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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-resolved SFG vibrational spectroscopy of CO on Ru(001) during desorption

    Femtosecond broadband vibrational spectroscopy enables us to detect the internal strech vibration of CO molecules adsorbed on Ru(001) with extremly high sensitivity. Recently we have analyzed the transient spectral changes under conditions of laser-induced desorption of about 40% of the adlayer per laser shot. Due to the high temperatures reached in the experiment the frustrated rotational mode and the Ru-CO stretch vibration can be excited in addition to the frustrated parallel translational mode. Whereas coupling of the CO stretch to the molecule-surface stretch mode is negligable, the contribution to the frequency shift and linewidth from the frustrated rotation is much larger. We conclude that coupling to the frustrated rotation plays a crucial role to account for the experimental spectra, in addition to the frustrated translation. Phys.Rev.Lett. With our IR-broadband SFG setup we can also excite hot band transitions (v=1->2) for isolated (i.e. weakly coupled) CO molecules. With increasing coverage dipole-dipole coupling lead to rapid energy delocalization within the excited adlayer. (, Chem.Phys. Lett.).

Novel surface chemistry with femtosecond lasers: CO oxidation on Ru(001)

    Under UHV conditions ruthenium is a bad catalyst for the oxidation of carbon monoxide. Thus heating of a Ru(001) surface on which CO and atomic oxygen are co-adsorbed leads exclusively to desorption of CO. In contrast, excitation with intense femtosecond infrared laser pulses enables also the formation of CO2. Our recent time-resolved investigations of the sub-picosecond reaction dynamics and density functional calculations reveal that this novel reaction pathway is initiated by hot substrate electrons which populate an antibonding unoccupied state of the O-Ru bond. The vibrationally activated oxygen will eventually react with coadsorbed CO on an ultrashort time scale. On the other hand, desorption of CO molecules is caused by coupling of the adsorbate to the phonon bath of the Ru substrate. Further detailed information, press release.

Dynamics of 'hot' holes in graphite

    As part of a collaboration with the group of Tobias Hertel we have investigate the electron dynamics in highly oriented pyrolytic graphite (HOPG). Recently we have observed for the first time the ultrafast relaxation of photogenerated hot holes using time-resolved photoemission. In this experiment a probe laser pulse with a photon energy tuned above the sample work function induced one-photon photoemission. A time-delayed pump laser pulse induced a depletion of the photoemission yield due to photogenerated holes below the Fermi level and a corresponding increase of the electron population above the Fermi level.
 

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