@misc{SchickEckertPontiusetal.2016, author = {Schick, Daniel and Eckert, Sebastian and Pontius, Niko and Mitzner, Rolf and F{\"o}hlisch, Alexander and Holldack, Karsten and Sorgenfrei, Florian}, title = {Versatile soft X-ray-optical cross-correlator for ultrafast applications}, series = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Zweitver{\"o}ffentlichungen der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {1331}, issn = {1866-8372}, doi = {10.25932/publishup-43696}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-436962}, pages = {054304-1 -- 054304-8}, year = {2016}, abstract = {We present an X-ray-optical cross-correlator for the soft (> 150 eV) up to the hard X-ray regime based on a molybdenum-silicon superlattice. The cross-correlation is done by probing intensity and position changes of superlattice Bragg peaks caused by photoexcitation of coherent phonons. This approach is applicable for a wide range of X-ray photon energies as well as for a broad range of excitation wavelengths and requires no external fields or changes of temperature. Moreover, the cross-correlator can be employed on a 10 ps or 100 fs time scale featuring up to 50\% total X-ray reflectivity and transient signal changes of more than 20\%. (C) 2016 Author(s).}, language = {en} } @article{GiangrisostomiOvsyannikovSorgenfreietal.2018, author = {Giangrisostomi, Erika and Ovsyannikov, Ruslan and Sorgenfrei, Florian and Zhang, Teng and Lindblad, Andreas and Sassa, Yasmine and Cappel, Ute B. and Leitner, Torsten and Mitzner, Rolf and Svensson, Svante and Martensson, Nils and F{\"o}hlisch, Alexander}, title = {Low Dose Photoelectron Spectroscopy at BESSY II}, series = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, volume = {224}, journal = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0368-2048}, doi = {10.1016/j.elspec.2017.05.011}, pages = {68 -- 78}, year = {2018}, abstract = {The implementation of a high-transmission, angular-resolved time-of-Right electron spectrometer with a 1.25 MHz pulse selector at the PM4 soft X-ray dipole beamline of the synchrotron BESSY II creates unique capabilities to inquire electronic structure via photoelectron spectroscopy with a minimum of radiation dose. Solid-state samples can be prepared and characterized with standard UHV techniques and rapidly transferred from various preparation chambers to a 4-axis temperature-controlled measurement stage. A synchronized MHz laser system enables excited-state characterization and dynamical studies starting from the picosecond timescale. This article introduces the principal characteristics of the PM4 beamline and LowDosePES end-station. Recent results from graphene, an organic hole transport material for solar cells and the transition metal dichalcogenide MoS2 are presented to demonstrate the instrument performances.}, language = {en} } @article{PontiusBeyeTrabantetal.2018, author = {Pontius, Niko and Beye, Martin and Trabant, Christoph and Mitzner, Rolf and Sorgenfrei, Florian and Kachel, Torsten and Woestmann, Michael and Roling, Sebastian and Zacharias, Helmut and Ivanov, Rosen and Treusch, Rolf and Buchholz, Marcel and Metcalf, Pete and Schuessler-Langeheine, Christian and F{\"o}hlisch, Alexander}, title = {Probing the non-equilibrium transient state in magnetite by a jitter-free two-color X-ray pump and X-ray probe experiment}, series = {Structural dynamics}, volume = {5}, journal = {Structural dynamics}, number = {5}, publisher = {American Institute of Physics}, address = {Melville}, issn = {2329-7778}, doi = {10.1063/1.5042847}, pages = {8}, year = {2018}, abstract = {We present a general experimental concept for jitter-free pump and probe experiments at free electron lasers. By generating pump and probe pulse from one and the same X-ray pulse using an optical split-and-delay unit, we obtain a temporal resolution that is limited only by the X-ray pulse lengths. In a two-color X-ray pump and X-ray probe experiment with sub 70 fs temporal resolution, we selectively probe the response of orbital and charge degree of freedom in the prototypical functional oxide magnetite after photoexcitation. We find electronic order to be quenched on a time scale of (30 +/- 30) fs and hence most likely faster than what is to be expected for any lattice dynamics. Our experimental result hints to the formation of a short lived transient state with decoupled electronic and lattice degree of freedom in magnetite. The excitation and relaxation mechanism for X-ray pumping is discussed within a simple model leading to the conclusion that within the first 10 fs the original photoexcitation decays into low-energy electronic excitations comparable to what is achieved by optical pump pulse excitation. Our findings show on which time scales dynamical decoupling of degrees of freedom in functional oxides can be expected and how to probe this selectively with soft X-ray pulses. Results can be expected to provide crucial information for theories for ultrafast behavior of materials and help to develop concepts for novel switching devices. (C) 2018 Author(s).}, language = {en} } @article{KuehnMuellerSorgenfreietal.2019, author = {K{\"u}hn, Danilo and M{\"u}ller, Moritz and Sorgenfrei, Florian and Giangrisostomi, Erika and Jay, Raphael Martin and Ovsyannikov, Ruslan and Martensson, Nils and Sanchez-Portal, Daniel and F{\"o}hlisch, Alexander}, title = {Directional sub-femtosecond charge transfer dynamics and the dimensionality of 1T-TaS2}, series = {Scientific reports}, volume = {9}, journal = {Scientific reports}, number = {488}, publisher = {Nature Publ. Group}, address = {London}, issn = {2045-2322}, doi = {10.1038/s41598-018-36637-0}, pages = {9}, year = {2019}, abstract = {For the layered transition metal dichalcogenide 1T-TaS2, we establish through a unique experimental approach and density functional theory, how ultrafast charge transfer in 1T-TaS2 takes on isotropic three-dimensional character or anisotropic two-dimensional character, depending on the commensurability of the charge density wave phases of 1T-TaS2. The X-ray spectroscopic core-hole-clock method prepares selectively in-and out-of-plane polarized sulfur 3p orbital occupation with respect to the 1T-TaS2 planes and monitors sub-femtosecond wave packet delocalization. Despite being a prototypical two-dimensional material, isotropic three-dimensional charge transfer is found in the commensurate charge density wave phase (CCDW), indicating strong coupling between layers. In contrast, anisotropic two-dimensional charge transfer occurs for the nearly commensurate phase (NCDW). In direct comparison, theory shows that interlayer interaction in the CCDW phase - not layer stacking variations - causes isotropic three-dimensional charge transfer. This is presumably a general mechanism for phase transitions and tailored properties of dichalcogenides with charge density waves.}, language = {en} } @article{KuehnGiangrisostomiJayetal.2019, author = {K{\"u}hn, Danilo and Giangrisostomi, Erika and Jay, Raphael Martin and Sorgenfrei, Florian and F{\"o}hlisch, Alexander}, title = {The influence of x-ray pulse length on space-charge effects in optical pump/x-ray probe photoemission}, series = {New journal of physics : the open-access journal for physics}, volume = {21}, journal = {New journal of physics : the open-access journal for physics}, publisher = {IOP Publ. Ltd.}, address = {Bristol}, issn = {1367-2630}, doi = {10.1088/1367-2630/ab2f5c}, pages = {12}, year = {2019}, abstract = {Pump-probe photoelectron spectroscopy (PES) is a versatile tool to investigate the dynamics of transient states of excited matter. Vacuum space-charge effects can mask these dynamics and complicate the interpretation of electron spectra. Here we report on space-charge effects in Au 4f photoemission from a polycrystalline gold surface, excited with moderately intense 90 ps (FWHM) soft x-ray probe pulses, under the influence of the Coulomb forces exerted by a pump electron cloud, which was produced by intense 40 fs laser pulses. The experimentally observed kinetic energy shift and spectral broadening of the Au 4f lines, measured with highly-efficient time-of-flight spectroscopy, are in good agreement with simulations utilizing a mean-field model of the electrostatic pump electron potential. This confirms that the line broadening is predominantly caused by variations in the take-off time of the probe electrons without appreciable influence of local scattering events. Our findings might be of general interest for pump-probe PES with picosecond-pulse-length sources.}, language = {en} } @article{FondellEckertJayetal.2017, author = {Fondell, Mattis and Eckert, Sebastian and Jay, Raphael Martin and Weniger, Christian and Quevedo, Wilson and Niskanen, Johannes and Kennedy, Brian and Sorgenfrei, Florian and Schick, Daniel and Giangrisostomi, Erika and Ovsyannikov, Ruslan and Adamczyk, Katrin and Huse, Nils and Wernet, Philippe and Mitzner, Rolf and F{\"o}hlisch, Alexander}, title = {Time-resolved soft X-ray absorption spectroscopy in transmission mode on liquids at MHz repetition rates}, series = {Structural dynamics}, volume = {4}, journal = {Structural dynamics}, publisher = {American Institute of Physics}, address = {Melville}, issn = {2329-7778}, doi = {10.1063/1.4993755}, pages = {11}, year = {2017}, abstract = {We present a setup combining a liquid flatjet sample delivery and a MHz laser system for time-resolved soft X-ray absorption measurements of liquid samples at the high brilliance undulator beamline UE52-SGM at Bessy II yielding unprecedented statistics in this spectral range. We demonstrate that the efficient detection of transient absorption changes in transmission mode enables the identification of photoexcited species in dilute samples. With iron(II)-trisbipyridine in aqueous solution as a benchmark system, we present absorption measurements at various edges in the soft X-ray regime. In combination with the wavelength tunability of the laser system, the set-up opens up opportunities to study the photochemistry of many systems at low concentrations, relevant to materials sciences, chemistry, and biology. (C) 2017 Author(s).}, language = {en} } @article{SchickEckertPontiusetal.2016, author = {Schick, Daniel and Eckert, Sebastian and Pontius, Niko and Mitzner, Rolf and F{\"o}hlisch, Alexander and Holldack, Karsten and Sorgenfrei, Florian}, title = {Versatile soft X-ray-optical cross-correlator for ultrafast applications}, series = {Structural dynamics}, volume = {3}, journal = {Structural dynamics}, publisher = {American Institute of Physics}, address = {Melville}, issn = {2329-7778}, doi = {10.1063/1.4964296}, pages = {054304-1 -- 054304-8}, year = {2016}, abstract = {We present an X-ray-optical cross-correlator for the soft (> 150 eV) up to the hard X-ray regime based on a molybdenum-silicon superlattice. The cross-correlation is done by probing intensity and position changes of superlattice Bragg peaks caused by photoexcitation of coherent phonons. This approach is applicable for a wide range of X-ray photon energies as well as for a broad range of excitation wavelengths and requires no external fields or changes of temperature. Moreover, the cross-correlator can be employed on a 10 ps or 100 fs time scale featuring up to 50\% total X-ray reflectivity and transient signal changes of more than 20\%. (C) 2016 Author(s).}, language = {en} } @article{Dell'AngelaAnniyevBeyeetal.2015, author = {Dell'Angela, Martina and Anniyev, Toyli and Beye, Martin and Coffee, Ryan and F{\"o}hlisch, Alexander and Gladh, J{\"o}rgen and Kaya, Sarp and Katayama, Tetsuo and Krupin, Oleg and Nilsson, Anders and Nordlund, Dennis and Schlotter, William F. and Sellberg, Jonas A. and Sorgenfrei, Florian and Turner, Joshua J. and {\"O}str{\"O}m, Henrik and Ogasawara, Hirohito and Wolf, Martin and Wurth, Wilfried}, title = {Vacuum space charge effects in sub-picosecond soft X-ray photoemission on a molecular adsorbate layer}, series = {Structural dynamics}, volume = {2}, journal = {Structural dynamics}, number = {2}, publisher = {American Institute of Physics}, address = {Melville}, issn = {2329-7778}, doi = {10.1063/1.4914892}, pages = {10}, year = {2015}, abstract = {Vacuum space charge induced kinetic energy shifts of O 1s and Ru 3d core levels in femtosecond soft X-ray photoemission spectra (PES) have been studied at a free electron laser (FEL) for an oxygen layer on Ru(0001). We fully reproduced the measurements by simulating the in-vacuum expansion of the photoelectrons and demonstrate the space charge contribution of the high-order harmonics in the FEL beam. Employing the same analysis for 400 nm pump-X-ray probe PES, we can disentangle the delay dependent Ru 3d energy shifts into effects induced by space charge and by lattice heating from the femtosecond pump pulse. (C) 2015 Author(s).}, language = {en} } @article{ObergGladhAnniyevetal.2015, author = {Oberg, H. and Gladh, J{\"o}rgen and Anniyev, Toyli and Beye, Martin and Coffee, Ryan and F{\"o}hlisch, Alexander and Katayama, T. and Kaya, Sarp and LaRue, Jerry and Mogelhoj, Andreas and Nordlund, Dennis and Ogasawara, Hirohito and Schlotter, William F. and Sellberg, Jonas A. and Sorgenfrei, Florian and Turner, Joshua J. and Wolf, Martin and Wurth, W. and Ostrom, Henrik and Nilsson, Anders and Norskov, Jens K. and Pettersson, Lars G. M.}, title = {Optical laser-induced CO desorption from Ru(0001) monitored with a free-electron X-ray laser: DFT prediction and X-ray confirmation of a precursor state}, series = {Surface science}, volume = {640}, journal = {Surface science}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0039-6028}, doi = {10.1016/j.susc.2015.03.011}, pages = {80 -- 88}, year = {2015}, abstract = {We present density functional theory modeling of time-resolved optical pump/X-ray spectroscopic probe data of CO desorption from Ru(0001). The BEEF van der Waals functional predicts a weakly bound state as a precursor to desorption. The optical pump leads to a near-instantaneous (<100 fs) increase of the electronic temperature to nearly 7000 K. The temperature evolution and energy transfer between electrons, substrate phonons and adsorbate is described by the two-temperature model and found to equilibrate on a timescale of a few picoseconds to an elevated local temperature of similar to 2000K. Estimating the free energy based on the computed potential of mean force along the desorption path, we find an entropic barrier to desorption (and by time-reversal also to adsorption). This entropic barrier separates the chemisorbed and precursor states, and becomes significant at the elevated temperature of the experiment (similar to 1.4 eV at 2000 K). Experimental pump-probe X-ray absorption/X-ray emission spectroscopy indicates population of a precursor state to desorption upon laser-excitation of the system (Dell'Angela et al., 2013). Computing spectra along the desorption path confirms the picture of a weakly bound transient state arising from ultrafast heating of the metal substrate. (C) 2015 Elsevier B.V. All rights reserved.}, language = {en} } @article{PontiusKachelSchuesslerLangeheineetal.2011, author = {Pontius, N. and Kachel, T. and Sch{\"u}ssler-Langeheine, C. and Schlotter, W. F. and Beye, Martin and Sorgenfrei, Florian and Chang, C. F. and F{\"o}hlisch, Alexander and Wurth, W. and Metcalf, P. and Leonov, I. and Yaresko, A. and Stojanovic, N. and Berglund, Martin and Guerassimova, N. and Duesterer, S. and Redlin, H. and Duerr, H. A.}, title = {Time-resolved resonant soft x-ray diffraction with free-electron lasers femtosecond dynamics across the Verwey transition in magnetite}, series = {Applied physics letters}, volume = {98}, journal = {Applied physics letters}, number = {18}, publisher = {American Institute of Physics}, address = {Melville}, issn = {0003-6951}, doi = {10.1063/1.3584855}, pages = {3}, year = {2011}, abstract = {Resonant soft x-ray diffraction (RSXD) with femtosecond (fs) time resolution is a powerful tool for disentangling the interplay between different degrees of freedom in strongly correlated electron materials. It allows addressing the coupling of particular degrees of freedom upon an external selective perturbation, e. g., by an optical or infrared laser pulse. Here, we report a time-resolved RSXD experiment from the prototypical correlated electron material magnetite using soft x-ray pulses from the free-electron laser FLASH in Hamburg. We observe ultrafast melting of the charge-orbital order leading to the formation of a transient phase, which has not been observed in equilibrium.}, language = {en} } @article{Dell'AngelaAnniyevBeyeetal.2013, author = {Dell'Angela, M. and Anniyev, Toyli and Beye, Martin and Coffee, Ryan and F{\"o}hlisch, Alexander and Gladh, J. and Katayama, T. and Kaya, S. and Krupin, O. and LaRue, J. and Mogelhoj, A. and Nordlund, D. and Norskov, J. K. and Oberg, H. and Ogasawara, H. and Ostrom, H. and Pettersson, Lars G. M. and Schlotter, W. F. and Sellberg, J. A. and Sorgenfrei, Florian and Turner, J. J. and Wolf, M. and Wurth, W. and Nilsson, A.}, title = {Real-time observation of surface bond breaking with an X-ray Laser}, series = {Science}, volume = {339}, journal = {Science}, number = {6125}, publisher = {American Assoc. for the Advancement of Science}, address = {Washington}, issn = {0036-8075}, doi = {10.1126/science.1231711}, pages = {1302 -- 1305}, year = {2013}, abstract = {We used the Linac Coherent Light Source free-electron x-ray laser to probe the electronic structure of CO molecules as their chemisorption state on Ru(0001) changes upon exciting the substrate by using a femtosecond optical laser pulse. We observed electronic structure changes that are consistent with a weakening of the CO interaction with the substrate but without notable desorption. A large fraction of the molecules (30\%) was trapped in a transient precursor state that would precede desorption. We calculated the free energy of the molecule as a function of the desorption reaction coordinate using density functional theory, including van der Waals interactions. Two distinct adsorption wells-chemisorbed and precursor state separated by an entropy barrier-explain the anomalously high prefactors often observed in desorption of molecules from metals.}, language = {en} } @article{KatayamaAnniyevBeyeetal.2013, author = {Katayama, T. and Anniyev, Toyli and Beye, Martin and Coffee, Ryan and Dell'Angela, M. and F{\"o}hlisch, Alexander and Gladh, J. and Kaya, S. and Krupin, O. and Nilsson, A. and Nordlund, D. and Schlotter, W. F. and Sellberg, J. A. and Sorgenfrei, Florian and Turner, J. J. and Wurth, W. and {\"O}str{\"o}m, H. and Ogasawara, H.}, title = {Ultrafast soft X-ray emission spectroscopy of surface adsorbates using an X-ray free electron laser}, series = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, volume = {187}, journal = {Journal of electron spectroscopy and related phenomena : the international journal on theoretical and experimental aspects of electron spectroscopy}, number = {1}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0368-2048}, doi = {10.1016/j.elspec.2013.03.006}, pages = {9 -- 14}, year = {2013}, abstract = {We report on an experimental system designed to probe chemical reactions on solid surfaces on a sub-picosecond timescale using soft X-ray emission spectroscopy at the Linac Coherent Light Source (LCLS) free electron laser (FEL) at the SLAC National Accelerator Laboratory. We analyzed the O 1s X-ray emission spectra recorded from atomic oxygen adsorbed on a Ru(0001) surface at a synchrotron beamline (SSRL, BL13-2) and an FEL beamline (LCLS, SXR). We have demonstrated conditions that provide negligible amount of FEL induced damage of the sample. In addition we show that the setup is capable of tracking the temporal evolution of electronic structure during a surface reaction of submonolayer quantities of CO molecules desorbing from the surface.}, language = {en} } @article{BeyeAnniyevCoffeeetal.2013, author = {Beye, Martin and Anniyev, Toyli and Coffee, Ryan and Dell'Angela, Martina and F{\"o}hlisch, Alexander and Gladh, J. and Katayama, T. and Kaya, S. and Krupin, O. and Mogelhoj, A. and Nilsson, A. and Nordlund, D. and Norskov, J. K. and Oberg, H. and Ogasawara, H. and Pettersson, Lars G. M. and Schlotter, W. F. and Sellberg, J. A. and Sorgenfrei, Florian and Turner, J. J. and Wolf, M. and Wurth, Wilfried and Ostrom, H.}, title = {Selective ultrafast probing of transient hot chemisorbed and precursor States of CO on Ru(0001)}, series = {Physical review letters}, volume = {110}, journal = {Physical review letters}, number = {18}, publisher = {American Physical Society}, address = {College Park}, issn = {0031-9007}, doi = {10.1103/PhysRevLett.110.186101}, pages = {6}, year = {2013}, abstract = {We have studied the femtosecond dynamics following optical laser excitation of CO adsorbed on a Ru surface by monitoring changes in the occupied and unoccupied electronic structure using ultrafast soft x-ray absorption and emission. We recently reported [M. Dell'Angela et al. Science 339, 1302 (2013)] a phonon-mediated transition into a weakly adsorbed precursor state occurring on a time scale of >2 ps prior to desorption. Here we focus on processes within the first picosecond after laser excitation and show that the metal-adsorbate coordination is initially increased due to hot-electron-driven vibrational excitations. This process is faster than, but occurs in parallel with, the transition into the precursor state. With resonant x-ray emission spectroscopy, we probe each of these states selectively and determine the respective transient populations depending on optical laser fluence. Ab initio molecular dynamics simulations of CO adsorbed on Ru(0001) were performed at 1500 and 3000 K providing insight into the desorption process.}, language = {en} } @article{BeyeSchreckSorgenfreietal.2013, author = {Beye, Martin and Schreck, S. and Sorgenfrei, Florian and Trabant, C. and Pontius, N. and Sch{\"u}ßler-Langeheine, C. and Wurth, W. and F{\"o}hlisch, Alexander}, title = {Stimulated X-ray emission for materials science}, series = {Nature : the international weekly journal of science}, volume = {501}, journal = {Nature : the international weekly journal of science}, number = {7466}, publisher = {Nature Publ. Group}, address = {London}, issn = {0028-0836}, doi = {10.1038/nature12449}, pages = {191 -- +}, year = {2013}, abstract = {Resonant inelastic X-ray scattering and X-ray emission spectroscopy can be used to probe the energy and dispersion of the elementary low-energy excitations that govern functionality in matter: vibronic, charge, spin and orbital excitations(1-7). A key drawback of resonant inelastic X-ray scattering has been the need for high photon densities to compensate for fluorescence yields of less than a per cent for soft X-rays(8). Sample damage from the dominant non-radiative decays thus limits the materials to which such techniques can be applied and the spectral resolution that can be obtained. A means of improving the yield is therefore highly desirable. Here we demonstrate stimulated X-ray emission for crystalline silicon at photon densities that are easily achievable with free-electron lasers(9). The stimulated radiative decay of core excited species at the expense of non-radiative processes reduces sample damage and permits narrow-bandwidth detection in the directed beam of stimulated radiation. We deduce how stimulated X-ray emission can be enhanced by several orders of magnitude to provide, with high yield and reduced sample damage, a superior probe for low-energy excitations and their dispersion in matter. This is the first step to bringing nonlinear X-ray physics in the condensed phase from theory(10-16) to application.}, language = {en} } @article{deJongKukrejaTrabantetal.2013, author = {de Jong, S. and Kukreja, R. and Trabant, C. and Pontius, N. and Chang, C. F. and Kachel, T. and Beye, Martin and Sorgenfrei, Florian and Back, C. H. and Braeuer, B. and Schlotter, W. F. and Turner, J. J. and Krupin, O. and Doehler, M. and Zhu, D. and Hossain, M. A. and Scherz, A. O. and Fausti, D. and Novelli, F. and Esposito, M. and Lee, W. S. and Chuang, Y. D. and Lu, D. H. and Moore, R. G. and Yi, M. and Trigo, M. and Kirchmann, P. and Pathey, L. and Golden, M. S. and Buchholz, Marcel and Metcalf, P. and Parmigiani, F. and Wurth, W. and F{\"o}hlisch, Alexander and Schuessler-Langeheine, Christian and Duerr, H. A.}, title = {Speed limit of the insulator-metal transition in magnetite}, series = {Nature materials}, volume = {12}, journal = {Nature materials}, number = {10}, publisher = {Nature Publ. Group}, address = {London}, issn = {1476-1122}, doi = {10.1038/NMAT3718}, pages = {882 -- 886}, year = {2013}, abstract = {As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown(1), magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator-metal, or Verwey, transition has long remained inaccessible(2-8). Recently, three- Fe- site lattice distortions called trimeronswere identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase(9). Here we investigate the Verwey transition with pump- probe X- ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator-metal transition. We find this to be a two- step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5 +/- 0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics(10).}, language = {en} }