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O: Fachverband Oberflächenphysik

O 12: Surface dynamics

O 12.6: Talk

Monday, March 26, 2012, 17:15–17:30, H 2013

Dynamics of Vibrational Modes of Pb Adsorbate Layers on Si(111): Mode Conversion and De-Excitation — •Annika Kalus, Simone Wall, Sung Sakong, Tim Frigge, Anja Hanisch-Blicharski, Peter Kratzer, Martin Kammler, and Michael Horn-von Hoegen — Universität Duisburg-Essen

Ultrafast time resolved reflection high energy electron diffraction was used to study the vibrational dynamics of adsorbate layers upon excitation with a fs laser pulse in a pump probe setup. We employed the Debye-Waller effect I=I₀exp(−⟨(uk)²⟩) to follow the temporal evolution of the vibrational amplitude u of the adsorbate atoms. The SIC(√3×√3) reconstruction of Pb on Si(111) was used as model system and excited by 50 fs, 800 nm laserpulses at a fluence of 4 mJ/cm². We observe two time constants of 100 ps and 3 ns for the vibrational de-excitation. With the k dependence of the Debye-Waller effect we identified the fast decay as a mode with a dominant parallel amplitude while the slow mode exhibits a dominant vertical amplitude. Both time constants can be explained with classical molecular dynamics simulations. The electronic and phononic band structures of the substrate plus adsorbate layer were calculated using density functional theory. The hot electrons created by the laser pulse initially excite solely optical phonons. These parallel modes convert on a 100 ps time scale to modes perpendicular to the surface that dissipate their energy into the Si substrate on a ns time scale. This surprisingly long lifetime is explained by the small overlap between the low-energy vertical Pb modes with an energy of 2 - 4 meV and the Si acoustic phonons.