Abstract: Electron transfer processes at surfaces and interfaces play a crucial role in diverse fields of materials sciences. In this talk, I will discuss surfaces of three-dimensional topological insulators and interfaces between metals and organic semiconductors. Time-resolved two-photon photoemission (2PPE), a method that combines femtosecond pump-probe techniques with photoelectron spectroscopy, can provide detailed information about the ultrafast dynamics of electrons excited into surface and interface-specific states of these systems. For investigations of metal/organic interfaces, PTCDA/Ag(111) has proven to be an excellent model system. It will be shown that the interface state, located between the Fermi level of the metal and the molecular LUMO of this and related systems, can efficiently mediate the electron transfer between the metal and the organic semiconductor. Three-dimensional topological insulators belong to a new class of materials which are characterized by an insulating bulk and a metallic surface state with a Dirac-cone-like energy dispersion. It will be shown that mid-infrared pump pulses permit a direct excitation of the topological surface state of Sb2Te3 across the Dirac point. The optical coupling leads to an asymmetric transient population in momentum space which reflects a macroscopic electric surface current. By observing the population decay with time-delayed UV probe pulses, we directly access the dynamics of the photocurrent and its topologically protected properties. Biosketch: Prof. Ulrich Höfer received a doctoral degree in physics in 1989 from the Technical University of Munich, Germany. After spending two years as a visiting scientist at the IBM Thomas J. Watson Research Center in Yorktown Heights, New York, he joined the Max-Planck-Institute for Quantum Optics in Garching, Germany. In 1999 he became a full professor for experimental physics at the Philipps University in Marburg. His main research interests are laser spectroscopy of surfaces and interfaces, coherence and ultrafast phenomena at surfaces, and the dynamics of elementary adsorbate reactions. Since 2013 he is the spokesman of a newly established collaborative research center on the structure and dynamics of internal interfaces.
Address: University of Science and Technology of China, No.96, JinZhai Road Baohe District, Hefei, Anhui, 230026, P.R.China.