title: Time-resolved observation of band-gap shrinking and electron-lattice thermalization within X-ray excited gallium arsenide.
abstract: Femtosecond X-ray irradiation of solids excites energetic photoelectrons that thermalize on a timescale of a few hundred femtoseconds. The thermalized electrons exchange energy with the lattice and heat it up. Experiments with X-ray free-electron lasers have unveiled so far the details of the electronic thermalization. In this work we show that the data on transient optical reflectivity measured in GaAs irradiated with femtosecond X-ray pulses can be used to follow electron-lattice relaxation up to a few tens of picoseconds. With a dedicated theoretical framework, we explain the so far unexplained reflectivity overshooting as a result of band-gap shrinking. We also obtain predictions for a timescale of electron-lattice thermalization, initiated by conduction band electrons in the temperature regime of a few eVs. The conduction and valence band carriers were then strongly non-isothermal. The presented scheme is of general applicability and can stimulate further studies of relaxation within X-ray excited narrow band-gap semiconductors.
authors.affiliation: Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany.|Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152, 31-342 Kraków, Poland.
authors.forename: Beata
authors.initials: B
authors.lastname: Ziaja
authors.affiliation: Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany.
authors.forename: Nikita
authors.initials: N
authors.lastname: Medvedev
authors.affiliation: Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany.
authors.forename: Victor
authors.initials: V
authors.lastname: Tkachenko
authors.affiliation: Department of Physics and Center for Free-Electron Laser Science CFEL, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.
authors.forename: Theophilos
authors.initials: T
authors.lastname: Maltezopoulos
authors.affiliation: Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany.|Department of Physics and Center for Free-Electron Laser Science CFEL, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany.|DESY Photon Science, Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany.
authors.forename: Wilfried
authors.initials: W
authors.lastname: Wurth