SwePub - search: WFRF:(Coffee R.)

Enumeration	Reference	Cover	Find
1.	Barty, A., et al. (author) Self-terminating diffraction gates femtosecond X-ray nanocrystallography measurements 2012 In: Nature Photonics. - 1749-4885 .- 1749-4893. ; 6:1, s. 35-40 Journal article (peer-reviewed)abstract X-ray free-electron lasers have enabled new approaches to the structural determination of protein crystals that are too small or radiation-sensitive for conventional analysis1. For sufficiently short pulses, diffraction is collected before significant changes occur to the sample, and it has been predicted that pulses as short as 10 fs may be required to acquire atomic-resolution structural information1, 2, 3, 4. Here, we describe a mechanism unique to ultrafast, ultra-intense X-ray experiments that allows structural information to be collected from crystalline samples using high radiation doses without the requirement for the pulse to terminate before the onset of sample damage. Instead, the diffracted X-rays are gated by a rapid loss of crystalline periodicity, producing apparent pulse lengths significantly shorter than the duration of the incident pulse. The shortest apparent pulse lengths occur at the highest resolution, and our measurements indicate that current X-ray free-electron laser technology5 should enable structural determination from submicrometre protein crystals with atomic resolution.
2.	Bolognesi, P., et al. (author) A study of the dynamical energy flow in uracil 2015 In: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 635 Journal article (peer-reviewed)abstract The time resolved photoionization of C 1s in uracil following excitation of the neutral molecule by 260 nm pulses has been studied at LCLS.
3.	Berrah, N., et al. (author) Femtosecond-resolved observation of the fragmentation of buckminsterfullerene following X-ray multiphoton ionization 2019 In: Nature Physics. - : Springer Science and Business Media LLC. - 1745-2473 .- 1745-2481. ; 15, s. 1279-1283 Journal article (peer-reviewed)abstract X-ray free-electron lasers have, over the past decade, opened up the possibility of understanding the ultrafast response of matter to intense X-ray pulses. In earlier research on atoms and small molecules, new aspects of this response were uncovered, such as rapid sequences of inner-shell photoionization and Auger ionization. Here, we studied a larger molecule, buckminsterfullerene (C60), exposed to 640eV X-rays, and examined the role of chemical effects, such as chemical bonds and charge transfer, on the fragmentation following multiple ionization of the molecule. To provide time resolution, we performed femtosecond-resolved X-ray pump/X-ray probe measurements, which were accompanied by advanced simulations. The simulations and experiment reveal that despite substantial ionization induced by the ultrashort (20fs) X-ray pump pulse, the fragmentation of C60 is considerably delayed. This work uncovers the persistence of the molecular structure of C60, which hinders fragmentation over a timescale of hundreds of femtoseconds. Furthermore, we demonstrate that a substantial fraction of the ejected fragments are neutral carbon atoms. These findings provide insights into X-ray free-electron laser-induced radiation damage in large molecules, including biomolecules.
4.	Hartmann, N., et al. (author) Attosecond time-energy structure of X-ray free-electron laser pulses 2018 In: Nature Photonics. - : Springer Science and Business Media LLC. - 1749-4885 .- 1749-4893. ; 12:4, s. 215-220 Journal article (peer-reviewed)abstract The time-energy information of ultrashort X-ray free-electron laser pulses generated by the Linac Coherent Light Source is measured with attosecond resolution via angular streaking of neon 1s photoelectrons. The X-ray pulses promote electrons from the neon core level into an ionization continuum, where they are dressed with the electric field of a circularly polarized infrared laser. This induces characteristic modulations of the resulting photoelectron energy and angular distribution. From these modulations we recover the single-shot attosecond intensity structure and chirp of arbitrary X-ray pulses based on self-amplified spontaneous emission, which have eluded direct measurement so far. We characterize individual attosecond pulses, including their instantaneous frequency, and identify double pulses with well-defined delays and spectral properties, thus paving the way for X-ray pump/X-ray probe attosecond free-electron laser science. © 2018 The Author(s).
5.	Heider, R., et al. (author) Megahertz-compatible angular streaking with few-femtosecond resolution at x-ray free-electron lasers 2019 In: Physical Review A. - 2469-9926. ; 100:5 Journal article (peer-reviewed)abstract Highly brilliant, coherent, femtosecond x-ray pulses delivered by free-electron lasers (FELs) constitute one of the pillars of modern ultrafast science. Next generation FEL facilities provide up to megahertz repetition rates and pulse durations down to the attosecond regime utilizing self-amplification of spontaneous emission. However, the stochastic nature of this generation mechanism demands single-shot pulse characterization to perform meaningful experiments. Here we demonstrate a fast yet robust online analysis technique capable of megahertz-rate mapping of the temporal intensity structure and arrival time of x-ray FEL pulses with few-femtosecond resolution. We performed angular streaking measurements of both neon photo- and Auger electrons and show their applicability for a direct time-domain feedback system during ongoing experiments. The fidelity of the real-time pulse characterization algorithm is corroborated by resolving isolated x-ray pulses and double pulse trains with few-femtosecond substructure, thus paving the way for x-ray-pump-x-ray-probe FEL science at repetition rates compatible with the demands of LCLS-II and European XFEL.
6.	Lomb, Lukas, et al. (author) Radiation damage in protein serial femtosecond crystallography using an x-ray free-electron laser 2011 In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 84:21, s. 214111-1-214111-6 Journal article (peer-reviewed)abstract X-ray free-electron lasers deliver intense femtosecond pulses that promise to yield high resolution diffraction data of nanocrystals before the destruction of the sample by radiation damage. Diffraction intensities of lysozyme nanocrystals collected at the Linac Coherent Light Source using 2 keV photons were used for structure determination by molecular replacement and analyzed for radiation damage as a function of pulse length and fluence. Signatures of radiation damage are observed for pulses as short as 70 fs. Parametric scaling used in conventional crystallography does not account for the observed effects.
7.	Lutman, A. A., et al. (author) Polarization control in an X-ray free-electron laser 2016 In: Nature Photonics. - : Springer Science and Business Media LLC. - 1749-4885 .- 1749-4893. ; 10:7, s. 468-472 Journal article (peer-reviewed)abstract X-ray free-electron lasers are unique sources of high-brightness coherent radiation. However, existing devices supply only linearly polarized light, precluding studies of chiral dynamics. A device called the Delta undulator has been installed at the Linac Coherent Light Source (LCLS) to provide tunable polarization. With a reverse tapered planar undulator line to pre-microbunch the beam and the novel technique of beam diverting, hundreds of microjoules of circularly polarized X-ray pulses are produced at 500-1,200 eV. These X-ray pulses are tens of femtoseconds long, have a degree of circular polarization of 0.98(+0.02)(-0.04) at 707 eV and may be scanned in energy. We also present a new two-colour X-ray pump-X-ray probe operating mode for the LCLS. Energy differences of Delta E/E = 2.4% are supported, and the second pulse can be adjusted to any elliptical polarization. In this mode, the pointing, timing, intensity and wavelength of the two pulses can be modified.
8.	Rolles, D., et al. (author) Femtosecond x-ray photoelectron diffraction on gas-phase dibromobenzene molecules 2014 In: Journal of Physics B: Atomic, Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 47:12 Journal article (peer-reviewed)abstract We present time-resolved femtosecond photoelectron momentum images and angular distributions of dissociating, laser-aligned 1,4-dibromobenzene (C6H4Br2) molecules measured in a near-infrared pump, soft-x-ray probe experiment performed at an x-ray free-electron laser. The observed alignment dependence of the bromine 2p photoelectron angular distributions is compared to density functional theory calculations and interpreted in terms of photoelectron diffraction. While no clear time-dependent effects are observed in the angular distribution of the Br(2p) photoelectrons, other, low-energy electrons show a pronounced dependence on the time delay between the near-infrared laser and the x-ray pulse.
9.	Sanchez-Gonzalez, A., et al. (author) Accurate prediction of X-ray pulse properties from a free-electron laser using machine learning 2017 In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 8 Journal article (peer-reviewed)abstract Free-electron lasers providing ultra-short high-brightness pulses of X-ray radiation have great potential for a wide impact on science, and are a critical element for unravelling the structural dynamics of matter. To fully harness this potential, we must accurately know the X-ray properties: intensity, spectrum and temporal profile. Owing to the inherent fluctuations in free-electron lasers, this mandates a full characterization of the properties for each and every pulse. While diagnostics of these properties exist, they are often invasive and many cannot operate at a high-repetition rate. Here, we present a technique for circumventing this limitation. Employing a machine learning strategy, we can accurately predict X-ray properties for every shot using only parameters that are easily recorded at high-repetition rate, by training a model on a small set of fully diagnosed pulses. This opens the door to fully realizing the promise of next-generation high-repetition rate X-ray lasers.
10.	Sanchez-Gonzalez, A., et al. (author) Auger electron and photoabsorption spectra of glycine in the vicinity of the oxygen K-edge measured with an X-FEL 2015 In: Journal of Physics B-Atomic Molecular and Optical Physics. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 48:23 Journal article (peer-reviewed)abstract We report the first measurement of the near oxygen K-edge auger spectrum of the glycine molecule. Our work employed an x-ray free electron laser as the photon source operated with input photon energies tunable between 527 and 547 eV. Complete electron spectra were recorded at each photon energy in the tuning range, revealing resonant and non-resonant auger structures. Finally ab initio theoretical predictions are compared with the measured above the edge auger spectrum and an assignment of auger decay channels is performed.
11.	Farrell, J. P., et al. (author) Ultrafast X-ray probe of nucleobase photoprotection 2012 In: Quantum Electronics and Laser Science Conference. - 9781467318396 ; , s. 6327153- Conference paper (peer-reviewed)abstract We will present first results of a UV-pump X-ray-probe study of the photoprotection mechanism of thymine. The experiment used element specific Auger spectroscopy and was carried out at the LCLS.
12.	Frasinski, L. J., et al. (author) Dynamics of Hollow Atom Formation in Intense X-Ray Pulses Probed by Partial Covariance Mapping 2013 In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 111:7, s. 073002- Journal article (peer-reviewed)abstract When exposed to ultraintense x-radiation sources such as free electron lasers (FELs) the innermost electronic shell can efficiently be emptied, creating a transient hollow atom or molecule. Understanding the femtosecond dynamics of such systems is fundamental to achieving atomic resolution in flash diffraction imaging of noncrystallized complex biological samples. We demonstrate the capacity of a correlation method called partial covariance mapping'' to probe the electron dynamics of neon atoms exposed to intense 8 fs pulses of 1062 eV photons. A complete picture of ionization processes competing in hollow atom formation and decay is visualized with unprecedented ease and the map reveals hitherto unobserved nonlinear sequences of photoionization and Auger events. The technique is particularly well suited to the high counting rate inherent in FEL experiments.
13.	Glownia, James M., et al. (author) Time-resolved pump-probe experiments at the LCLS 2010 In: Optics Express. - 1094-4087. ; 18:17, s. 17620-17630 Journal article (peer-reviewed)abstract The first time-resolved x-ray/optical pump-probe experiments at the SLAC Linac Coherent Light Source (LCLS) used a combination of feedback methods and post-analysis binning techniques to synchronize an ultrafast optical laser to the linac-based x-ray laser. Transient molecular nitrogen alignment revival features were resolved in time-dependent x-ray-induced fragmentation spectra. These alignment features were used to find the temporal overlap of the pump and probe pulses. The strong-field dissociation of x-ray generated quasi-bound molecular dications was used to establish the residual timing jitter. This analysis shows that the relative arrival time of the Ti:Sapphire laser and the x-ray pulses had a distribution with a standard deviation of approximately 120 fs. The largest contribution to the jitter noise spectrum was the locking of the laser oscillator to the reference RF of the accelerator, which suggests that simple technical improvements could reduce the jitter to better than 50 fs.
14.	Johansson, Linda C, 1983, et al. (author) Lipidic phase membrane protein serial femtosecond crystallography. 2012 In: Nature methods. - : Springer Science and Business Media LLC. - 1548-7105 .- 1548-7091. ; 9:3, s. 263-265 Journal article (peer-reviewed)abstract X-ray free electron laser (X-FEL)-based serial femtosecond crystallography is an emerging method with potential to rapidly advance the challenging field of membrane protein structural biology. Here we recorded interpretable diffraction data from micrometer-sized lipidic sponge phase crystals of the Blastochloris viridis photosynthetic reaction center delivered into an X-FEL beam using a sponge phase micro-jet.
15.	Mucke, Melanie, et al. (author) Covariance mapping of two-photon double core hole states in C2H2 and C2H6 produced by an x-ray free electron laser 2015 In: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 17 Journal article (peer-reviewed)abstract Few-photon ionization and relaxation processes in acetylene (C2H2) and ethane (C2H6) were investigated at the linac coherent light source x-ray free electron laser (FEL) at SLAC, Stanford using a highly efficient multi-particle correlation spectroscopy technique based on a magnetic bottle. The analysis method of covariance mapping has been applied and enhanced, allowing us to identify electron pairs associated with double core hole (DCH) production and competing multiple ionization processes including Auger decay sequences. The experimental technique and the analysis procedure are discussed in the light of earlier investigations of DCH studies carried out at the same FEL and at third generation synchrotron radiation sources. In particular, we demonstrate the capability of the covariance mapping technique to disentangle the formation of molecular DCH states which is barely feasible with conventional electron spectroscopy methods.
16.	Perosa, G., et al. (author) Femtosecond Polarization Shaping of Free-Electron Laser Pulses 2023 In: Physical Review Letters. - 0031-9007. ; 131:4 Journal article (peer-reviewed)abstract We demonstrate the generation of extreme-ultraviolet (XUV) free-electron laser (FEL) pulses with timedependent polarization. To achieve polarization modulation on a femtosecond timescale, we combine two mutually delayed counterrotating circularly polarized subpulses from two cross-polarized undulators. The polarization profile of the pulses is probed by angle-resolved photoemission and above-threshold ionization of helium; the results agree with solutions of the time-dependent Schrodinger equation. The stability limit of the scheme is mainly set by electron-beam energy fluctuations, however, at a level that will not compromise experiments in the XUV. Our results demonstrate the potential to improve the resolution and element selectivity of methods based on polarization shaping and may lead to the development of new coherent control schemes for probing and manipulating core electrons in matter.
17.	Reid, A. H., et al. (author) Beyond a phenomenological description of magnetostriction 2018 In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 9 Journal article (peer-reviewed)abstract Magnetostriction, the strain induced by a change in magnetization, is a universal effect in magnetic materials. Owing to the difficulty in unraveling its microscopic origin, it has been largely treated phenomenologically. Here, we show how the source of magnetostriction-the underlying magnetoelastic stress-can be separated in the time domain, opening the door for an atomistic understanding. X-ray and electron diffraction are used to separate the subpicosecond spin and lattice responses of FePt nanoparticles. Following excitation with a 50-fs laser pulse, time-resolved X-ray diffraction demonstrates that magnetic order is lost within the nanoparticles with a time constant of 146 fs. Ultrafast electron diffraction reveals that this demagnetization is followed by an anisotropic, three-dimensional lattice motion. Analysis of the size, speed, and symmetry of the lattice motion, together with ab initio calculations accounting for the stresses due to electrons and phonons, allow us to reveal the magnetoelastic stress generated by demagnetization.
18.	Rohringer, N., et al. (author) Stimulated X-Ray Raman Scattering with Free-Electron Laser Sources 2016 In: X-Ray Lasers 2014. - Cham : Springer. - 0930-8989. - 9783319195216 - 9783319195209 ; , s. 201-207 Book chapter (peer-reviewed)abstract Stimulated electronic x-ray Raman scattering is the building block for several proposed x-ray pump probe techniques, that would allow the study of electron dynamics at unprecedented timescales.We present high spectral resolution data on stimulated electronic x-ray Raman scattering in a gas sample of neon using a self-amplified spontaneous emission x-ray free-electron laser. Despite the limited spectral coherence and broad bandwidth of these sources, high-resolution spectra can be obtained by statistical methods, opening the path to coherent stimulated x-ray Raman spectroscopy. An extension of these ideas to molecules and the results of a recent experiment in CO are discussed.
19.	Stern, S., et al. (author) Toward atomic resolution diffractive imaging of isolated molecules with X-ray free-electron lasers 2014 In: Faraday Discussions. - : Royal Society of Chemistry (RSC). - 1364-5498. ; 171, s. 393-418 Journal article (peer-reviewed)abstract We give a detailed account of the theoretical analysis and the experimental results of an X-ray-diffraction experiment on quantum-state selected and strongly laser-aligned gasphase ensembles of the prototypical large asymmetric rotor molecule 2,5-diiodobenzonitrile, performed at the Linac Coherent Light Source [Phys. Rev. Lett. 112, 083002 (2014)]. This experiment is the first step toward coherent diffractive imaging of structures and structural dynamics of isolated molecules at atomic resolution, i.e., picometers and femtoseconds, using X-ray free-electron lasers.
20.	Zhaunerchyk, Vitali, et al. (author) Disentangling formation of multiple-core holes in aminophenol molecules exposed to bright X-FEL radiation 2015 In: Journal of Physics B. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 48:24 Journal article (peer-reviewed)abstract Competing multi-photon ionization processes, some leading to the formation of double core hole states, have been examined in 4-aminophenol. The experiments used the linac coherent light source (LCLS) x-ray free electron laser, in combination with a time-of-flight magnetic bottle electron spectrometer and the correlation analysis method of covariance mapping. The results imply that 4-aminophenol molecules exposed to the focused x-ray pulses of the LCLS sequentially absorb more than two x-ray photons, resulting in the formation of multiple core holes as well as in the sequential removal of photoelectrons and Auger electrons (so-called PAPA sequences).
21.	Zhaunerchyk, Vitali, et al. (author) Using covariance mapping to investigate the dynamics of multi-photon ionization processes of Ne atoms exposed to X-FEL pulses 2013 In: Journal of Physics B. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 46:16, s. 164034- Journal article (peer-reviewed)abstract We report on a detailed investigation into the electron emission processes of Ne atoms exposed to intense femtosecond x-ray pulses, provided by the Linac Coherent Light Source Free Electron Laser (FEL) at Stanford. The covariance mapping technique is applied to analyse the data, and the capability of this approach to disentangle both linear and nonlinear correlation features which may be hidden on coincidence maps of the same data set is demonstrated. Different correction techniques which enable improvements on the quality of the spectral features extracted from the covariance maps are explored. Finally, a method for deriving characteristics of the x-ray FEL pulses based on covariance mapping in combination with model simulations is presented.
22.	Allahabadi, H, et al. (author) Assessing Trustworthy AI in Times of COVID-19: Deep Learning for Predicting a Multiregional Score Conveying the Degree of Lung Compromise in COVID-19 Patients 2022 In: IEEE transactions on technology and society. - 2637-6415. ; 3:4, s. 272-289 Journal article (peer-reviewed)
23.	Barillot, T., et al. (author) Correlation-Driven Transient Hole Dynamics Resolved in Space and Time in the Isopropanol Molecule 2021 In: Physical Review X. - : American Physical Society. - 2160-3308. ; 11:3 Journal article (peer-reviewed)abstract The possibility of suddenly ionized molecules undergoing extremely fast electron hole (or hole) dynamics prior to significant structural change was first recognized more than 20 years ago and termed charge migration. The accurate probing of ultrafast electron hole dynamics requires measurements that have both sufficient temporal resolution and can detect the localization of a specific hole within the molecule. We report an investigation of the dynamics of inner valence hole states in isopropanol where we use an x-ray pump-x-ray probe experiment, with site and state-specific probing of a transient hole state localized near the oxygen atom in the molecule, together with an ab initio theoretical treatment. We record the signature of transient hole dynamics and make the first tentative observation of dynamics driven by frustrated Auger-Meitner transitions. We verify that the effective hole lifetime is consistent with our theoretical prediction. This state-specific measurement paves the way to widespread application for observations of transient hole dynamics localized in space and time in molecules and thus to charge transfer phenomena that are fundamental in chemical and material physics.
24.	Beye, M., et al. (author) Selective Ultrafast Probing of Transient Hot Chemisorbed and Precursor States of CO on Ru(0001) 2013 In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 110:18 Journal article (peer-reviewed)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.
25.	Cryan, James P., et al. (author) Auger Electron Angular Distribution of Double Core-Hole States in the Molecular Reference Frame 2010 In: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 105:8, s. 083004- Journal article (peer-reviewed)abstract The Linac Coherent Light Source free electron laser is a source of high brightness x rays, 2×1011 photons in a ∼5 fs pulse, that can be focused to produce double core vacancies through rapid sequential ionization. This enables double core vacancy Auger electron spectroscopy, an entirely new way to study femtosecond chemical dynamics with Auger electrons that probe the local valence structure of molecules near a specific atomic core. Using 1.1 keV photons for sequential x-ray ionization of impulsively aligned molecular nitrogen, we observed a rich single-site double core vacancy Auger electron spectrum near 413 eV, in good agreement with ab initio calculations, and we measured the corresponding Auger electron angle dependence in the molecular frame.
26.	Cryan, J P, et al. (author) Molecular frame Auger electron energy spectrum from N2 2012 In: Journal of Physics B. - : IOP Publishing. - 0953-4075 .- 1361-6455. ; 45:5, s. 055601- Journal article (peer-reviewed)abstract Here we present the first angle-resolved, non-resonant (normal) Auger spectra for impulsively aligned nitrogen molecules. We have measured the angular pattern of Auger electron emission following K -shell photoionization by 1.1 keV photons from the Linac Coherent Light Source (LCLS). Using strong-field-induced molecular alignment to make molecular frame measurements is equally effective for both repulsive and quasi-bound final states. The capability to resolve Auger emission angular distributions in the molecular frame of reference provides a new tool for spectral assignments in congested Auger electron spectra that takes advantage of the symmetries of the final diction states. Based on our experimental results and theoretical predictions, we propose the assignment of the spectral features in the Auger electron spectrum.
27.	Dell'Angela, M., et al. (author) Real-Time Observation of Surface Bond Breaking with an X-ray Laser 2013 In: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 339:6125, s. 1302-1305 Journal article (peer-reviewed)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.
28.	Dell'Angela, M., et al. (author) Vacuum space charge effects in sub-picosecond soft X-ray photoemission on a molecular adsorbate layer 2015 In: Structural Dynamics. - : AIP Publishing. - 2329-7778. ; 2:2 Journal article (peer-reviewed)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.
29.	Hartmann, G., et al. (author) Circular dichroism measurements at an x-ray free-electron laser with polarization control 2016 In: Review of Scientific Instruments. - : AIP Publishing. - 0034-6748 .- 1089-7623. ; 87:8 Journal article (peer-reviewed)abstract A non-destructive diagnostic method for the characterization of circularly polarized, ultraintense, short wavelength free-electron laser (FEL) light is presented. The recently installed Delta undulator at the LCLS (Linac Coherent Light Source) at SLAC National Accelerator Laboratory (USA) was used as showcase for this diagnostic scheme. By applying a combined two-color, multi-photon experiment with polarization control, the degree of circular polarization of the Delta undulator has been determined. Towards this goal, an oriented electronic state in the continuum was created by non-resonant ionization of the O2 1s core shell with circularly polarized FEL pulses at hν 700 eV. An also circularly polarized, highly intense UV laser pulse with hν 3.1 eV was temporally and spatially overlapped, causing the photoelectrons to redistribute into so-called sidebands that are energetically separated by the photon energy of the UV laser. By determining the circular dichroism of these redistributed electrons using angle resolving electron spectroscopy and modeling the results with the strong-field approximation, this scheme allows to unambiguously determine the absolute degree of circular polarization of any pulsed, ultraintense XUV or X-ray laser source. © 2016 Author(s).
30.	Katayama, T., et al. (author) Ultrafast soft X-ray emission spectroscopy of surface adsorbates using an X-ray free electron laser 2013 In: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048 .- 1873-2526. ; 187, s. 9-14 Journal article (peer-reviewed)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.
31.	LaRue, Jerry, et al. (author) Real-Time Elucidation of Catalytic Pathways in CO Hydrogenation on Ru 2017 In: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 8:16, s. 3820-3825 Journal article (peer-reviewed)abstract The direct elucidation of the reaction pathways in heterogeneous catalysis has been challenging due to the short-lived nature of reaction intermediates. Here, we directly measured on ultrafast time scales the initial hydrogenation steps of adsorbed CO on a Ru catalyst surface, which is known as the bottleneck reaction in syngas and CO2 reforming processes. We initiated the hydrogenation of CO with an ultrafast laser temperature jump and probed transient changes in the electronic structure using real-time X-ray spectroscopy. In combination with theoretical simulations, we verified the formation of CHO during CO hydrogenation.
32.	McFarland, B.K., et al. (author) Experimental strategies for optical pump - soft x-ray probe experiments at the LCLS 2014 In: Journal of Physics: Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 488, s. 012015- Journal article (peer-reviewed)
33.	McFarland, B. K., et al. (author) Probing nucleobase photoprotection with soft x-rays 2013 In: XVIIITH INTERNATIONAL CONFERENCE ON ULTRAFAST PHENOMENA. - : EDP Sciences. - 9782759809561 ; , s. 07004- Conference paper (peer-reviewed)abstract Nucleobases absorb strongly in the ultraviolet region, leading to molecular excitation into reactive states. The molecules avoid the photoreactions by funnelling the electronic energy into less reactive states on an ultrafast timescale via non-Born-Oppenheimer dynamics. Current theory on the nucleobase thymine discusses two conflicting pathways for the photoprotective dynamics. We present our first results of our free electron laser based UV-pump soft x-ray-probe study of the photoprotection mechanism of thymine. We use the high spatial sensitivity of the Auger electrons emitted after the soft x-ray pulse induced core ionization. Our transient spetra show two timescales on the order of 200 fs and 5 ps, in agreement with previous (all UV) ultrafast experiments. The timescales appear at different Auger kinetic energies which will help us to decipher the molecular dynamics.
34.	McFarland, B. K., et al. (author) Ultrafast X-ray Auger probing of photoexcited molecular dynamics 2014 In: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 5, s. 4235- Journal article (peer-reviewed)abstract Molecules can efficiently and selectively convert light energy into other degrees of freedom. Disentangling the underlying ultrafast motion of electrons and nuclei of the photoexcited molecule presents a challenge to current spectroscopic approaches. Here we explore the photoexcited dynamics of molecules by an interaction with an ultrafast X-ray pulse creating a highly localized core hole that decays via Auger emission. We discover that the Auger spectrum as a function of photoexcitation-X-ray-probe delay contains valuable information about the nuclear and electronic degrees of freedom from an element-specific point of view. For the nucleobase thymine, the oxygen Auger spectrum shifts towards high kinetic energies, resulting from a particular C-O bond stretch in the pi pi* photoexcited state. A subsequent shift of the Auger spectrum towards lower kinetic energies displays the electronic relaxation of the initial photoexcited state within 200 fs. Ab-initio simulations reinforce our interpretation and indicate an electronic decay to the n pi* state.
35.	Öberg, Henrik, et al. (author) 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 2015 In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 640, s. 80-88 Journal article (peer-reviewed)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.

Skapa referenser, mejla, bekava och länka

Permalink

Träfflista för sökning "WFRF:(Coffee R.) "

Refine your search

Year