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Disentanglement of electron dynamics and space-charge effects in time-resolved photoemission from h-BN/Ni(111)

Date Do, 07.07.2011 - Do, 07.07.2011
Time 10.15
Speaker Dominik Leuenberger, Universität Zürich, Surface Physics Group
Location Universität Bern, Institut für Angewandte Physik, Gebäude exakte Wissenschaften, Hörsaal B116, Sidlerstrasse 5, 3012 Bern
Program We present time-resolved photoemission spectroscopy data from Ni(111) surface capped with a monolayer of hexagonal boron nitride (h-BN/Ni(111)). These data were taken using high laser fluence in order to study hot carrier and magnetization dynamics in ferromagnetic nickel covered with an insulating capping layer. In the past decade time-resolved photoemission has evolved to one of the most powerful instruments for the investigation of transient structural, electronic and magnetic responses of a condensed-matter system to the pertubation by intense femtosecond pulses. At high excitation densities a high photoelectron background severely distorts the spectra in time- resolved experiments. However after emission on their drift to the detector, all electrons interact strongly exchanging kinetic energy due to mutual Coulomb repulsion, leading to shifts and broadening of the spectral distribution, a phenomenon called space-charge effects.
Similar observations were made in experiments using undulator based 3rd generation synchrotron radiation or free- electron lasers, where in particular high-brilliance free-electron laser sources provide photons in the extreme ultraviolet up to the soft x-ray regime with extremely high pulse densities and high photoelectron yields. We develop a simple but successful model which allows one to disentangle space-charge effects from the electron underlying dynamics probed in multi-photon transitions from a model system on a femtosecond timescale. In addition we will discuss ultrafast demagnetization effects and an excitonic excitation in a single layer hexagonal boron nitride film on top of a ferromagnetic Ni(111) surface.
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