lab (headed by Georg Heyne) is well organized and continues to provide excellent service for the institute..

1.1 Research of the Department

The research of the Department of Physical Chemistry is focused on the dynamics of elementary processes at surfaces, interfaces and in solids. Our goal is to develop a microscopic understanding of the dynamics of molecular and electronic processes as well as the interactions between various (electronic, spin and lattice) degrees of freedom. The general strategy is to address these problems from several sides using complementary approaches, in particular by the development and application of various spectroscopic techniques dedicated to the specific physical questions. Research is performed by small groups with specific expertise. Although the research of each individual group is mostly curiosity driven our complementary approach creates various synergies between the different groups.

The research of the department is currently structured into two main areas, (I) ultrafast dynamics of elementary processes in solids and at interfaces, and (II) molecular processes at surfaces and is carried out by the research groups listed below. Furthermore, the Max-Planck- Research Group (MPRG) of Ralph Ernstorfer on “Structural and Electronic Surface Dynamics” is closely associated with the department and complements the research on ultrafast dynamics of solids by time-resolved electron diffraction techniques

Ultrafast Dynamics in Solids and at Interfaces
  • Dynamics of Correlated Materials (Martin Wolf)
  • Electron Dynamics at Interfaces (Julia Stähler)
  • Electronic Structure of Surfaces and Interfaces (Karsten Horn)
  • Terahertz Physics (Tobias Kampfrath)
  • Time-resolved Second Harmonic Generation Spectroscopy (Alexey Melnikov)
Molecular Processes at Surfaces
  • Nanoscale Science (Leonhard Grill)
  • Nanoscale Surface Chemistry (Takashi Kumagai)
  • Interfacial Molecular Spectroscopy (Kramer Campen)
  • Complex systems (Alexander Mikhailov)
  • Spatiotemporal Self-organization (Markus Eiswirth)
Max-Planck-Research Group (MPRG)
  • Structural and Electronic Surface Dynamics (Ralph Ernstorfer)

The first line of research aims at studying the dynamics of elementary processes on the relevant time scales of the process by ultrafast laser spectroscopy (typically with femto- or picoseconds time resolution). The department applies a broad spectrum of established as well as newly developed techniques; these are used to study the electronic excitations and low energy excitations in solids, dynamics of electron transfer processes, vibrational dynamics at interfaces, as well as optically induced phase transitions. The second line of research is the investigation of elementary molecular processes either at the single molecule level, or by employing various schemes of optical excitations including photo-induced surface reactions. Scanning probe microscopy (in part combined with optical excitation) permits imaging, manipulation and spectroscopy as well as inducing and probing chemical processes of individual molecules. Further activities address problems of molecular biophysics and electrochemistry. Here, theoretical studies of molecular machines are performed, complemented by studies of nonlinear dynamics and pattern formation in electrochemical systems.

The promotion of young scientists is an important goal and several measures are taken to help them developing their career (e.g. IMPRS graduate school, PhD and department workshops, regular status discussions, nomination for invited talks). In particular, junior staff scientists and postdocs are guided to gain experience in grant applications and establish their scientific network. Currently, the department cooperates in several EU funded projects, four collaborative research centers (Sfb 658, Sfb 910, Sfb 951, Sfb 1109) and a research unit (FOR 1700) funded by the Deutsche Forschungsgemeinschaft. These projects are complemented by several individual research grants of young scientist (see section 3 for a complete list of projects).

1.2 Selected Research Highlights

The following research highlights are a selection of results from research projects as well as instrument development in the department obtained during the last two years:

  • Progress was achieved in the area of time and angle-resolved resolved photoemission spectroscopy (trARPES, (hν = 6 eV)) applied to various correlated materials (high-Tc superconductors, Fe-pnictides and charge density wave (CDW) systems). A systematic study of the tri-telluride CDW system demonstrated direct probing of the transient modulation of the CDW gap as well as vibrational coherent control of the amplitude mode using a three-pulse excitation scheme. Recently, ultrafast melting of the spin-density-wave phase in Cr was studied with trARPES using XUV light obtained by high harmonics generation. [Phys. Rev. Lett. 108, 097002 (2012), ibid. 107, 097002 (2011)]
  • The ultrafast evolution of the photoinduced insulator-to-metal transition in VO2 was probed with a white light supercontinuum. Using the optical phonons of the insulating phase as a marker, an ultrafast change of the lattice potential symmetry (i.e. the restoring forces) was identified on a timescale much faster than the structural phase transition leading to a transient excited state which differs from the equilibrium metallic state of VO2. (Nature Commun. 3, 721 (2012), Phys. Rev. B 87, 115126 (2013))
  • A high repetition rate (500 kHz) laser system for high harmonics generation of XUV laser pulses (hν = 20 – 40 eV) in combination with a state-of-the-art ARPES experiment is currently being developed. This system employs multiple stages of fiber lasers, a slab laser amplifier and optical parametric chirped pulse amplification (OPCPA) to generate < 20 fs pulses with more that 20W output power in the visible.
  • Bilayer graphene band calculations show a feature that is very desirable but absent in the monolayer – a band gap. Transport experiments, however, have so far failed to detect a clear gap. Using angle-resolved photoemission, this contradiction is explained by showing that small twists in the relative bilayer arrangement lead to the coexistence of massive Dirac particles (expected in the bilayer) with a massless particle band that crosses the ideal bilayer gap. [Nature Materials 12, 887(2013)].
  • It is still not clear why quasicrystals, which have perfect atomic order yet lack translational periodicity, assume their complex structures. Using bulk-sensitive hard x-ray photoemission evidence for a large pseudogap near the Fermi level is obtained, supporting a (Hume-Rothery) mechanism for quasicrystal formation. Quasicrystals apparently form a metallic phase at the surface that masks, when using surface sensitive photoemission, the true bulk electronic structure. [Phys. Rev. Lett. 109, 216403(2012)].
  • The ultrafast quasiparticle dynamics at the ZnO(10-10) single crystal surface following above-band gap excitation was probed by time-resolved two-photon photoelectron spectroscopy, exhibiting ultrafast electron cooling in the conduction band by electronphonon scattering and followed by formation of a surface-bound exciton.
  • The transport of very short bunches of spin-polarized electrons (spin-current pulses) in magnetic heterostructures has been demonstrated employing THz emission spectroscopy as a probe. Using femtosecond laser excitation, the spin transport was launched from a ferromagnetic Fe thin film into a nonmagnetic cap layer of low (Ru) or high mobility (Au), which results in spin trapping (Ru) or ballistic traversal (Au). The results are potentially useful for future spintronics circuitry operated at highest (THz) frequencies [Nature Nanotech. 8, 256 (2013)].
  • A high-precision optical setup for time-resolved linear and non-linear magneto-optical spectroscopy has been developed, operating in back pump-front probe scheme with 20 fs time resolution. Femtosecond spin current pulses were demonstrated in epitaxial Fe/Au/Fe/MgO(001) multilayers [Phys. Rev. Lett. 107, 076601 (2011)]
  • Manipulation of the spectrum or temporal shape of a light pulse has been achieved by coupling femtosecond laser pulses into waveguides. For example, by suddenly tilting the waveguide’s dispersion relation the spectrum can be compressed, a process which is reversible, features high conversion efficiency and could find application as a magnifying lens for optical spectra. Furthermore, the concept for an ultrafast optical delay line could be demonstrated [Phys. Rev. Lett. 108, 033902 (2012), ibid. 108, 213901 (2012)].
  • A breakthrough has been achieved using femtosecond time-resolved x-ray spectroscopy (RIXS) at the LCLS free electron laser in Stanford to probe the electronic structure of CO molecules, as their chemisorption state on Ru(0001) changes upon exciting the substrate by using a femtosecond optical laser pulse. The observed electronic structure changes are consistent with a transient weakening of the CO-metal bond without notable desorption, indicationg the existence of two distinct adsorption wells, a chemisorbed and a precursor state, separated by an entropy barrier [Science 339, 1302 (2013); Phys. Rev. Lett. 110, 186101 (2013)]
  • Hierarchical "bottom-up" covalent binding of molecular building blocks in a well-defined pattern was demonstrated by sequentially supplying reactive sites on molecular building blocks. Copolymer networks were formed with high spatial selectivity. After dissociation of Br substituents from molecular building blocks on gold surfaces the polymerization to straight chains along the step edges was demonstrated, resulting in a pre-alignment along a given direction over the entire sample. [Nature Chemistry 4, 215 (2012), Angew. Chem. Int. Ed. 51, 5096 (2012)]
  • The electrical current through a single molecule could be measured at different voltages over a large range. In this way, the conductance properties of an individual polymer could be correlated with its electronic states for the first time. Comparison with calculation reveals that the conductance depends on the precise atomic structure and the bending of the molecule in the STM junction. [Nature Nanotech. 7, 713 (2012)]
  • The intramolecular H-atom transfer reaction (tautomerization) within a single porphycene molecule on Cu(110) was controlled directly by low-temperature STM. The potential landscape of this process can be precisely tuned by putting single Cu adatoms nearby or by changing the orientation of neighboring molecules. Furthermore, the mechanism of thermally and vibrationally-induced tautomerization was deduced from isotope effects and the bias voltage and tunneling current dependence [Nature Chemistry (in press 2013), Phys. Rev. Lett. (in press 2013)]
  • A new experimental setup for sum frequency generation (SFG) spectroscopy has been developed allowing studies of liquid solid interfaces as well as probing low-frequency vibrations (e.g. surface phonons) down to ~700 cm-1. The dissociative adsorption of water (D2O) on α-Al2O3(0001) has been studied both in UHV and under ambient conditions by (1) characterizing the fragments via the OD stretch vibration and (2) the accompanying surface reconstruction using the Al-O surface phonon SFG spectral response.
  • By combined experimental and computational studies of the non-hydrogen bonded (free) OH groups at the air/water interface, it was shown that these are structurally and dynamically heterogeneous on sub-picosecond timescales, and that 2/3 of their vibrational relaxation proceeds via intramolecular energy transfer and 1/3 via reorientation [J Phys. Chem. B, 116, 9467 (2012), ibid. 117, 11753 (2013), Proc. Nat. Acad. Sci (in press 2013)]
  • Coarse-grained elastic-network numerical investigations of two macromolecules, playing a fundamental role in the cells, have been performed. For the molecular motor myosin, its strain-sensor behavior, previously found in single-molecule experiments, was explained and communication between important functional domains of the protein could be elucidated [Biophys. J. 102, 542-551 (2012)].
  • Many protein machines operate as active inclusions in lipid bilayers forming biological membranes. Fast and efficient methods have been developed for numerical simulations of lipid bilayers in membranes with active protein inclusions indicating the hydrodynamic effects should play a principal role in interactions between active membrane inclusions [J. Chem. Phys. 137, 055101 (2012) and J. Chem. Phys. 138, 195101 (2013)].
 
2. Progress Report

2.1 Ultrafast Dynamics in Solids and at Interfaces

Elementary processes in solids and at interfaces such as transfer of charge and spin, energy dissipation, or electron-phonon coupling are the underlying microscopic basis of much more complex phenomena, ranging from surface reactions to phase transitions in solids. The study of the non-equilibrium, ultrafast dynamics of such fundamental processes provides mechanistic insights into the interplay and energy exchange between electron, spin and lattice degrees of freedom. It is the strategy of the department to elucidate the dynamics of elementary processes from various sides using complementary approaches and techniques. These approaches are implemented by several groups in the department, which perform realtime studies of solids and interfaces on ultrafast time-scales, complemented by studies of the electronic structure.

2.1.1 Transient Electronic Structure of Correlated Materials

Correlated electron materials exhibit exotic electronic and magnetic properties, characterized by broken-symmetry ground states such as metal-to-insulator instabilities, unconventional superconductivity, and various cooperative ordering phenomena. One of the major challenges in this field is to understand the ground and excited state properties on a microscopic level and to disentangle the competing interactions and correlations of charge, spin, orbital and lattice degrees of freedom, which act on multiple length, energy and time scales.

Ultrafast laser spectroscopy provides a tool to access elementary scattering and relaxation processes by optically exciting the electronic system and subsequently probing the evolution of the transient electronic structure by an appropriate spectroscopic technique. For example, in a material undergoing an insulator-to-metal transition, optical excitation can induce a transient melting of the band gap whereby the timescale of the gap closing is characteristic for a mechanism driven by purely electronic correlations (Mott transition) or by ion motions (Peierls instability). The technique of time- and angle-resolved photoemission spectroscopy (trARPES) extends the benefits of momentum-resolved photoelectron spectroscopy into the time domain and provides direct access to the transient evolution of the electronic structure after optical excitation. Furthermore, the collective dynamics of lattice or spin excitations can be studied through their influence on the quasiparticle band structure.

As prototypical charge density wave (CDW) system the group of Martin Wolf has investigated the material class of rare-earth tri-tellurides, RTe3 (RTe3, R = Te, Ho, Dy) using trARPES with a 6 eV fs laser probe (in collaboration with U. Bovensiepen, Duisburg and Z.-X. Shen, Stanford). Using a position sensitive detector for two-dimensional imaging of the photoelectron momentum, the transient changes of the Fermi surface and the opening and closing of the CDW gap could be mapped directly on a femtosecond time scale (see Fig. 1). By employing a novel three pulse “pump-reexcite-probe” photoemission scheme, the dynamics of the upper and lower CDW band edges could be resolved in great detail, indicating that the CDW gap modulation (amplitude mode) originates from a complex lattice motion whereby at least two coupled phonon modes are involved. A more detailed analysis of the band collapse reveals a transient reduction of the curvature of the Fermi surface, which is attributed to a transient change of the Te5p orbital overlap in the excited state. This leads to reduced coupling between neighbouring Te chains and thus to a modification in the dimensionality of the 2D band structure. Furthermore, vibrational coherent control has been demonstrated for the amplitude mode using double pulse excitation.

Fig.1 (top) Transient evolution of the CDW gap in TbTe3 following optical excitation with a hν = 1.5 eV fs pulse probed by time-resolved ARPES. (left) The upper and lower band edges and the order parameter (CDW gap size) are periodically modulated with two frequencies, 2.23 THz and 1.77 THz, respectively, indicating a complex dynamics of the amplitude mode.

Further studies using trARPES with hν = 6 eV have probed the temperature-dependent relaxation times of photoexcited electrons and holes in antiferromagnetic Fe-pnictide, EuFe2As2, and have attributed their dynamics with the spin density wave gap and the singleparticle band at the zone center, respectively [1]. The recovery of magnetic order after ultrafast excitation occurs four times slower compared to electron-phonon equilibration due to a smaller phase space for spin-dependent relaxation.

To overcome the limited accessible k-space due to the low kinetic energy of photoelectron using 6 eV laser ARPES, considerable efforts are currently undertaken to set up an advanced trARPES experiment based on a high repetition laser source (100 kHz – 1 MHz) for high harmonics generation (HHG) of XUV laser pulses (hν = 20 – 40 eV) in combination with a state-of-the-art ARPES ultrahigh vacuum system. In addition to the possibility of accessing several Brillouin zones, the HHG-based trARPES will provide much enhanced counting statistics. This experiment employs novel fibre and OPCPA laser technology and is developed in close collaboration with the MPRG group of Ralph Ernstorfer (see report MPRG). On the way to develop HHG-based trARPES experiments, the group has recently performed two studies using the access to exisiting HHG setups, which operate at 1-10 kHz based on established TiSapphire laser technology. In collaboration with M. Weinelt, Freie Universität Berlin, the prototypical spin density wave (SDW) material chromium has been studied. An ultrafast melting of the SDW phase was revealed from the response of the related backfolded band, which can be compared to a transient change in electronic temperature. Very recently, the charge density wave (CDW) material TiSe2 was studied with HHG-based trARPES in a beamtime at the ARTEMIS light source at Rutherford Appleton Lab (UK). The mechanism responsible for the CDW phase transition is still heavily debated. For TiSe2 temperature dependent measurements were conducted as well as long-wavelength excitation to investigate the peculiar physics of this system.

In the group of Julia Stäher, the ultrafast electron and lattice dynamics of vanadium dioxide (VO2) across the insulator-to-metal transition were investigated by means of transient optical spectroscopy using a white light supercontinuum, and by time-resolved two-photon photoelectron spectroscopy. VO2 undergoes a phase transition from a monoclinic, insulating phase at low temperatures and a rutile, metallic phase above Tcrit = 340 K. This transition can also be induced by photoexcitation, enabling its investigation on ultrafast time scales. Using Raman-active optical phonons of the monoclinic phase as a sensor, it could be shown that the symmetry of the restoring forces is lost within few femtoseconds, even before ionic motion occurs. Re-excitation of the non-equilibrated system, i.e. a pump-probe experiment of the excited state, furthermore unveils that the VO2 has not reached the metallic phase yet at these early times after excitation. The optical response resembles that of the thermodynamically stable phase only after picoseconds [2].
[1]   L. Rettig et al., Phys. Rev. Lett. 108, 097002 (2012)
[2]   S. Wall, D. Wegkamp, L. Foglia, K. Appavoo, J. Nag, R. F. Haglund, J. Stähler, M. Wolf, Nature Commun., 3, 721 (2012); S. Wall et al., Phys. Rev. B   87, 115126 (2013).

2.1.2 Ultrafast Carrier and Exciton Dynamics in Inorganic/Organic Hybrid Systems

The combination of inorganic semiconductors with organic molecules to hybrid systems (HIOS) promises superior functionality of the interface compared to a bare linear combination of the single material properties. Applications such as organic LEDs or solar cells would not only benefit from the high charge carrier mobility and stability of the inorganic compound in combination with the tunable optical properties of the organic molecules, but could also make use of interfacial hybrid states that facilitate, for example, charge or energy transfer between the constituents.

A promising candidate for such applications is zinc oxide (ZnO) due to its wide band gap (3.4 eV), n-type conductivity and abundance. Despite several decades of research, a full understanding of the surface properties of ZnO remains elusive. The group of Julia Stähler has investigated the electronic structure and ultrafast carrier dynamics of ZnO, its (10-10) surface, and interfaces with organic molecular layers with femtosecond (fs) time-resolved two-photon photoemission (2PPE) and optical spectroscopy. The non-polar ZnO(10-10) surface exhibits a downward surface band bending when terminated with atomic hydrogen,
Fig. 2: (a) Two-photon-photoemission (2PPE) and scattering processes at the ZnO(10-10) surface. (b) Work function change due to adsorption of hydrogen (blue line) and pyridine (purple line, markers DFT calculation) (c) Ultrafast electron dynamics at the ZnO surface as probed by 2PPE spectroscopy. False colors represent additional electrons excited by the pump pulse. Hot carriers in the CB relax on fs timescales by optical phonon emission. After a few 100 fs, additional electrons are observed below EF, indicating that a surface exciton has formed.
leading to the formation of a charge accumulation layer below the Fermi level EF with a density on the order of 10-13 cm-2, as shown in Fig. 2a. This goes along with a work function decrease of up to ΔΦ = -0.6 eV due to charge donation from the hydrogen. Significantly stronger reduction of Φ can be achieved using the dipolar molecule pyridine with negative electron affinity (Fig. 2b). Here, due to the lack of Fermi level pinning, a huge work function decrease of ΔΦ = -2.9 eV could be demonstrated in excellent agreement with ab-initio calculations by the group of >I>Patrick Rinke, Theory Department [1].

As illustrated in Fig. 2a, above band gap excitation of ZnO leads to the creation of excited electrons in the conduction band (CB). The subsequent ultrafast relaxation dynamics (see processes 1 & 2 in Fig. 2a) are monitored in time-resolved 2PPE by a second, time-delayed fs laser pulse (hνprobe). Fig.2c displays in false colors how the excited electron population relaxes on fs timescales by first electron-phonon scattering towards lower energies, and finally ends up in an excitonic surface state (SX) below EF. The binding energy of more than 200 meV with respect to the bulk CB minimum is responsible for its long lifetime exceeding several 100 ps. Remarkably, such SX is not observed for SrTiO3 (STO), another n-type transparent conducting oxide that exhibits also a 2D electron gas at its surface. Here, however, the charge density is at least one order of magnitude larger than for ZnO, which screens the electron-hole Coulomb attraction.

The combination of ZnO with organic and optically active molecules is a crucial step towards application. The group has characterized the interface of the organic molecule SP6 (2,7- bis(biphenyl-4-yl-)2',7'-ditertbutyl-9,9'-spirobifluorene) with several oxide surfaces by means of non-resonant Raman spectroscopy down to the monolayer level [2]. First optical transient transmission experiments probing the LUMO → LUMO+1 resonance of the molecules point at efficient charge separation at the interface to ZnO. These are complemented by 2PPE studies of the electronic structure of the SP6/ZnO(10-10) interface.
[1]   O. T. Hofmann, J.-C. Deinert, Y. Xu, P. Rinke, J. Stähler, M. Wolf, M. Scheffler, J. Chem. Phys. 139, 174701 (2013).
[2]   J. Stähler, O. T. Hofmann, P. Rinke, S. Blumstengel, F. Henneberger, Y. Li, T. F. Heinz, Chem. Phys. Lett.   584, 74 (2013).

2.1.3 Electronic Structure of Surfaces and Interfaces

Among the low dimensional materials, graphene has an exceptional status since its electronic structure brings together solid state physics and quantum electrodynamics. Also many realworld applications are envisaged and thus an enormous research effort is under way.

The group of Karsten Horn has investigated graphene oxidation, for example using NO2 and SO2, and fluorination by interaction with PF3 and XeF2. The latter is a particularly interesting process, as “half-fluorinated graphene” has been found, where a fluorine atom is attached to every second carbon atom, is an insulating phase with a large band gap. A metastable phase exists which transitions back to the ground state under emission of blue light. The graphene bilayer has received specific attention since it is supposed to exhibit a small band gap, desirable for electronic switching applications. This has, however, failed to appear in electronic transport measurements. Photoemission data from our collaboration with groups at the Advanced Light Source (Lawrence Berkeley Laboratory), and TU Chemnitz, reveal the reasons for this: they show the coexistence of massive Dirac Fermions with massless ones which bridge the gap predicted from tight binding calculations (see Fig. 3, e.g. features C1
Fig. 3 Top: Experimental band dispersion of hole-doped (left), nearlyneutral (center), and electron-doped (right) bilayer graphene with overlaid lines from tight-binding band structure calculations for AB stacking, doping level n (units 1013 cm–2) and Coulomb energy U (units eV). Bottom: Schematic band structure of bilayer graphene with AB, AA, and twisted AA stacking.
and C2) [1]. An analysis of the data shows that small (~ 0.2°) twists between the two layers, unavoidable even in the highest quality samples, lead to a gradual variation of the relative lateral arrangement of the carbon atoms in the two layers, (A - B and A - A stack bottom ing), which induces the above coexistence of massive and massless states in the band structure, as shown schematically in Fig. 3 (bottom).

Furthermore, graphene’s interaction with metals has been studied, for intercalation of metals (Cu, Co, Mn) in between graphene on Ir(111). Here, novel phases of these materials could be examined, since the intercalated atoms assume the (larger) lattice constant of the iridium substrate. Graphene deposited on ferromagnetic materials has been predicted to act as spin filter; hence we have studied magnetic interaction in such junctions using x-ray magnetic circular dichroism studies, for example in nickelgrapheme- cobalt “sandwich” layers, where the magnetic coupling between nickel and cobalt across the graphene layer was established. With the aim of graphene functionalization, the adsorption of ammonia and water on graphene has been studied on different substrates (Ni and Ir) showing that the strength of graphene’s interaction with its substrate has a marked effect on the nature of the chemisorption interaction.

Further studies on Al-Pd-Mn and Al-Cu-Fe quasicrystals using bulk-sensitive hard x-ray photoemission (HXPES) have provided evidence for the existence of a large pseudogap near the Fermi level (characteristic for a Hume-Rothery mechanism), which is not observed in surface sensitive low energy photoemission because the spectrum is affected by a metallic phase formed near the surface that masks the true bulk electronic structure [2].
[1]   K.S. Kim et al., Nature Materials 12, 887 (2013).
[2]   J. Nayak et al., Phys. Rev. Lett.   109, 216403 (2012).

2.1.4 Terahertz Physics: Low-energy Excitations and Control by THz pulses

The terahertz (THz) frequency range is of central relevance from a fundamental-scientific as well as from an application-related point of view. First, many elementary excitations in physical systems have transition energies on the order of 10 meV, for example quasi-free electrons in solids, crystal lattice vibrations, and excitons in semiconductors. As 1 THz corresponds to a photon energy of 4.1 meV, these modes can be probed resonantly and with sub-picosecond time resolution using THz electromagnetic field pulses. With recent advances in THz pulse generation providing electric-field amplitudes of ~1 MV cm–1 it has become possible to even drive and control such resonances on sub-picosecond time scales [1]. Second, bit rates in current information technology may soon approach the THz frequency range. Therefore, it is important to develop ultrafast techniques to manipulate electric currents or light at THz frequencies, for example with modulators and frequency shifters.

The group of Tobias Kampfrath makes use of ultrashort THz and optical laser pulses to investigate the interplay of low-energy excitations in complex materials and to control the properties of matter and light at the highest frequencies. Currently, the group focuses on electron spins in magnetically ordered solids, and studies new schemes to manipulate the dynamics and transport of magnetization. This goal also addresses basic questions of spinphonon interaction and spin-orbit coupling.

Fig. 4: (a) Schematic of generation, manipulation, and detection of THz spin currents. A femtosecond laser pulse launches a spin current from a Fe thin film (10nm) to a metallic cap layer (2nm) of either Ru or Au. While transport is slowed down in Ru (low electron mobility), it is fast in Au (high electron mobility). The spin current is converted into a charge current (owing to the inverse spin Hall effect), resulting in the emission of THz radiation whose detection allows extraction of the spin current dynamics. (b) Spin currents in Fe/Au and Fe/Ru as determined from the THz transients emitted by these heterostructures. The dynamics in Fe/Ru is much slower than in Fe/Au showing that adding layers of different mobility are a route toward manipulation of the shape of spin current bursts.

Ultrafast spin transport: Future electronics will potentially not only make use of the electron charge as an information carrier, but also employ the electron spin (up/down) to encode the value of a bit. Successful implementation of such “spintronics” requires the transfer of electron spins through space as well as the manipulation of the spin state. These elementary operations should proceed at a pace exceeding that of today’s computers, that is, at THz frequencies. To study and control ultrafast spin transport, a spin-polarized current pulse was launched in a Fe thin film through illumination with a femtosecond laser pulse [Fig. 4(a)]. Such a spin current arises because the laser pulse promotes spin-up electrons from d-type states with low band velocity into sp-like states with high velocity, whereas the minority spindown electrons remain d-type and, thus, slow. To manipulate these bursts, the Fe film was contacted with another nonmagnetic film of either low (Ru) or high (Au) electron mobility. As a result, the spin transport in the Fe/Au heterostructure should proceed much faster than in Fe/Ru. To probe the spin flow in a contactless manner, the inverse spin Hall effect is used that (through spin-orbit coupling) converts the longitudinal spin current into a transverse charge current, thereby leading to the emission of a detectable THz electromagnetic transient (see Fig. 4a). The measurements indeed verify the expected behavior: the spin current in Fe/Au exhibits a much faster dynamics than in the low-mobility Fe/Ru structure (see Fig. 4b). These findings are relevant because they show that the inverse spin Hall effect is still operative even at THz frequencies and that ultrafast spin currents can be delayed in a relatively simple manner [2].

Ultrafast spin manipulation: To manipulate the magnetization of a ferromagnetic metal film on ultrafast time scales, the sample is usually illuminated with a femtosecond laser pulse, thereby depositing energy in the electronic subsystem. The resulting reduction of magnetization on a 100-fs time scale is not yet understood, despite considerable experimental and theoretical efforts since the first experiments in 1996. To freeze out the complex electronic degrees of freedom, we recently focused on the ferrimagnetic insulator yttrium iron garnet (YIG, electronic band gap 2.8eV). Energy is brought into the system by exclusively pumping optical phonons with an intense THz pulse, thereby leaving the electronic subsystem unchanged. First results demonstrate a fast demagnetization with a 1.1 ps time constant, which is extremely surprising because spin-lattice relaxation in YIG is known to occur on much slower time scales of 1 ns and more. The results indicate that the coupling of spins with optical phonons is orders of magnitude stronger than with acoustic phonons. Currently a detailed study is under way to understand the microscopic origin of this observation.

Surface/interface sensitivity: So far, THz pulses have exclusively addressed the bulk properties of samples rather than their interfaces. The group of Tobias Kampfrath currently investigates the possibility to measure currents flowing at interfaces of topological insulators. Similar to the scheme of Fig. 4a, currents are launched by a circular polarized fs laser pulse in surface states of topological insulators and then measured by detecting the emitted THz electromagnetic transient (THz emission spectroscopy).

Manipulation of light on-the-fly: Besides controlling spins as information carriers, also new schemes for light manipulation have been tested. The basic idea is to couple light into a photonic structure and then vary its photonic mode structure by applying a femtosecond control pulse. The light inside the structure will then adiabatically follow these changes and finally emerge with a modified shape. To test this scheme, a photonic crystal was used and light at the telecommunication band around 200 THz coupled into it. By applying a spatially shaped femtosecond control pulse, free charge carriers were induced in selected regions of the photonic crystal, thereby (i) shifting and (ii) even tilting the dispersion relation (frequency vs wave vector) of the structure. As a result, the probe pulse followed these changes adiabatically, resulting in (i) a blueshift (by up to 0.3 THz) and (ii) a compression of the pulse spectrum (by up to 10%) with high efficiency (up to 80%). The frequency shifting is potentially useful for shifting signal trains in optical telecommunication to different frequency channels, whereas the spectral compression may find application as a spectral lens [3].
[1]   T. Kampfrath, K. Tanaka, K.A. Nelson, Nature Photon. 7, 680 (2013).
[2]   T. Kampfrath et al., Nature Nanotech.   8, 256 (2013).
[3]   D. M. Beggs, T. F. Krauss, L. Kuipers, T. Kampfrath, Phys. Rev. Lett.   108, 033902 (2012); D. M. Beggs et al., Phys. Rev. Lett.   108, 213901 (2012).

2.1.5 Ultrafast Spin Dynamics in Epitaxial Metallic Multilayers

Ultrafast spin dynamics induced by transport of photoexcited spin polarized carriers is of fundamental interest for magnetic applications like spintronics and data storage. To study the underlying elementary processes on femtosecond time scales, the group of Alexey Melnikov has developed a time domain approach that probes the spin dynamics induced by hot carriers (HC) in a back-pump-front-probe scheme. In a first experiment the transport of spin polarized HC through a Au/Fe/MgO(001) stack has been demonstrated [1]: Thereby, optical excitation of hot carriers in the Fe film is followed by superdiffusive transport to the Au surface where the carrier density and spin polarization are detected by magneto-induced second harmonic (SH) generation. Later on it has been shown that the ballistic HC fraction can be controlled by the Fe layer thickness and the duration of the ballistic spin current pulse can be as short as 30 fs. The spin dynamics in Fe was studied by a combination of SH and magneto-optical Kerr effect.

To proceed towards a new concept of metal-based elements for femtosecond spintronics, Fe/Au/Fe/MgO(001) stacks with different thickness of Fe layers were used, which allow for parallel or anti-parallel alignment of the magnetizations ME and MC in emitter and collector (see Fig. 5). The SH electric field consists of Eeven and Eodd components which are even and odd with respect to magnetization reversal. The relative pump-induced variation Δodd = ΔEodd/Eodd characterize variations of the magnetization at the interfaces. Since optical phases are sensitive to interface properties, Eodd generated at Fe/Au and Fe/MgO interfaces can interfere constructively or destructively, which leads, respectively, to large or small magnetic contrast ρ ≈ 2 Eodd/Eeven. In the first case Δodd is sensitive to the average (sum) of the magnetizations at both interfaces (Fig. 5b) while in the second case Δodd monitors variations of the difference between two interface magnetizations (Fig. 5a).

Fig. 5. Pump-induced changes of the odd second harmonic (SH) field Δodd which probes the spin dynamics in Fe/Au/Fe structures for (a) a 8 nm-thick emitter and small collector magnetic contrast ρ=5% (a) and (b) a 5 nmthick emitter and large contrast ρ=70%. The parallel (P) and anti-parallel (AP) orientations of magnetizations in the emitter (ME) and collector (MC) are shown in the experimental schemes in the insets. Inset (a): The large ballistic propagation length of majority HC in Fe λFe>>λFe leads to more effective emission of majority HC (thick green vs. thin red arrows) and to the accumulation of minority HC at the Fe/Au interface of the collector. However, in Au λAu<<λAu and hot carrier transport is ballistic (fast) for minority and diffusive (slow) for majority carriers (long red vs. short green arrows).

In Fe/Au/Fe structures with parallel alignment, minority HC which traverse the Au layer within 40 fs [1] are accumulated at the collector Fe/Au interface. This reduces the interface MC and thus the absolute value of Eodd, which results in positive Δodd (Fig. 5b) due to destructive interference of interface contributions. In the case of antiparallel alignment, ballistic HC (minority HC with respect to ME) are majority HC with respect to MC and thus are not accumulated at the interface. The interface demagnetization increasing Δodd (Fig. 5b) occurs upon the arrival of diffusive HC (now with negative spin) on a timescale of 200 fs [1]. Finally, the HC transport leads to the demagnetization of the Fe/Au interface and occurs on the timescale of ballistic or diffusive HC transport for parallel or antiparallel configurations, respectively.

In Fe/Au/Fe structures with large magnetic contrast (Fig. 5b), the role of ballistic HC providing small changes of interface MC due to their small concentration is not significant and we can consider only diffusive (majority with respect to ME) HC. The reversal of ME permits to alternate between positive and negative variations of MC on a femtosecond timescale, which is promising for the future development of spintronics devices.
[2]   A. Melnikov et al., hys. Rev. Lett.   107, 076601 (2011).

2.2 Molecular Processes at Interfaces

Understanding molecular processes at a microscopic level of single molecule reactions, interfacial charge and energy transfer and vibrational dynamics provides fundamental insight into surface reactions. Studies of molecular processes at interfaces are performed by several groups in the department which employ complementary techniques with high spatial resolution as well as chemical sensitivity using vibrational or x-ray spectroscopy. In these studies, surface reactions and molecular rearrangements are stimulated by thermal activation, excitation by light or interfacial charge transfer. These are complemented by computational studies of biomolecular machines and spatiotemporal pattern formation in electrochemical systems.

2.2.1 Nanoscience with Functional Molecules

The research activities in the group of Leonhard Grill focus on the investigation and manipulation of single functional molecules on surfaces by scanning tunnelling microscopy (STM), preferentially at low temperatures of 5 K. This method makes it possible to image single molecules with very high spatial resolution, and to manipulate them by chemical forces, tunnelling electrons or the electric field in the junction.

An important class of functional molecules are molecular switches that exhibit at least two stable states with characteristic physical and/or chemical properties. The research group has studied various types of molecular switches on metal surfaces. The first example [1] follows previous work of the group on azobenzene derivatives with four tert-butyl side groups, but here these groups were attached at slightly different positions of the benzene rings (para instead of meta position). These molecules are found to be prochiral on a Au(111) surface, and enantiomerically pure islands are observed. In manipulation experiments chirality switching is observed, where single molecules change from one enantiomer to the other, probably by a twofold internal rotation [1]. In another study, the group has investigated covalently connected multiple switching systems where the coupling of the different switching units is of particular interest. It could be shown that bisazobenzene molecules with two switching units can be deposited onto a Au(111) surface under clean ultrahigh vacuum conditions, and that they self-organize in large ordered islands of different arrangements depending on their chemical structure. While lateral manipulation can be achieved, no switching processes could be induced by voltage pulses over different parts of the molecules and in different environments. When changing the switching unit and studying imine derivatives on Au(111), it was found that the molecular layer gradually transforms from a nearly complete trans- to a nearly complete cis-monolayer with increasing molecular coverage [2].

Another focus in the field of functional molecules is on so-called nanomachines, which are objects with dimensions of few nanometers that perform work – a key vision in molecular nanotechnology. Functional molecules are of particular interest in this regard and the research group has studied so-called motorized nano-cars which should move on a surface upon illumination, essentially as a result of an isomerization process of the “motor” unit. After the very difficult synthesis, the intact deposition of such a complex molecule under ultrahigh vacuum conditions represents a central challenge. It could be shown that intact molecules can be sublimed in vacuum onto a Cu(111) surface and that they can be imaged there as individual entities [3]. The molecular appearance in the STM images is in good agreement with the molecular dimensions in the gas phase, and according to their chemical structure, two typical conformations are identified. However, lateral motion on the surface, in particular by activating the molecular motor, has not been achieved so far.

Fig. 6: Hierarchical growth following sequential thermal activation. (a) Scheme of the activation mechanism. Arrows indicate the different growth directions of the two sequential steps. (b–d) STM images (8 × 8 nm2 in (b,c) and 10 × 10 nm2 in (d)) of trans-Br2I2TPP molecules on Au(111): after deposition (at 80 K, (b)), after heating to 120°C (c) and after further heating to 250°C (d). The corresponding chemical structures are indicated.

In addition to the fundamental understanding of physical processes and chemical reactions of single molecules on a surface, the research group is interested in the linking of molecular building blocks on surfaces by covalent bonds, i.e. on-surface polymerization that is based on previous work by the group (Nature Nanotech. 2, 687 (2007)). An important issue in this regard is the precise location of the molecular activation process, i.e. the dissociation of Br substituents from the molecular building blocks on the surface. By comparing flat and stepped gold surfaces, the research group could precisely identify the kink sites at the step edges as the catalytically active sites because there is a high preference for activation for that side of the molecules that points towards these sites [4]. After heating the surface, the step edges ccreate polymers that run parallel over the surface, which is of interest for a pre-alignment in future polymerization processes. Another study has focused on the complexity of the molecular structures produced by on-surface polymerization. All approaches so far have relied on a single step process, thus resulting in very simple structures. The research group has extended this method by introducing a hierarchical growth process based on a sequential activation of the molecular building blocks as sketched in Fig.1a [5]. The molecular building blocks (trans-Br2I2TPP molecules) exhibit two types of halogen substituents (bromine and iodine) that are dissociated from the molecular core at different temperatures, due to their characteristic binding energies. This results in a programmed reactivity in which in the first step, only the iodine atoms and in the second step the bromine atoms are cleaved from the molecule. Starting from the intact molecules (Fig.1b, the two halogen species appear at different height), this leads to polymer chains after the first step (Fig.1c). It is important to note that these polymers were linked exclusively at the former iodine sites, which is visible in the bright lobe, i.e. iodine, at the terminus and darker lobes, i.e. bromine, sideways in Fig.1c. After a second heating process, the polymer chains are connected sideways in a zipping mechanism, resulting in a two-dimensional network (Fig.1d). In addition to these homomolecular polymers, also copolymers could be formed by mixing two different building blocks and the resulting structure could not be formed in a conventional single-step linking process [5].

The research group has used the expertise in on-surface polymerization for the formation of graphene nanoribbons (following a recipe by R. Fasel and co-workers) on a Au(111) surface [6]. However, the objective was here not the polymerization process itself, but the characterization of charge transport through the thus formed molecular wires. By pulling individual graphene nanoribbons off the surface with the STM tip, the current decay along the polymer, which is the key property for the characterization of charge transport, can be determined. In particular, this was done for various electron energies, thus for the first time correlating the conductance of individual molecules with their electronic structure from the HOMO over the gap up to the LUMO. It was found that the charge transport is most efficient if the electron energy matches either the HOMO or the LUMO level, which are both delocalized along the ribbon [6].
[1]   S. Selvanathan, M. V. Peters, S. Hecht, and L. Grill, J. Phys.: Condens. Matter 24, 354013 (2012).
[2]   C. Gahl et al., J. Am. Chem. Soc.   135, 4273 (2013).
[3]   P.-T. Chiang et al., ACS Nano   6, 592 (2012).
[4]   A. Saywell, J. Schwarz, S. Hecht, and L. Grill, Angew. Chem. Int. Ed. 51, 5096 (2012).
[5]   L. Lafferentz et al., Nature Chemistry   4, 215 (2012).
[6]   M. Koch, F. Ample, C. Joachim, and L. Grill, Nature Nanotech.   7, 713 (2012).

2.2.2 Molecular Manipulation and Spectroscopy at the Nanoscale

Molecular processes and functions are fundamental in nature and also play a key role for molecular devices in future nanotechnology. Low-temperature scanning tunneling microscopy (LT-STM) permits not only to directly observe adsorbate dynamics at the single-molecule level, but also to precisely manipulate adsorbates and to control chemical reactions of single molecules by using chemical forces between the STM tip and adsorbates, the injection of tunneling electrons as well as the electric field in the junction. The research in the group of Takashi Kumagai focuses on the investigation of individual molecules and molecular ensembles using LT-STM and tip-enhanced Raman scattering.

Direct observation and control of single molecule dynamics: It is known that local environments of individual molecules have a significant impact on chemical processes in condensed phases via the deformation of the potential landscape. However, such local influences have rarely examined at the level of individual molecules in experiments and the effects of nearby single atoms or molecules on chemical reactions have not been studied so far.

Takashi Kumagai and coworkers have achieved precise control of an intramolecular hydrogen transfer reaction (tautomerization) in single porphycene molecules adsorbed on Cu(110) [1].
Fig.7 Porphycene molecules on a Cu(110) surface a STM images of a single porphycene molecule. b The optimized structure determined by the density functional theory calculations. Porphycene molecule favors cis configuration on Cu(110). c Schematic illustration of the cis-cis tautomerization.
Single porphycene molecules were imaged at 5 K and found to have a cis configuration in which the inner Hatoms are located on one side in the cavity (Fig.7a and b). Although the molecule is stationary at low bias voltages, the cis-cis tautomerization is induced at higher voltages and the STM image shows a flipping between the two states (Fig.7a and c). The efficiency of tautomerization depends on the lateral STM tip position with respect to a molecule, i.e., atomically precise location of the electron injection into the molecule, and shows maxima when the electron is injected over the position where the inner H-atoms exist. The tautomerization is also thermally induced at elevated temperatures and a barrier of 168±5 meV is determined from an Arrhenius plot. Furthermore, an isotope effect using deuterium-substituted porphycene in which the inner H-atoms are replaced by deuterium revealed that the STMinduced tautomerization is triggered by vibrational excitation via inelastic electron tunneling processes.

Remarkably, the probability for tautomerization can be precisely tuned by placing a single Cu adatom nearby a porphycene molecule. Cu adatoms are controlled by STM manipulation and the rate of tautomerization is significantly affected depending on the relative position between the adatom and molecule. The results demonstrate the high sensitivity of an elementary reaction not only to the presence but also to the exact position of individual atoms with respect to the molecule, surprisingly also at rather large distances much larger than a van der Waals radius [1].

The group extended this study to molecular assemblies and observed cooperative effects in the tautomerization process. Tautomerization is almost quenched in the dimer, but it becomes active in a specific molecules within larger clusters. It is revealed that even the hydrogen arrangement in the cavity of a neighboring molecule influences the tautomerization, causing positive and negative cooperativity. The results highlight the importance of local environments in the vicinity of individual molecules, and demonstrate the potential to regulate a single-molecule function. It is expected that this control over chemical reactions by subtle changes in the atomic-scale environment can be extended to other systems and will thus improve the understanding of fundamental molecular processes. It might even allow the tuning of molecular processes in functional nanostructures, which would pave the way towards information processing at the single-molecule level.

Tip-enhanced Raman spectroscopy: Tip-enhanced Raman spectroscopy (TERS) is one of the possible techniques to probe both adsorbate geometries and local vibrations, which provides fruitful insight into physical and chemical processes on surfaces. In TERS an STM tip is employed to generate a plasmonic field to enhance the Raman scattering of adsorbates as well as imaging their local structure with sub-molecular resolution. Within the last few years a TERS setup in ultra-high vacuum (UHV) has been developed to achieve a local spectroscopy at the single-molecule level [2]. Current experiments investigate graphene nano-ribbon (GNR) on a Au(111) surface, an attractive material in nano-science and technology. Local defects and alkali doping of GNR are expected to have a significant impact on its properties and the understanding of such influences is of importance for potential device applications.
[1]   ST. Kumagai et al., Nature Chemistry, (in press 2013).
[2]   B. Pettinger et al., Ann. Rev. .Phys. Chem.   63, 379 (2012).

2.2.3 Real-time Observation of Photoinduced Surface Reactions

A longstanding dream has been to follow the dynamics of chemical reactions in real time and directly observe each elementary step. While for photoinduced reactions in the gas phase a remarkable level of sophistication has been reached, similar breakthroughs have not be achieved for heterogeneously catalyzed reactions at surfaces. Fundamental questions in surface reaction dynamics adress how charge and energy is transferred between the adsorbates and the surface, or how the electronic structure is rearranged within the molecular unit during the elementary reaction steps. The real-time observation of chemical bond formation at surfaces requires techniques which are sensitive to the chemical state and simultaneously enable real-time probing of elementary steps with femtosecond time resolution.

Fig. 8 (left) Schematic illustration of elementary steps in the desorption process of carbon monoxide. The reaction is stimulated by an ultrashort optical laser and probed with soft x-ray spectroscopy using femtosecond x-ray pulses. (right) Free energy diagram along the reaction pathway for CO on Ru(0001) exhibiting a chemisorption well and a shallow precursor which are separated by a temperature dependent barrier. [2]

Both x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) have the unique ability to provide an atom-specific probe of the electronic structure. With the advent of femtosecond x-ray lasers these techniques can now be transformed into time-resolved probes of transient chemical species at surfaces. For real-time probing of surface reactions, the chemical process must be initiated by a time-correlated ultrashort laser pulse, a concept which has been studied previously in the Department by the group of Martin Wolf: Femtosecond laser excitation of an adsorbate-covered metal surface can serve as a ultrafast “trigger” of surface reactions, whereby the non-adiabatic coupling between transiently excited metal electrons and adsorbate vibrational degrees of freedom mediates chemical processes such as associate desorption of the reactants [1].

Our collaboration with Anders Nilsson, and groups from Stanford, Stockholm, Hamburg and Berlin has performed a series of surface dynamics experiments, exploiting the unique capabilities of the Stanford x-ray free electron laser, LCLS, to probe the atom specific electronic structure changes during surface reactions induced by a strong laser pulse. Fig. 8 (left) schematically illustrates the basic principle of these experiments, exemplified for the simple case of desorption. The first experiment addressed desorption of CO from Ru(0001) induced by a 400 nm laser pulse [2]: By combining XAS and XES in the O 1s region both occupied and unoccupied states of the CO-metal bond (5σ/1π, dπ, 2π* states) were probed as a function of time delay between the optical and x-ray laser pulses, demonstrating for the first time the feasibility of time-resolved resonant inelastic x-ray scattering (RIXS) in surface chemistry. Remarkably, a substantial transient weakening of the CO-Ru bond was observed, persisting for ~20 ps whereby ~30% of the adsorbed CO molecules resembled an electronic structure close to free CO. From DFT calculations including van-der Waals contributions of the free energy along the reaction coordinate (potential of mean force) a consistent picture could be obtained: The desorption dynamics of CO from Ru(0001) is governed by a transient energy landscape with a chemisorption well and a shallow precursor state, which are separated by a transient (entropic) barrier [2]. During desorption, a fraction of the vibrational excited CO molecules are transferred into the precursor state and are stabilized by the barrier prior to desorption (see Fig. 8 right). It should be noted that such insight into the dynamics of an excited (i.e. reacting) adlayer could not be revealed with established techniques like molecular beam scattering.

Further analysis demonstrated that by using resonant XES, selective excitation of different ensembles within the excited CO adlayer becomes possible, which exhibit different dynamical behavior and coupling to substrate electrons and phonons [3].

More recently, the technique of time-resolved RIXS has been successfully applied at LCLS to also study associative surface reactions such as ultrafast photoinduced CO2 formation and hydrogenation of CO on ruthenium. Future planning includes the development of a dedicated surface science endstation with two XES spectrometers for molecular orientation sensitive detection and optimized sensitivity.
[1]   H-J. Freund , G. Meijer, M. Scheffler, R. Schlögl and M. Wolf, Angew. Chem. Int. Ed., 50, 10064 (2011).
[2]   M. Dell’Angela et al., Science   339, 1302 (2013).
[3]   M. Beye et al., Phys. Rev. Lett.   110, 186101 (2013).

2.2.4 Interfacial Molecular Spectroscopy

The structure, reactivity and dynamics of molecules at interfaces are addressed by the group of Kramer Campen using the interface specific nonlinear optical technique of vibrational sum frequency (VSF) spectroscopy. Furthermore, vibrational and structural dynamics are directly probed by time-resolved infrared pump / VSF probe experiments. Over the last two years three systems have been studied: the air/water interface, CH4 and C2H4 dissociation and reaction at the Ru(0001) surface in UHV, and water dissociation and reaction at the α-Al2O3(0001) surface both in UHV and under ambient conditions. For UHV studies a molecular beam source was developed to prepare translational and vibrational non-thermal distributions of impinging molecules, e.g. for dissociative adsorption. Under ambient conditions conventional VSF spectroscopy has been extended either by directly sampling structural dynamics, in the case of the air/water interface, or by probing low-frequency modes (e.g. the surface phonons of α-Al2O3) not previously observed.

Air/Water Interface: Prior VSF and simulation studies have shown that, from the H2O molecule point of view, interfaces between liquid water and hydrophobic surfaces have a large population of water molecules with one non-hydrogen bonded (free) OH groups. Recent theoretical work has made clear that quantitative understanding of hydrophobic solvation requires an understanding of the ultrafast structural dynamics of these free OH groups [1]. We have recently applied both experiment, i.e. time resolved VSF spectroscopy (in collaboration with M. Bonn, MPI for Polymer Science), and simulation (in collaboration with A. Vila Verde, MPI of Colloids and Interfaces), to understand the dynamics of the free OH. Taken together, this work suggests: (1) The free OH rotates 3x faster than hydrogen bonded OH groups either at the air/water interface or in bulk water [2, 3]. (2) The free OH is structurally heterogeneous on picosecond timescales: free OH groups closer to the vapor have a different orientational distribution and persist longer before rotating down towards the liquid than free OH groups closer to bulk water [2-4]. (3) Relaxation of vibrationally excited free OH groups proceeds by a combination of energy transfer between the free and hydrogen bonded OH within a single water molecule (2/3 of relaxation occurs via this pathway) and structural relaxation (1/3) in which the excited free OH rotates towards the liquid and forms a hydrogen bond [5].
 
Fig. 9  Left: Measured VSF spectra showing the decrease in CH2 spectral amplitude on heating above 350 K.
Right: Change in CH2 resonance amplitudes plotted as a function of sample heating temperature on an Arrhenius plot illustrating CH2 to CH conversion. In the low coverage limit the Ea for CH2 dehydrogenation is 14 kJ/mol

Methane and Ethylene at Ru(0001): Interaction of CH4 and C2H4 with metal (oxide) surfaces may lead to their decomposition and the formation of higher hydrocarbons. To study this chemistry we have characterized the interaction of CH4 and C2H4 with the Ru(0001) surface in UHV using temperature programmed desorption (TPD) and VSF spectroscopy as a function of sample temperature and carbon coverage. To overcome the dissociation barrier of CH4 a molecular beam source was employed with CH4. By probing the CH spectral response (2800-3100 cm-1) during the decomposition of both CH4 and C2H4 we have tracked the relative stability of all one and two carbon, CH containing species. We find that both the relative stabilities and rates of interconversion of the various hydrocarbon species present are strongly depending on surface coverage and temperature (Fig. 9 left). A dramatic change in reactivity was observed above 350 K. Both computation (in collaboration with S. Levchenko, Theory Department) and experiment clarify that below 350 K adsorbed hydrogen blocks energetically favorable surface sites and that above 350 K recombinative desorption of H2 sets in. This coverage dependence of hydrocarbon reactivity is manifested by the thermal stability of CCH2/CCH species and the thermal activation for the conversion of CH2 to CH (Fig. 9 right).

Water Dissociation and Surface Reconstruction at α-Al2O3(0001): Most properties of Al2O3 surfaces change dramatically on exposure to even submonolayer concentrations of water. To gain insight into the mechanism(s) of such change, we here probe the elementary steps of single molecule water dissociative adsorption on α-Al2O3(0001) and the changes in surface structure they induce. Water adsorption was studied by preparing a well defined α-Al2O3(0001) surface in UHV and then dosing this surface, using the molecular beam source, with D2O seeded in He. Next the spectral response of the frequency range corresponding to dissociated water molecules was characterized as a function of sample temperature. This data show five resonances whose relative frequencies, and intensities as a function of experimental geometry, are consistent with computation (collaboration with P. Saalfrank, University of Potsdam). Based on this agreement, these five modes are assigned to fragments resulting from three different predicted water dissociative adsorption channels. By tracking the thermal stabilities of these fragments, it was demonstrated that models of water surface reactivity must explicitly account for surface coverage effects.

While the water OD stretch is a useful probe of dissociative adsorption, it tells us little about concurrent surface reconstruction. To probe surface reconstruction both within and outside UHV we have extended VSF spectroscopy to infrared frequencies as low as 750 cm-1 to probe also α-Al2O3 surface phonons. As expected, the amplitudes/line shapes of these modes are dramatically changed by treatments known to dehydrate/rehydrate the surface. Normal mode calculation confirms our assignment and allows a full microscopic description of each mode. This work demonstrates that surface reconstruction outside of UHV can be tracked by optical probing of surface phonons.

Electron solvation at interfaces: The reactivity of excess electrons in aqueous environments is highly relevant in various disciplines ranging from atmospheric chemistry to photosynthesis. The group of Julia Stähler has studied excess electrons at ice-vacuum interfaces created by photoexcitation of the metal template and also by low-energy electron impact. Their relaxation and lifetime of several seconds was determined using two-photon photoelectron (2PPE) spectroscopy. In addition to pure charging, permanent changes to the surface dipole and therefore work function (up to 1 eV) were also observed, which may result from a buildup of OH-1 induced by the trapped electrons. In combination with simple model calculations, these work function modifications were also used to identify the impact of work function distributions on photoelectron experiments [6].
[1]   D. Chandler, Nature, 437, 640 (2005).
[2]   C.S. Hsieh et al., Phys. Rev. Lett.   107, 116102 (2011).
[3]   A. Vila Verde, P.G. Bolhuis, R.K. Campen, J. Phys. Chem. B   116, 9467 (2012).
[4]   Y. Tong, A. Vila Verde, R.K. Campen, J. Phys. Chem. B   117, 11753 (2013).
[5]   C.S. Hsieh, R.K. Campen, M. Okuno, E.H.G. Backus, Y. Nagata, M. Bonn, PNAS   (in press 2013).
[6]   D. Wegkamp, M. Meyer, C. Richter, M. Wolf, and J. Stähler, Appl. Phys. Lett.   103, 151603 (2013).

2.2.5 Computational Dynamics of Protein Machines

The group of Alexander Mikhailov performs computational studies of biomolecular systems. Protein machines play a fundamental role in biological cells. Operating as molecular motors, they transport load over filaments and microtubules or generate mechanical force. They can perform operations with other molecules, such as DNA or RNA, and cut, glue or unwind them. Protein machines can act as enzymes, facilitating reaction events, or as pumps transporting ions across lipid bilayers. In all their functions, the operation of such molecular devices is based on the ability of proteins to fold into a definite conformation and to perform ordered mechanochemical motions, induced by binding and detachment of ligands or by chemical reactions with them. All machines require energy for their operation and this energy is usually provided through ATP molecules. Understanding the dynamics of protein machines is essential in biophysics of a cell; it can also open a way for engineering of synthetic molecular devices with similar properties.

The cycles of protein machines involve slow conformational motions on the scales of milliseconds or longer. Therefore, they cannot be reproduced in current full molecular dynamics simulations and coarse-grained descriptions are required. One such description consists in modeling a protein as an elastic networks (EN) formed by particles (amino acids) with effective elastic interactions between them. EN models are popular and often used in the context of normal-mode analysis. A special feature of the investigations in the group of Alexander Mikhailov is that complex nonlinear dynamics is considered in the EN models of various real protein machines through numerical simulations. The research is undertaken in cooperation with partners in Japan, Canada, Belgium and Taiwan.

Myosin is the molecular motor responsible for muscle contraction and for transport along actin filaments in biological cells. It has been extensively investigated in the group of T. Yanagida, Osaka University, and at RIKEN Quantitative Biology Center in Japan. Their recent single-molecule experiments have shown that this protein acts as a “strain sensor”, so that its affinity towards the actin filament is strongly modulated by the applied external forces; this behavior is of principal importance for the motor function. Numerical analysis of responses of myosin molecules to external forces could not only confirm the experimental results, but also disclose the nature of the strain-sensor behavior in this macromolecule [1].

Actin is a structural protein able to form, through polymerization, long filaments which build the skeleton of a cell and are also used for intracellular transport by myosin motors. The filaments are permanently growing at one end and dissolving at the other end through the process of “treadmilling”. While it was known that ATP is needed for filament growth, its role in the polymerization process remained unclear. The EN simulations have revealed that, when ATP binds to an actin monomer, this stabilizes its closed conformation already present as a metastable conformational state in the ATP-free molecules. Such closed conformation much better fits the growing filament end and thus the polymerization rate is greatly enhanced. The metastable conformational states of actin were previously detected in singlemolecule FRET experiments by T. Yanagida with coworkers.

      Fig. 10. The cycle of a membrane machine. (A) The ligand binds to the machine; (B) the machine conformation changes from the open state to the closed state; (C) the reaction takes place and the ligand is released; (D) the machine returns to its open state.
 

Many protein machines operate as active inclusions in lipid bilayers forming biological membranes. In such cases, additional complications arise because of the necessity to incorporate lipids and the solvent into a model. Together with R. Kapral, Toronto University, and researchers from the National Central University in Taiwan, fast and efficient methods for such simulations, combining the EN description for proteins with a coarse-grained description for lipids and the multiparticle collision dynamics for the solvent, were developed [2,3]. Fig.10 shows a cycle of a model protein machine attached to a lipid bilayer. An important result is that strong hydrodynamic flows in the lipid bilayer, induced by operating machines, were found, indicating that hydrodynamic effects should play a principal role in interactions between active membrane inclusions. The swimming behavior of active machines in biomembranes has been further discussed [4].

Furthermore, a statistical analysis based on the NMR data for conformational ensembles in a set of 1500 different proteins was analyzed and used for improvement of the accuracy of the EN method (with Y. Dehouck, ULB in Belgium). In addition to the work on biomolecular systems the group has also investigated complex dynamics in networks.
[1]   M. Düttmann, Y. Togashi, T. Yanagida, A. S. Mikhailov, Biophys. J. 102, 542 (2012).
[2]   M.-J. Huang, R. Kapral, A. S. Mikhailov, H.-Y. Chen, J. Chem. Phys.   137, 055101 (2012).
[3]   M.-J. Huang, R. Kapral, A. S. Mikhailov, H.-Y. Chen, J. Chem. Phys.   138, 195101 (2013).
[4]   M.-J. Huang, H.-Y. Chen, A. S. Mikhailov, Eur. Phys. J. E   35, 119 (2012).

2.2.6 Electrochemical Dynamics

The co-existence of distinct timescales is an important feature in most natural and man-made systems exhibiting kinetic instabilities and oscillatory behavior. The group of Markus Eiswirth has concentrated on different aspects of non-linear dynamics in electrochemical systems from both experimental and theoretical points of view. For electrochemical reactions taking place at the solid/liquid interface, experimentally recorded time-series are often subjected to a long-term surface deactivation process that acts as a slowly evolving bifurcation parameter. Recently, mechanistic aspects associated to this process during the catalytic electro-oxidation of small organic molecules have been investigated. Experiments and numerical simulations were carried out in cooperation with Hamilton Varela, University of Sao Paulo at Sao Carlos, Brazil, and Jaeyoung Lee, Ertl Center for Electrochemistry and Catalysis, Gwangju, Korea).

Fig. 11 shows results for the electro-oxidation of formaldehyde on platinum in terms of the time-trace of the electrode potential, U, obtained under a slow galvanodynamic sweep [1]. The deliberate increase of the applied current mimics the spontaneous and slow poisoning process, also observed in many other systems. Therefore, the oscillatory patterns depicted in (c - h) and monitored at different applied currents appear spontaneously after setting a given fixed current. For a certain kind of oscillation or waveform, the mean electrode potential increases in time (Fig. 11 a). The system thus consists of two parts: the core oscillator, associated to the main dynamics or the oscillations themselves, and a slowly evolving drift. The coupled system is characterized by the co-existence of two disparate time-scales. After each cycle, the surface would ideally return to its original state and, under those conditions, oscillations would persist as long as the reaction proceeds. Conversely, the slowly evolving parameter may arise from the fact that the surface is not completely restored to its initial conditions after one oscillatory cycle. Since the drift in acidic media is always accompanied by an increase in the mean electrode potential, it is likely that the surface is slowly getting oxidized after each cycle.

Fig. 11(top): Time-traces of (a) the mean electrode potential, and of (b) the electrode potential, during the galvanodynamic (10.42 μA cm–2 s–1) electro-oxidation of formaldehyde on platinum (c-h expanded time axis). Electrolyte: 0.5 M H2SO4 aqueous solution with 0.1 mol L–1 of HCHO. (Bottom): Reaction scheme of electrooxidation of methanol and formaldehyde on platinum [1].

Based on the above conjectures and also on in situ infrared and online mass spectrometry, one can describe the interplay between slow and fast processes in terms of the surface coverage of adsorbates. The main aspects are summarized in the reaction scheme in Fig. 11 (bottom). Generally speaking, the electro-oxidation of organic molecules on platinum proceeds via parallel pathways: the direct pathway, with an active intermediate (adsorbed formate in this scheme) transforming to carbon dioxide relatively fast, and the indirect route, where adsorbed carbon monoxide is oxidized at comparatively high overpotentials in a Langmuir- Hinshelwood step with adsorbed oxygenated species. Steps 1 to 6 in this scheme are associated to oscillations and thus belong to the core subsystem.

The adsorbed species PtO(H)x represents a generic oxygenated adsorbate that participates in the oxidation of species such as adsorbed carbon monoxide. Further oxidation of the platinum surface leads to the so-called place-exchange process, in which surface oxygen atoms are inserted in the platinum lattice in order to allow further surface oxidation to take place. Subsurface oxygen is in principle unavailable for Langmuir-Hinshelwood steps such as the illustrated electro-oxidation of adsorbed carbon monoxide. As a consequence, the increase in the amount of Osub-Pt accompanying the increase in the electrode potential is equivalent to a decrease in the overall number of surface sites, which in turn, causes an increase in the actual current density (in contrast to the applied current, which is strictly constant). Therefore, coupled to the core oscillator, the spontaneous drift that slowly evolves in time is suggested to result from the increase of sub-surface oxygen.

After assigning the slowly evolving parameter as the coverage of a surface-blocking species, incorporated this process has been incorporated in a generic model for this family of oscillators. The resulting model consists of four ordinary differential equations, and it was investigated over a wide parameter range. Besides the bifurcation analysis, the system was studied by means of high-resolution period and Lyapunov diagrams. It was observed that the system’s dynamics becomes simpler as the irreversible poisoning evolves, as evidenced by the changes in the structure of the bifurcation diagram. Nevertheless, periodic cascades are preserved in a confined region of the resistance vs. potential diagram.
[1]   M. F. Cabral, R. Nagao, E. Sitta, M. Eiswirth, H. Varela, Phys. Chem. Chem. Phys. 15, 1437 (2013).

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