Dresden 2017 – scientific programme

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O: Fachverband Oberflächenphysik

O 75: Ultrafast Electron and Spin Dynamics

O 75.15: Talk

Wednesday, March 22, 2017, 18:30–18:45, TRE Phy

Lattice Response of Pb/Si(111) Layers and Islands upon Femtosecond Optical Excitation — •Tobias Witte, Tim Frigge, Bernd Hafke, Boris Krenzer, and Michael Horn-von Hoegen — University of Duisburg-Essen, Lotharstr. 1, 47048 Duisburg, Germany

Employing time resolved reflection high energy rlectron diffraction (tr-RHEED) the non-equilibrium dynamics of the phonon system after optical excitation of ultrathin Pb films and islands on Si(111) has been investigated. The sample is pumped by fs-IR-laserpulses, while the transient response is probed by a 30 keV electron pulse. Surface sensitivity is achieved via gracing incidence of the high energy electrons. fs-laserpulses cause impulsive excitation of the electron system of the Pb films and islands, respectively. The hot electron system thermalizes and heats the lattice system by electron-phonon coupling. The thermal motion of the (surface-) atoms is observed through the transient Debye-Waller effect. As Pb is known to exhibit strong electron-phonon coupling in the bulk the time constant for the energy transfer to the lattice system is - in the framework of the two temperature model (TTM) - expected to be in the sub-ps regime. In contrast, a rather slow time constant of τ = 3.2 ps was observed at the surface. Moreover, incoherent lattice excitation of the islands occurs even slower at a timeconstant of τ = 4.6 ps. These results demonstrate how low-dimensionality and quantum-size effects reduce the energy flow mediated by electron-phonon coupling.