15:00-16:00
Ultrafast Electron Dynamics in Quantum Well States of Pb/Si(111) Observed by Time-resolved Photoemission
Patrick S. Kirchmann(Department of Physics, Free University Berlin)
- The advent of nanotechnology calls for a comprehensive understanding
of finite size effects such as the energy relaxation of excited
electrons in 2D quantum well states (QWS) of ultrathin metal films.
Angle-resolved photoemission [1] reveals occupied QWS in ultrathin Pb
(111) films grown epitaxially on Cu(111) which are characterized by a
nearly free electron band structure where the quantized 6pz-Pb band
is well described by the effective electron mass of bulk Pb. In
contrast, Pb epitaxially grown on √3x√3 reconstructed Pb/
Si(111) exhibits almost non-dispersing QWS [2] indicating increased
electron localization parallel to the surface.
We employed femtosecond time-resolved two photoemission spectroscopy
(2PPE) using tunable laser pulses in relaxation of optically excited
electrons in unoccupied QWS of Pb/Si(111). The band gap of Si
decouples the QWS from the substrate, which creates the interesting
situation where some QWS fall into the band gaph showing a decreased
decay rate while other QWS above the gap decay much faster. We
performed 2PPE utilizing IR pump pulses, insufficient for optical
excitation across the Si band gap, to separate the influence of the
electronic excitation of the Si subtrate from the energy relaxation
exclusive to the QWS. The difference in decay rates observed upon
visible and infrared excitation will be discussed.
[1] J.H. Dil, H.W. Kim, S. Gokhale, M. Tallarida, K. Horn, Phys. Rev. B70, 045405 (2004)
[2] J.H. Dil, J.W. Kim, Th. Kampen, K. Horn, A. R. H.F. Etterna, Phys. Rev. B73, 161308 (R) (2006).
