Berlin 2008 – scientific programme

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O: Fachverband Oberflächenphysik

O 11: Time-Resolved Spectroscopy I

O 11.6: Talk

Monday, February 25, 2008, 14:45–15:00, MA 043

Time-resolved Electron Diffraction studies on the ultrafast temperature response of Bi and Pb on Si(111) — •Boris Krenzer, Anja Hanisch, Simone Möllenbeck, Tobias Pelka, and Michael Horn-von Hoegen — Department of Physics, University of Duisburg-Essen, 47048 Duisburg, Germany

Recent progress in developing ultrashort electron pulses has opened the wide field of investigating structural dynamics of surfaces upon short-pulsed laser excitations. Because the diffraction pattern is related to the atomic positions in a solid and its surface, direct investigation of transient structures on a femtosecond timescale became possible. Additionally, the diffracted intensity is affected by the thermal excitation of the solid and its surface. Thus, a time-resolved electron diffraction experiment yields information on the transient structure and the transient vibrational energy contained in the system at the same time.

Here we report on the fs-excitation of ultra-thin epitaxial Bi- and Pb-films on a Si(111)-substrate. For the Bi-film the initial temperature rise upon laser excitation is rather slow with a time-constant of 20 ps, which is in accordance to the predictions of the two-temperature model (2TM). Because Pb has a three orders of magnitude larger electron-phonon coupling constant the observed initial surface temperature rise is faster than for Bi but slower than the prediction of the 2TM. This discrepancy is attributed to the limited time-resolution of the experiment. However, the Pb/Si(111) is an ideal system to quantify the experimental temporal resolution and to test methods for improving the time-resolution.