Electron Dynamics
Surface Femtochemistry
Vibrational Spectroscopy
Molecular Switches

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Prof. Dr. Martin Wolf
Abt. Physikalische Chemie
Fritz-Haber Institut der MPG
Faradayweg 4-6
14195 Berlin, Germany
phone:+49 30 8413-5111
Fax: +49 30 8413-5106

hitherto (2010):
Department of Physics
Free University Berlin
Arnimallee 14
D - 14195 Berlin


Time evolution of the electronic structure of TaS2 through the insulator-metal transition


    Following the electronic structure during an
    insulator-to-metal transition in real time

    In daily life switching of light bulbs requires a conductivity change in an electrical switch. However, in a specific class of materials, the change of conductivity can be triggered a trillion times faster by the light itself, without need for a mechanical contact. Here, we show that a switch can be driven by optically excited electrons directly and does not require the slower movement of the atoms.

    In our experiment a infrared femtosecond laser pulse excites the insulating phase of 1T-TaS2 (see Figure, left panel) and generates hot electrons. This material is known to undergo a metal-to-insulator transition driven by electron-electron interaction (Mott insulator). We analyze the electronic structure directly with time-resolved photoelectron spectroscopy by photoemitting valence electrons by a time-delayed UV laser pulse (right panel). We find that the optical excitation induces an ultrafast transformation to a conducting phase, as evidenced by the instantaneous collapse of the electronic band gap (central panel). The hot electron distribution decays rapidly within a few hundred femtosecond and the band gap is subsequently re-established. In addition, the hot electrons launch a nuclear motion of the lattice atoms which vibrate coherently after the instantaneous excitation. Since these vibrations do not alter the band gap, the material remains insulating and represents the fastest push-button realized so far (right panel). Nevertheless the breathing of the lattice structure modifies the binding energy of electronic states by electron-phonon coupling. We observe this breathing of the electronic structure directly in the time domain. This represents a novel approach to electron-phonon interaction because one can now measure the change in the electron band structure during propagation of the phonon.

    The employed experimental approach distinguishes between effects of electrons and atoms and is thus very promising to further fundamental understanding in solid state physics in general, but might also be relevant for the development for future optoelectronic applications like optically actuated switches operating in the femtosecond regime.

    (Further information, Phys. Rev. Lett.)

Dynamic promotion of a surface reaction: Femtosecond-laser induced associative desorption of molecular hydrogen from Ru(001)  
    Promotion and poisoning effects, i.e. enhancement and hindrance of a chemical reaction on a solid metal surface, are well known phenomena in catalytic reactions. Modification of the electronic structure of the system like changes of the reaction barrier or energetic shifts in the potential energy landscape is facilitated through coadsorbates besides the actual reactants. While these changes typically occur in a stationary manner, our experiments with atomically adsorbed H and D on a Ru(001) surface show a dynamic promotion of the recombination reaction in which a hydrogen molecule is formed and desorbs into the gas phase.

    H2-RekombinationWe use intense near infrared femtosecond laser pulses to initiate the Hads + Hads -> H2,gas reaction. Our time-resolved measurements of the recombination yield reveal that hot substrate electrons transfer energy into the reactants within few 100 fs. Since this energy flow crucially depends on the mass of the reacting hydrogen atom, H is more readily excited than a respective D reactant. Thus, by separating the time scales of excitation, one might think of tailoring specifically wanted reaction products. In addition, experiments with isotopically mixed adsorbate layers (H and D statistically distributed on the Ru substrate) exhibit that the mere presence of the lighter H enhances the recombination of two heavier D atoms. Our findings imply that even under thermal equilibrium conditions these dynamic promotion effects play a role. In moments of favorable surroundings conditions caused by thermal fluctuations, chemical reactions might predominantly proceed. Only our excitation method with ultrafast laser pulses enable us to time-resolve the initial steps of such a surface reaction.
    (Further information, Phys. Rev. Lett.)

Coherent Optical Phonons and Parametrically Coupled Magnons Induced by Femtosecond Laser Excitation of the Gd(0001) Surface 

    Up to now magnetic recording faces a strict limit in writing speed of few GHz (109 Hz) which corresponds to the time required to invert an elementary magnetic dipole, i.e. a single spin. By studying the surface of the rare earth ferromagnet Gadolinium (Gd) we found that the surface magnetization Msurf follows a lattice vibration at a much higher frequency of 3 THz (1012 Hz). This new time scale for coherent magnetization dynamics suggests the application of coherent spin lattice coupling in comparable systems for an increase in magnetic recording speed up to a 1000 times compared to present approaches.
    In our pump-probe experiment we have studied a single crystal Gd surface by time-resolved second harmonic generation, an optical method that measures the lattice dynamics and the magnetization dynamics simultaneously. Two requirements have to be fulfilled to achieve coupling of these high frequency lattice vibration and the spin subsystem in a phase locked manner: The first one is excitation of lattice vibration by means of fs-laser pulses, which occurs for the Gd surface due to its specific electronic structure. The second one is a ferromagnetic system that is able to follow the lattice with the same phase, which is the case for Gd due to its THz spin waves. The interaction of the lattice and the spin system occurs as the lattice constant oscillates while the lattice vibrates and the strength of ferromagnetic order is determined inter alia by the interatomic distance. Thus, the Gd surface magnetization is found to oscillate several orders of magnitude faster than expected from the precessional GHz limit.

    These experiments have been performed in cooperation with K. Starke.

    (the movie;
    Phys. Rev. Lett.;
    press release of the Freie Universität Berlin)

Ultrafast electron transfer and solvation dynamics in ice films on Cu(111)

    Water, a polar solvent, can stabilize excess electrons. This process is referred to as solvation and is of key relevance in chemistry and biology. In liquid water, the ultrafast dynamics of electron solvation have been studied in great detail with femtosecond laser techniques. We have now succeeded in unrevealing the analogous process in thin ice layers adsorbed on a metal surface. Using time-resolved two-photon-photoemission (2PPE), we are able to resolve all elementary steps following electron injection from the metal substrate into the conduction band of ice. Photoinjected electrons localize in the ice within 100 fs and are stabilized by reorganization of the surrounding water molecules. With increasing degree of solvation the electrons are less strongly coupled to the substrate and hence the rate of electron transfer back to the metal decreases. In particular, we observe a striking influence of the ice layer structure on the solvation dynamics. Since the substrate acts as a template for the ice layer this study opens the perspective for systematic investigations of the relation between the structure and solvation dynamics in low dimensional systems.
    (Further information, Phys. Rev. Lett.)

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