@article{YangZhuWolfetal.2018,
  author    = {Yang, Jie and Zhu, Xiaolei and Wolf, Thomas J. A. and Li, Zheng and Nunes, Jo{\~a}o Pedro Figueira and Coffee, Ryan and Cryan, James P. and G{\"u}hr, Markus and Hegazy, Kareem and Heinz, Tony F. and Jobe, Keith and Li, Renkai and Shen, Xiaozhe and Veccione, Theodore and Weathersby, Stephen and Wilkin, Kyle J. and Yoneda, Charles and Zheng, Qiang and Martinez, Todd J. and Centurion, Martin and Wang, Xijie},
  title     = {Imaging CF3I conical intersection and photodissociation dynamics with ultrafast electron diffraction},
  series = {Science},
  volume    = {361},
  journal   = {Science},
  number    = {6397},
  publisher = {American Assoc. for the Advancement of Science},
  address   = {Washington},
  issn      = {0036-8075},
  doi       = {10.1126/science.aat0049},
  pages     = {64 -- 67},
  year      = {2018},
  abstract  = {Conical intersections play a critical role in excited-state dynamics of polyatomic molecules because they govern the reaction pathways of many nonadiabatic processes. However, ultrafast probes have lacked sufficient spatial resolution to image wave-packet trajectories through these intersections directly. Here, we present the simultaneous experimental characterization of one-photon and two-photon excitation channels in isolated CF3I molecules using ultrafast gas-phase electron diffraction. In the two-photon channel, we have mapped out the real-space trajectories of a coherent nuclear wave packet, which bifurcates onto two potential energy surfaces when passing through a conical intersection. In the one-photon channel, we have resolved excitation of both the umbrella and the breathing vibrational modes in the CF3 fragment in multiple nuclear dimensions. These findings benchmark and validate ab initio nonadiabatic dynamics calculations.},
  language  = {en}
}
@article{WolfHolzmeierWagneretal.2017,
  author    = {Wolf, Thomas J. A. and Holzmeier, Fabian and Wagner, Isabella and Berrah, Nora and Bostedt, Christoph and Bozek, John and Bucksbaum, Phil and Coffee, Ryan and Cryan, James and Farrell, Joe and Feifel, Raimund and Martinez, Todd J. and McFarland, Brian and Mucke, Melanie and Nandi, Saikat and Tarantelli, Francesco and Fischer, Ingo and G{\"u}hr, Markus},
  title     = {Observing Femtosecond Fragmentation Using Ultrafast X-ray-Induced Auger Spectra},
  series = {Applied sciences},
  volume    = {7},
  journal   = {Applied sciences},
  number    = {7},
  publisher = {MDPI},
  address   = {Basel},
  issn      = {2076-3417},
  doi       = {10.3390/app7070681},
  pages     = {11},
  year      = {2017},
  abstract  = {Molecules often fragment after photoionization in the gas phase. Usually, this process can only be investigated spectroscopically as long as there exists electron correlation between the photofragments. Important parameters, like their kinetic energy after separation, cannot be investigated. We are reporting on a femtosecond time-resolved Auger electron spectroscopy study concerning the photofragmentation dynamics of thymine. We observe the appearance of clearly distinguishable signatures from thymines neutral photofragment isocyanic acid. Furthermore, we observe a time-dependent shift of its spectrum, which we can attribute to the influence of the charged fragment on the Auger electron. This allows us to map our time-dependent dataset onto the fragmentation coordinate. The time dependence of the shift supports efficient transformation of the excess energy gained from photoionization into kinetic energy of the fragments. Our method is broadly applicable to the investigation of photofragmentation processes.},
  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{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{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{WolfHolzmeierWagneretal.2017,
  author    = {Wolf, Thomas J. A. and Holzmeier, Fabian and Wagner, Isabella and Berrah, Nora and Bostedt, Christoph and Bozek, John and Bucksbaum, Philip H. and Coffee, Ryan and Cryan, James and Farrell, Joe and Feifel, Raimund and Martinez, Todd J. and McFarland, Brian and Mucke, Melanie and Nandi, Saikat and Tarantelli, Francesco and Fischer, Ingo and G{\"u}hr, Markus},
  title     = {Observing Femtosecond Fragmentation Using Ultrafast X-ray-Induced Auger Spectra},
  series = {Applied Sciences},
  volume    = {7},
  journal   = {Applied Sciences},
  number    = {7},
  publisher = {MDPI},
  address   = {Basel},
  issn      = {2076-3417},
  doi       = {10.3390/app7070681},
  year      = {2017},
  abstract  = {Molecules often fragment after photoionization in the gas phase. Usually, this process can only be investigated spectroscopically as long as there exists electron correlation between the photofragments. Important parameters, like their kinetic energy after separation, cannot be investigated. We are reporting on a femtosecond time-resolved Auger electron spectroscopy study concerning the photofragmentation dynamics of thymine. We observe the appearance of clearly distinguishable signatures from thymine′s neutral photofragment isocyanic acid. Furthermore, we observe a time-dependent shift of its spectrum, which we can attribute to the influence of the charged fragment on the Auger electron. This allows us to map our time-dependent dataset onto the fragmentation coordinate. The time dependence of the shift supports efficient transformation of the excess energy gained from photoionization into kinetic energy of the fragments. Our method is broadly applicable to the investigation of photofragmentation processes.},
  language  = {en}
}
@misc{WolfHolzmeierWagneretal.2017,
  author    = {Wolf, Thomas J. A. and Holzmeier, Fabian and Wagner, Isabella and Berrah, Nora and Bostedt, Christoph and Bozek, John and Bucksbaum, Philip H. and Coffee, Ryan and Cryan, James and Farrell, Joe and Feifel, Raimund and Martinez, Todd J. and McFarland, Brian and Mucke, Melanie and Nandi, Saikat and Tarantelli, Francesco and Fischer, Ingo and G{\"u}hr, Markus},
  title     = {Observing Femtosecond Fragmentation Using Ultrafast X-ray-Induced Auger Spectra},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-402692},
  pages     = {11},
  year      = {2017},
  abstract  = {Molecules often fragment after photoionization in the gas phase. Usually, this process can only be investigated spectroscopically as long as there exists electron correlation between the photofragments. Important parameters, like their kinetic energy after separation, cannot be investigated. We are reporting on a femtosecond time-resolved Auger electron spectroscopy study concerning the photofragmentation dynamics of thymine. We observe the appearance of clearly distinguishable signatures from thymine′s neutral photofragment isocyanic acid. Furthermore, we observe a time-dependent shift of its spectrum, which we can attribute to the influence of the charged fragment on the Auger electron. This allows us to map our time-dependent dataset onto the fragmentation coordinate. The time dependence of the shift supports efficient transformation of the excess energy gained from photoionization into kinetic energy of the fragments. Our method is broadly applicable to the investigation of photofragmentation processes.},
  language  = {en}
}
@article{YangGuehrVecchioneetal.2016,
  author    = {Yang, Jie and G{\"u}hr, Markus and Vecchione, Theodore and Robinson, Matthew Scott and Li, Renkai and Hartmann, Nick and Shen, Xiaozhe and Coffee, Ryan and Corbett, Jeff and Fry, Alan and Gaffney, Kelly and Gorkhover, Tais and Hast, Carsten and Jobe, Keith and Makasyuk, Igor and Reid, Alexander and Robinson, Joseph and Vetter, Sharon and Wang, Fenglin and Weathersby, Stephen and Yoneda, Charles and Centurion, Martin and Wang, Xijie},
  title     = {Diffractive imaging of a rotational wavepacket in nitrogen molecules with femtosecond megaelectronvolt electron pulses},
  series = {Nature Communications},
  volume    = {7},
  journal   = {Nature Communications},
  publisher = {Nature Publ. Group},
  address   = {London},
  issn      = {2041-1723},
  doi       = {10.1038/ncomms11232},
  pages     = {9},
  year      = {2016},
  abstract  = {Imaging changes in molecular geometries on their natural femtosecond timescale with sub-Angstrom spatial precision is one of the critical challenges in the chemical sciences, as the nuclear geometry changes determine the molecular reactivity. For photoexcited molecules, the nuclear dynamics determine the photoenergy conversion path and efficiency. Here we report a gas-phase electron diffraction experiment using megaelectronvolt (MeV) electrons, where we captured the rotational wavepacket dynamics of nonadiabatically laser-aligned nitrogen molecules. We achieved a combination of 100 fs root-mean-squared temporal resolution and sub-Angstrom (0.76 angstrom) spatial resolution that makes it possible to resolve the position of the nuclei within the molecule. In addition, the diffraction patterns reveal the angular distribution of the molecules, which changes from prolate (aligned) to oblate (anti-aligned) in 300 fs. Our results demonstrate a significant and promising step towards making atomically resolved movies of molecular reactions.},
  language  = {en}
}
@misc{YangGuehrVecchioneetal.2016,
  author    = {Yang, Jie and Guehr, Markus and Vecchione, Theodore and Robinson, Matthew Scott and Li, Renkai and Hartmann, Nick and Shen, Xiaozhe and Coffee, Ryan and Corbett, Jeff and Fry, Alan and Gaffney, Kelly and Gorkhover, Tais and Hast, Carsten and Jobe, Keith and Makasyuk, Igor and Reid, Alexander and Robinson, Joseph and Vetter, Sharon and Wang, Fenglin and Weathersby, Stephen and Yoneda, Charles and Wang, Xijie and Centurion, Martin},
  title     = {Femtosecond gas phase electron diffraction with MeV electrons},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-394989},
  pages     = {19},
  year      = {2016},
  abstract  = {We present results on ultrafast gas electron diffraction (UGED) experiments with femtosecond resolution using the MeV electron gun at SLAC National Accelerator Laboratory. UGED is a promising method to investigate molecular dynamics in the gas phase because electron pulses can probe the structure with a high spatial resolution. Until recently, however, it was not possible for UGED to reach the relevant timescale for the motion of the nuclei during a molecular reaction. Using MeV electron pulses has allowed us to overcome the main challenges in reaching femtosecond resolution, namely delivering short electron pulses on a gas target, overcoming the effect of velocity mismatch between pump laser pulses and the probe electron pulses, and maintaining a low timing jitter. At electron kinetic energies above 3 MeV, the velocity mismatch between laser and electron pulses becomes negligible. The relativistic electrons are also less susceptible to temporal broadening due to the Coulomb force. One of the challenges of diffraction with relativistic electrons is that the small de Broglie wavelength results in very small diffraction angles. In this paper we describe the new setup and its characterization, including capturing static diffraction patterns of molecules in the gas phase, finding time-zero with sub-picosecond accuracy and first time-resolved diffraction experiments. The new device can achieve a temporal resolution of 100 fs root-mean-square, and sub-angstrom spatial resolution. The collimation of the beam is sufficient to measure the diffraction pattern, and the transverse coherence is on the order of 2 nm. Currently, the temporal resolution is limited both by the pulse duration of the electron pulse on target and by the timing jitter, while the spatial resolution is limited by the average electron beam current and the signal-to-noise ratio of the detection system. We also discuss plans for improving both the temporal resolution and the spatial resolution.},
  language  = {en}
}
@article{YangGuehrVecchioneetal.2016,
  author    = {Yang, Jie and G{\"u}hr, Markus and Vecchione, Theodore and Robinson, Matthew Scott and Li, Renkai and Hartmann, Nick and Shen, Xiaozhe and Coffee, Ryan and Corbett, Jeff and Fry, Alan and Gaffney, Kelly and Gorkhover, Tais and Hast, Carsten and Jobe, Keith and Makasyuk, Igor and Reid, Alexander and Robinson, Joseph and Vetter, Sharon and Wang, Fenglin and Weathersby, Stephen and Yoneda, Charles and Wang, Xijie and Centurion, Martin},
  title     = {Femtosecond gas phase electron diffraction with MeV electrons},
  series = {Faraday discussions},
  volume    = {194},
  journal   = {Faraday discussions},
  publisher = {Royal Society of Chemistry},
  address   = {Cambridge},
  issn      = {1359-6640},
  doi       = {10.1039/c6fd00071a},
  pages     = {563 -- 581},
  year      = {2016},
  abstract  = {We present results on ultrafast gas electron diffraction (UGED) experiments with femtosecond resolution using the MeV electron gun at SLAC National Accelerator Laboratory. UGED is a promising method to investigate molecular dynamics in the gas phase because electron pulses can probe the structure with a high spatial resolution. Until recently, however, it was not possible for UGED to reach the relevant timescale for the motion of the nuclei during a molecular reaction. Using MeV electron pulses has allowed us to overcome the main challenges in reaching femtosecond resolution, namely delivering short electron pulses on a gas target, overcoming the effect of velocity mismatch between pump laser pulses and the probe electron pulses, and maintaining a low timing jitter. At electron kinetic energies above 3 MeV, the velocity mismatch between laser and electron pulses becomes negligible. The relativistic electrons are also less susceptible to temporal broadening due to the Coulomb force. One of the challenges of diffraction with relativistic electrons is that the small de Broglie wavelength results in very small diffraction angles. In this paper we describe the new setup and its characterization, including capturing static diffraction patterns of molecules in the gas phase, finding time-zero with sub-picosecond accuracy and first time-resolved diffraction experiments. The new device can achieve a temporal resolution of 100 fs root-mean-square, and sub-angstrom spatial resolution. The collimation of the beam is sufficient to measure the diffraction pattern, and the transverse coherence is on the order of 2 nm. Currently, the temporal resolution is limited both by the pulse duration of the electron pulse on target and by the timing jitter, while the spatial resolution is limited by the average electron beam current and the signal-to-noise ratio of the detection system. We also discuss plans for improving both the temporal resolution and the spatial resolution.},
  language  = {en}
}
@article{YangGuehrShenetal.2016,
  author    = {Yang, Jie and Guehr, Markus and Shen, Xiaozhe and Li, Renkai and Vecchione, Theodore and Coffee, Ryan and Corbett, Jeff and Fry, Alan and Hartmann, Nick and Hast, Carsten and Hegazy, Kareem and Jobe, Keith and Makasyuk, Igor and Robinson, Joseph and Robinson, Matthew Scott and Vetter, Sharon and Weathersby, Stephen and Yoneda, Charles and Wang, Xijie and Centurion, Martin},
  title     = {Diffractive Imaging of Coherent Nuclear Motion in Isolated Molecules},
  series = {Physical review letters},
  volume    = {117},
  journal   = {Physical review letters},
  publisher = {American Physical Society},
  address   = {College Park},
  issn      = {0031-9007},
  doi       = {10.1103/PhysRevLett.117.153002},
  pages     = {6},
  year      = {2016},
  abstract  = {Observing the motion of the nuclear wave packets during a molecular reaction, in both space and time, is crucial for understanding and controlling the outcome of photoinduced chemical reactions. We have imaged the motion of a vibrational wave packet in isolated iodine molecules using ultrafast electron diffraction with relativistic electrons. The time-varying interatomic distance was measured with a precision 0.07 angstrom and temporal resolution of 230 fs full width at half maximum. The method is not only sensitive to the position but also the shape of the nuclear wave packet.},
  language  = {en}
}