Berlin 2008 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 11: Time-Resolved Spectroscopy I

O 11.5: Vortrag

Montag, 25. Februar 2008, 14:30–14:45, MA 043

Ultrafast electron dynamics in Pb/Si(111) investigated by two-photon photoemission — •Patrick S. Kirchmann, Martin Wolf, and Uwe Bovensiepen — Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin-Dahlem

We studied the ultrafast electron relaxation dynamics of hot electrons in quantum well states (QWS) in ultrathin epitaxial Pb films on Si(111) [1] by femtosecond time-resolved two-photon photoemission spectroscopy.

Up to four unoccupied QWSs are identified, which exhibit a bi-exponential decay of the hot electron population. The slower decay is assigned to a delayed filling of the metallic QWS in the Pb adlayer by scattering from electronic states of the optically excited Si(111) substrate. The faster decay is assigned to e-e scattering within the Pb film. The overall trend of the extracted decay rates is governed by Fermi liquid theory. However, a detailed analysis reveals a well resolved dip in the decay rate which occurs precisely at the binding energy of the band bottom of the first unoccupied QWS. This local minimum of the decay rate is assigned to intra-subband scattering within the Pb film.

Thus, for a comprehensive description of the electron decay in a two-dimensional metal film not only the electron density and screening parameters as in Fermi liquid theory have to be considered. Here, we show that also the electron scattering processes in the quantized band structure have to be taken explicitly into account.

[1] P. S. Kirchmann et al., Phys. Rev. B 76, 075406 (2007)