Alvaro Sanchez-Gonzalez

Auger electron and photoabsorption spectra of glycine in the vicinity of the oxygen K-edge measured with an X-FEL

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.

Accurate prediction of X-ray pulse properties from a free-electron laser using machine learning

A. Sanchez-Gonzalez,1 P. Micaelli,1 C. Olivier,1 T. R. Barillot,1 M. Ilchen,2, 3 A. A. Lutman,4 A. Marinelli,4 T. Maxwell,4 A. Achner,3 M. Agåker,5 N. Berrah,6 C. Bostedt,4, 7 J. Buck,8 P. H. Bucksbaum,2, 9 S. Carron Montero,4, 10 B. Cooper,1 J. P. Cryan,2 M. Dong,5 R. Feifel,11 L. J. Frasinski,1 H. Fukuzawa,12 A. Galler,3 G. Hartmann,8, 13 N. Hartmann,4 W. Helml,4, 14 A. S. Johnson,1 A. Knie,13 A. O. Lindahl,2, 11 J. Liu,3 K. Motomura,12 M. Mucke,5 C. O’Grady,4 J-E. Rubensson,5 E. R. Simpson,1 R. J. Squibb,11 C. Såthe,15 K. Ueda,12 M. Vacher,16, 17 D. J. Walke,1 V. Zhaunerchyk,11 R. N. Coffee,4 and J. P. Marangos1 1Department of Physics, Imperial College, London, SW7 2AZ, United Kingdom 2Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA 3European XFEL GmbH, Holzkoppel 4, 22869 Schenefeld, Germany 4Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA 5Department of Physics and Astronomy, Uppsala University, Uppsala, 75120, Sweden 6Department of Physics, University of Connecticut, 2152 Hillside Road, U-3046, Storrs, CT 06269, USA 7Argonne National Laboratory, Lemont, IL 60439, USA 8Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, Hamburg, 22607, Germany 9Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305, USA 10California Lutheran University, 60 W Olsen Rd, Thousand Oaks, CA 91360, USA 11Department of Physics, University of Gothenburg, Origovägen 6B, 41296 Gothenburg, Sweden 12Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan 13Institut für Physik und CINSaT, Universität Kassel, Heinrich-Plett-Str. 40, 34132 Kassel, Germany 14Physics Department, TU Munich, James-Franck-Str. 1, 85748 Garching, Germany 15MAX IV Laboratory, Lund University, Box 118, SE-221 00 Lund, Sweden 16Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom 17Department of Chemistry Ångtröm, Uppsala University, Uppsala, 75120, SwedenFree-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.

Polarisation response of delay dependent absorption modulation in strong field dressed helium atoms probed near threshold

We present the first measurement of the vectorial response of strongly dressed helium atoms probed by an attosecond pulse train (APT) polarised either parallel or perpendicular to the dressing field polarisation. The transient absorption is probed as a function of delay between the APT and the linearly polarised 800 nm field of peak intensity . The APT spans the photon energy range 16–42 eV, covering the first ionisation energy of helium (24.59 eV). With parallel polarised dressing and probing fields, we observe modulations with periods of one half and one quarter of the dressing field period. When the polarisation of the dressing field is altered from parallel to perpendicular with respect to the APT polarisation we observe a large suppression in the modulation depth of the above ionisation threshold absorption. In addition to this we present the intensity dependence of the harmonic modulation depth as a function of delay between the dressing and probe fields, with dressing field peak intensities ranging from 2 × 1012 to 2 × 1014 . We compare our experimental results with a full-dimensional solution of the single-atom time-dependent (TD) Schrodinger equation obtained using the recently developed abinitio TD B-spline ADC method and find good qualitative agreement for the above threshold harmonics.

Stimulated X-Ray Raman Scattering with Free-Electron Laser Sources

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.

Coincidence timing of femtosecond optical pulses in an X-ray free electron laser

Femtosecond resolution pump-probe experiments are now routinely carried out at X-ray Free Electron Lasers, enabled by the development of cross-correlation “time-tools” which correct the picosecond-level jitter between the optical and X-ray pulses. These tools provide very accurate, <10 fs, measurement of the relative arrival time, but do not provide a measure of the absolute coincidence time in the interaction. Cross-correlation experiments using transient reflectivity in a crystal are commonly used for this purpose, and to date no quantitative analysis of the accuracy or stability of absolute coincidence time determination has been performed. We have performed a quantitative analysis of coincidence timing at the SACLA facility through a cross-correlation of 100 ± 10 fs, 400 nm optical pulses with 7 fs, 10.5 keV X-ray pulses via transient reflectivity in a cerium-doped yttrium aluminum garnet crystal. We have modelled and fit the transient reflectivity, which required a convolution with a 226 ± 12 fs unce...

X-ray Free Electron Laser Determination of Crystal Structures of Dark and Light States of a Reversibly Photoswitching Fluorescent Protein at Room Temperature.

The photochromic fluorescent protein Skylan-NS (Nonlinear Structured illumination variant mEos3.1H62L) is a reversibly photoswitchable fluorescent protein which has an unilluminated/ground state with an anionic and cis chromophore conformation and high fluorescence quantum yield. Photo-conversion with illumination at 515 nm generates a meta-stable intermediate with neutral trans-chromophore structure that has a 4 h lifetime. We present X-ray crystal structures of the cis (on) state at 1.9 Angstrom resolution and the trans (off) state at a limiting resolution of 1.55 Angstrom from serial femtosecond crystallography experiments conducted at SPring-8 Angstrom Compact Free Electron Laser (SACLA) at 7.0 keV and 10.5 keV, and at Linac Coherent Light Source (LCLS) at 9.5 keV. We present a comparison of the data reduction and structure determination statistics for the two facilities which differ in flux, beam characteristics and detector technologies. Furthermore, a comparison of droplet on demand, grease injection and Gas Dynamic Virtual Nozzle (GDVN) injection shows no significant differences in limiting resolution. The photoconversion of the on- to the off-state includes both internal and surface exposed protein structural changes, occurring in regions that lack crystal contacts in the orthorhombic crystal form.

A beamline for attosecond pump-probe experiments: towards tracking ultrafast electron dynamics in atoms and molecules

We present a technical review of a beamline built to perform pump-probe experiments with a temporal resolution of < 200 attosecond. This is designed specifically to use the technique of attosecond transient absorption spectroscopy (ATAS) to resolve ultra-fast electron dynamics in atoms and molecules. A non-collinear, interferometrically stable geometry is adopted to allow us to individually control the characteristics of each of the pump and probe arms independent from each other. With the use of an auxiliary interferometer to correct for long-term drifts between the pump and probe arms we measure better than 150as resolution for our time-corrected delay despite having separated beam paths of over 4m in length. In our first experiment we have focused on the time dependence of the electronic states of an atom in a strong laser field. An extreme ultra-violet (XUV) attosecond pulse train (APT) and a precisely synchronized 30fs IR pulse are used in this work. Delay-dependent absorption modulations are observed at even multiples (2 and 4) of the IR dressing field frequency as the pump-probe delay is scanned. We investigate the dependence of the 2ω order absorption modulation amplitude from the transient absorption of laser-dressed helium as the IR dressing field ellipticity is varied, and we discuss the issues in obtaining such results. We present qualitative data indicating a clear anisotropy in the response of the atom to an IR dressing field, and discuss how we will improve this measurement in future experiments.