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In this contribution I will discuss picosecond time-resolved photoemission experiments performed at the Berlin synchrotron facility (BESSY). A regenerative Ti:sapphire amplifier delivers micro-Joule pump pulses synchronized to the BESSY master clock. The excited state dynamics is probed by EUV and soft X-ray photoemission. Photoelectrons are detected after a hemispherical analyzer and gated according to their time-of-flight. The time resolution of the experiment is limited by the synchrotron bunch-length to 50 ps or 10 ps in normal or low-alpha mode, respectively.
With this pump-probe setup we have studied the response of the buckled surface dimers on (100)-silicon to the optically altered electron population in the dangling-bond bands. The lifetime of hot carriers in the surface conduction-band of several tens of picoseconds exceeds far beyond initial equilibration of electron and lattice temperatures of 4 ps. Time-dependent density functional theory reveals that while coherent phonons have for a long time died out, the surface potential is still modified by the altered population. Changes of the dimer buckling-angle manifest in a transient broadening of the silicon 2p surface core-level components, which resembles the population dynamics of the hot surface carriers. As a second example we will discuss a time-resolved study of the magnetic linear dichroism of the Gadolinium 4f core-level. Upon optical excitation of the 5d6s valence electrons the magnetic order in the 4f spin-system is reduced. Remarkably, the linear dichroism remains at 80% of the equilibrium contrast while the lattice temperature reaches the Curie temperature due to electron-phonon scattering. Contrasting itinerant ferromagnets, this shows that equilibration between the lattice and spin subsystems takes in Gd about 80 ps and is established in parallel with heat diffusion. Experiments were performed at the MBI-BESSY beamline in collaboration with U. Bovensiepen, C. Gahl, T. Gießel, M. Lisowski, A. Melnikov, H. Prima-Garcia, R. Schmidt, and R. Weber. Calculations were done by N. M. Bulgakova, J. van Heys, and E. Pehlke. |
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