Dresden 2011 – scientific programme

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O: Fachverband Oberflächenphysik

O 43: Surface Dynamics II

O 43.4: Talk

Wednesday, March 16, 2011, 12:00–12:15, WIL B122

How Fast Does a Lead Monolayer Cool? — •Annika Kalus, Simone Möllenbeck, Sung Sakong, Anja Hanisch-Blicharski, Martin Kammler, Peter Kratzer, and Michael Horn-von Hoegen — Department of Physics and Center for Nanointegration Duisburg-Essen (CeNIDE), Universität Duisburg-Essen, D-47057 Duisburg, Germany

We employ ultra fast time-resolved electron diffraction to analyze the vibrational dynamics of adsorbates on surfaces on a ps-timescale upon excitation by a fs laser pulse. Surface sensitivity is achieved by a reflection geometry of high energy electrons (RHEED) and time resolution by a pump probe setup.

Two different reconstructions of Pb on Si were prepared: the (√3×√3) reconstruction with a coverage of 4/3 monolayers (ML) and the (√7×√3) reconstruction with a coverage of 1.2 ML. All experiments were performed at 90K. Upon excitation with 50fs laser pulses at 800nm both reconstructions show a sharp drop of intensity in all reconstruction spots due to the Debye-Waller effect. The intensity reversibly recovers on two different time scales. The short time constant of 100ps is attributed to the deexcitation of the Si-Pb mode with an energy of 4meV and is in good agreement with molecular dynamic simulations. The long time constant of 3ns is assigned to a low frequency Pb-Pb mode with an energy of 2meV. The weak coupling of this mode to the Si substrate is explained by the small overlap of this mode with the density of phonon states in Si. The vibrational amplitude of these excited modes shows a strong non-linear dependence on the laser fluence.