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Type: article × Subject: Electrons × WGL Institute: MBI ×

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    In situ single-shot diffractive fluence mapping for X-ray free-electron laser pulses
    ([London] : Nature Publishing Group UK, 2018) Schneider, Michael; Günther, Christian M.; Pfau, Bastian; Capotondi, Flavio; Manfredda, Michele; Zangrando, Marco; Mahne, Nicola; Raimondi, Lorenzo; Pedersoli, Emanuele; Naumenko, Denys; Eisebitt, Stefan
    Free-electron lasers (FELs) in the extreme ultraviolet (XUV) and X-ray regime opened up the possibility for experiments at high power densities, in particular allowing for fluence-dependent absorption and scattering experiments to reveal non-linear light-matter interactions at ever shorter wavelengths. Findings of such non-linear effects are met with tremendous interest, but prove difficult to understand and model due to the inherent shot-to-shot fluctuations in photon intensity and the often structured, non-Gaussian spatial intensity profile of a focused FEL beam. Presently, the focused beam is characterized and optimized separately from the actual experiment. Here, we present the simultaneous measurement of XUV diffraction signals from solid samples in tandem with the corresponding single-shot spatial fluence distribution on the actual sample. Our in situ characterization scheme enables direct monitoring of the sample illumination, providing a basis to optimize and quantitatively understand FEL experiments.
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    Ultrafast Structural Changes in Chiral Molecules Measured with Free-Electron Lasers
    (Bristol : IOP Publ., 2020) Schmidt, P.; Music, V.; Hartmann, G.; Boll, R.; Erk, B.; Bari, S.; Allum, F.; Baumann, T.M.; Brenner, G.; Brouard, M.; Burt, M.; Coffee, R.; Dörner, S.; Galler, A.; Grychtol, P.; Heathcote, D.; Inhester, L.; Kazemi, M.; Larsson, M.; Li, Z.; Lutmann, A.; Manschwetus, B.; Marder, L.; Mason, R.; Moeller, S.; Osipov, T.; Otto, H.; Passow, C.; Rolles, D.; Rupprecht, P.; Schubert, K.; Schwob, L.; Thomas, R.; Vallance, C.; Von Korff Schmising, C.; Wagner, R.; Walter, P.; Wolf, T.J.A.; Zhaunerchyk, V.; Meyer, M.; Ehresmann, A.; Knie, A.; Demekhin, P.V.; Ilchen, M.
    (X-ray) free-electron lasers are employed to site specifically interrogate atomic fragments during ultra-fast photolysis of chiral molecules via time-resolved photoelectron circular dichroism. © 2020 Institute of Physics Publishing. All rights reserved.
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    Photodissociation of aligned CH3I and C6H3F2I molecules probed with time-resolved Coulomb explosion imaging by site-selective extreme ultraviolet ionization
    (Melville, NY : AIP Publishing LLC, 2018) Amini, Kasra; Savelyev, Evgeny; Brauße, Felix; Berrah, Nora; Bomme, Cédric; Brouard, Mark; Burt, Michael; Christensen, Lauge; Düsterer, Stefan; Erk, Benjamin; Höppner, Hauke; Kierspel, Thomas; Krecinic, Faruk; Lauer, Alexandra; Lee, Jason W. L.; Müller, Maria; Müller, Erland; Mullins, Terence; Redlin, Harald; Schirmel, Nora; Thøgersen, Jan; Techert, Simone; Toleikis, Sven; Treusch, Rolf; Trippel, Sebastian; Ulmer, Anatoli; Vallance, Claire; Wiese, Joss; Johnsson, Per; Küpper, Jochen; Rudenko, Artem; Rouzée, Arnaud; Stapelfeldt, Henrik; Rolles, Daniel; Boll, Rebecca
    We explore time-resolved Coulomb explosion induced by intense, extreme ultraviolet (XUV) femtosecond pulses from a free-electron laser as a method to image photo-induced molecular dynamics in two molecules, iodomethane and 2,6-difluoroiodobenzene. At an excitation wavelength of 267 nm, the dominant reaction pathway in both molecules is neutral dissociation via cleavage of the carbon-iodine bond. This allows investigating the influence of the molecular environment on the absorption of an intense, femtosecond XUV pulse and the subsequent Coulomb explosion process. We find that the XUV probe pulse induces local inner-shell ionization of atomic iodine in dissociating iodomethane, in contrast to non-selective ionization of all photofragments in difluoroiodobenzene. The results reveal evidence of electron transfer from methyl and phenyl moieties to a multiply charged iodine ion. In addition, indications for ultrafast charge rearrangement on the phenyl radical are found, suggesting that time-resolved Coulomb explosion imaging is sensitive to the localization of charge in extended molecules.
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    Compact intense extreme-ultraviolet source
    (Washington, DC : OSA, 2021) Major, Balázs; Ghafur, Omair; Kovács, Katalin; Varjú, Katalin; Tosa, Valer; Vrakking, Marc J. J.; Schütte, B.
    High-intensity laser pulses covering the ultraviolet to terahertz spectral regions are nowadays routinely generated in a large number of laboratories. In contrast, intense extreme-ultraviolet (XUV) pulses have only been demonstrated using a small number of sources including free-electron laser facilities [1-3] and long high-harmonic generation (HHG) beamlines [4-9]. Here we demonstrate a concept for a compact intense XUV source based on HHG that is focused to an intensity of $2 \times 10^{14}$ W/cm$^2$, with a potential increase up to $10^{17}$ W/cm$^2$ in the future. Our approach uses tight focusing of the near-infrared (NIR) driving laser and minimizes the XUV virtual source size by generating harmonics several Rayleigh lengths away from the NIR focus. Accordingly, the XUV pulses can be refocused to a small beam waist radius of 600 nm, enabling the absorption of up to four XUV photons by a single Ar atom in a setup that fits on a modest (2 m) laser table. Our concept represents a straightforward approach for the generation of intense XUV pulses in many laboratories, providing novel opportunities for XUV strong-field and nonlinear optics experiments, for XUV-pump XUV-probe spectroscopy and for the coherent diffractive imaging of nanoscale structures.
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    Three-Dimensional Shapes of Spinning Helium Nanodroplets
    (College Park, Md. : APS, 2018) Langbehn, Bruno; Sander, Katharina; Ovcharenko, Yevheniy; Peltz, Christian; Clark, Andrew; Coreno, Marcello; Cucini, Riccardo; Drabbels, Marcel; Finetti, Paola; Di Fraia, Michele; Giannessi, Luca; Grazioli, Cesare; Iablonskyi, Denys; LaForge, Aaron C.; Nishiyama, Toshiyuki; Oliver Álvarez de Lara, Verónica; Piseri, Paolo; Plekan, Oksana; Ueda, Kiyoshi; Zimmermann, Julian; Prince, Kevin C.; Stienkemeier, Frank; Callegari, Carlo; Fennel, Thomas; Rupp, Daniela; Möller, Thomas
    A significant fraction of superfluid helium nanodroplets produced in a free-jet expansion has been observed to gain high angular momentum resulting in large centrifugal deformation. We measured single-shot diffraction patterns of individual rotating helium nanodroplets up to large scattering angles using intense extreme ultraviolet light pulses from the FERMI free-electron laser. Distinct asymmetric features in the wide-angle diffraction patterns enable the unique and systematic identification of the three-dimensional droplet shapes. The analysis of a large data set allows us to follow the evolution from axisymmetric oblate to triaxial prolate and two-lobed droplets. We find that the shapes of spinning superfluid helium droplets exhibit the same stages as classical rotating droplets while the previously reported metastable, oblate shapes of quantum droplets are not observed. Our three-dimensional analysis represents a valuable landmark for clarifying the interrelation between morphology and superfluidity on the nanometer scale.
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    Interference structure of above-threshold ionization versus above-threshold detachment
    (Bristol : IOP, 2012) Korneev, Ph.A.; Popruzhenko, S.V.; Goreslavski, S.P.; Becker, W.; Paulus, G.G.; Fetić, B.; Milošević, D.B.
    Laser-induced electron detachment or ionization of atoms and negative ions is considered. In the context of the saddle-point evaluation of the strong-field approximation (SFA), the velocity maps of the direct electrons (those that do not undergo rescattering) exhibit a characteristic structure due to the constructive and destructive interference of electrons liberated from their parent atoms/ions within certain windows of time. This structure is defined by the above-threshold ionization rings at fixed electron energy and by two sets of curves in momentum space on which destructive interference occurs. The spectra obtained with the SFA are compared with those obtained by numerical solution of the time-dependent Schrödinger equation. For detachment, the agreement is excellent. For ionization, the effect of the Coulomb field is most pronounced for electrons emitted in a direction close to laser polarization, while for nearperpendicular emission the qualitative appearance of the spectrum is unaffected.
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    Real-time spatial characterization of micrometer-sized X-ray free-electron laser beams focused by bendable mirrors
    (Washington, DC : Soc., 2022) Mercurio, Giuseppe; Chalupský, Jaromír; Nistea, Ioana-Theodora; Schneider, Michael; Hájková, Věra; Gerasimova, Natalia; Carley, Robert; Cascella, Michele; Le Guyader, Loïc; Mercadier, Laurent; Schlappa, Justine; Setoodehnia, Kiana; Teichmann, Martin; Yaroslavtsev, Alexander; Burian, Tomáš; Vozda, Vojtĕch; Vyšín, Luděk; Wild, Jan; Hickin, David; Silenzi, Alessandro; Stupar, Marijan; Torben Delitz, Jan; Broers, Carsten; Reich, Alexander; Pfau, Bastian; Eisebitt, Stefan; La Civita, Daniele; Sinn, Harald; Vannoni, Maurizio; Alcock, Simon G.; Juha, Libor; Scherz, Andreas
    A real-time and accurate characterization of the X-ray beam size is essential to enable a large variety of different experiments at free-electron laser facilities. Typically, ablative imprints are employed to determine shape and size of μm-focused X-ray beams. The high accuracy of this state-of-the-art method comes at the expense of the time required to perform an ex-situ image analysis. In contrast, diffraction at a curved grating with suitably varying period and orientation forms a magnified image of the X-ray beam, which can be recorded by a 2D pixelated detector providing beam size and pointing jitter in real time. In this manuscript, we compare results obtained with both techniques, address their advantages and limitations, and demonstrate their excellent agreement. We present an extensive characterization of the FEL beam focused to ≈1 μm by two Kirkpatrick-Baez (KB) mirrors, along with optical metrology slope profiles demonstrating their exceptionally high quality. This work provides a systematic and comprehensive study of the accuracy provided by curved gratings in real-time imaging of X-ray beams at a free-electron laser facility. It is applied here to soft X-rays and can be extended to the hard X-ray range. Furthermore, curved gratings, in combination with a suitable detector, can provide spatial properties of μm-focused X-ray beams at MHz repetition rate.
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    Attosecond electron thermalization in laser-induced nonsequential multiple ionization: Hard versus glancing collisions
    (College Park, MD : Institute of Physics Publishing, 2008) Liu, X.; De Morisson Faria, C.F.; Becker, W.
    A recollision-based largely classical statistical model of laser-induced nonsequential multiple (N-fold) ionization of atoms is further explored. Upon its return to the ionic core, the first-ionized electron interacts with the other N - 1 bound electrons either through a contact or a Coulomb interaction. The returning electron may leave either immediately after this interaction or join the other electrons to form a thermalized complex which leaves the ion after the delay Δt, which is the sum of a thermalization time and a possible additional dwell time. Good agreement with the available triple and quadruple ionization data in neon and argon is obtained with the contact scenario and delays of Δt = 0.17 T and 0.265 T, respectively, with T the laser period. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
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    Towards time resolved core level photoelectron spectroscopy with femtosecond x-ray free-electron lasers
    (College Park, MD : Institute of Physics Publishing, 2008) Pietzsch, A.; Föhlisch, A.; Beye, M.; Deppe, M.; Hennies, F.; Nagasono, M.; Suljotil, E.; Wurth, W.; Gahl, C.; Dörich, K.; Melnikov, A.
    We have performed core level photoelectron spectroscopy on a W(110) single crystal with femtosecond XUV pulses from the free-electron laser at Hamburg (FLASH). We demonstrate experimentally and through theoretical modelling that for a suitable range of photon fluences per pulse, time-resolved photoemission experiments on solid surfaces are possible. Using FLASH pulses in combination with a synchronized optical laser, we have performed femtosecond time-resolved core-level photoelectron spectroscopy and observed sideband formation on the W 4f lines indicating a cross correlation between femtosecond optical and XUV pulses. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
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    Sub-15-fs X-ray pump and X-ray probe experiment for the study of ultrafast magnetization dynamics in ferromagnetic alloys
    (Washington, DC : Soc., 2021) Liu, Xuan; Merhe, Alaaeldine; Jal, Emmanuelle; Delaunay, Renaud; Jarrier, Romain; Chardonnet, Valentin; Hennes, Marcel; Chiuzbaian, Sorin G.; Légaré, Katherine; Hennecke, Martin; Radu, Ilie; Von Korff Schmising, Clemens; Grunewald, Særen; Kuhlmann, Marion; Lüning, Jan; Vodungbo, Boris
    In this paper, we present a new setup for the measurement of element-specific ultrafast magnetization dynamics in ferromagnetic thin films with a sub-15-fs time resolution. Our experiment relies on a split and delay approach which allows us to fully exploit the shortest X-rays pulses delivered by X-ray Free Electrons Lasers (close to the attosecond range), in an X-ray pump – X-ray probe geometry. The setup performance is demonstrated by measuring the ultrafast elemental response of Ni and Fe during demagnetization of ferromagnetic Ni and Ni80Fe20 (Permalloy) samples upon resonant excitation at the corresponding absorption edges. The transient demagnetization process is measured in both reflection and transmission geometry using, respectively, the transverse magneto-optical Kerr effect (T-MOKE) and the Faraday effect as probing mechanisms.