Berlin 2008 – scientific programme
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O: Fachverband Oberflächenphysik
O 72: Symposium: Beyond Optical Wavelengths: Time-Resolved Spectroscopy of Surface Dynamics with EUV and XUV Radiation II (Invited Speakers: Wilfried Wurth, Hermann Dürr, Shik Shin)
O 72.7: Talk
Thursday, February 28, 2008, 16:30–16:45, HE 101
Femtosecond time- and angle-resolved photoelectron spectroscopy: A tool to study non-equilibrium states of electronic structure — •Uwe Bovensiepen — Freie Universität Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin
During the last 20 years angle-resolved photoemission spectroscopy has turned from an established surface science technique to a powerful experimental system that maps the electronic structure of valence bands in solids. Intense optical excitation by femtosecond laser pulses drives this electronic subsystem in condensed matter into a non-equilibrium state and its energy content is characterized by electronic temperatures of several thousand Kelvin. The ion-ion interaction is determined by the screened electron-ion potential and the intense optical excitation can be expected to modify the screening contribution. We excite metallic structures by 1.5 eV laser pulse at absorbed fluences of 0.1 – 1 mJ/cm2 and probe the transient state of the electronic structure by 6 eV UV laser pulses in time- and angle-resolved photoemission. For the 5dz2 surface state of Gd(0001) and for the highest occupied quantum well state originating from the 6pz band in epitaxial Pb films on Si(111) we observe a transient binding energy increase by 50 – 100 meV before electron-phonon scattering has lowered the excess energy in the electronic subsystem. Thus, we conclude that the optical excitation leads to a stabilization of both these systems. First experiments on RTe3 charge density wave materials show that under comparable conditions even a transition from an insulating to a metallic state can be induced optically.