Dresden 2009 – scientific programme

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O: Fachverband Oberflächenphysik

O 55: Time-resolved spectroscopies II

O 55.4: Talk

Thursday, March 26, 2009, 15:45–16:00, SCH A315

Ultrafast dynamics of occupied quantum well states in Pb/Si(111) — •Laurenz Rettig, Patrick S. Kirchmann, Martin Wolf, and Uwe Bovensiepen — Freie Universität Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin

We investigated the ultrafast response of occupied quantum well states (QWS) in the two-dimensional model system Pb/Si(111) to an intense optical excitation using time-resolved photoemission spectroscopy (trPES). Employing pump photon energies of 1.5 eV and absorbed fluences up to 200 µJ/cm² a significant part of the valence electrons can be excited. The effect on the QWS is probed with 6.0 eV photon energy. We find a pronounced shift of up to 40 meV of the binding energy of the highest occupied QWS to higher binding energies, i.e. an energetic stabilization. A detailed analysis reveals that this shift is established within the laser pulse duration of ∼80 fs where most of the excess energy resides in the electronic system. Therefore, this gain in binding energy can be attributed to an electronic effect. The shift can be explained by a transient reduction of the electronic screening of the ion core potentials due to the excitation of electrons into unoccupied parts of the discrete QWS band structure. This can lead to a spatial redistribution of the electron density within the metal film, hence reducing the electron density at the core sites. Furthermore, a careful analysis of the transient binding energy reveals a small (2 meV) periodic modulation of the binding energy at a frequency of 2.18(5) THz which we attribute to a coherent longitudinal high energy phonon excitation within the lead film.