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- ItemTime-resolved site-selective imaging of predissociation and charge transfer dynamics: The CH3I B-band(Bristol : IOP Publ., 2020) Forbes, Ruaridh; Allum, Felix; Bari, Sadia; Boll, Rebecca; Borne, Kurtis; Brouard, Mark; Bucksbaum, Philip H.; Ekanayake, Nagitha; Erk, Benjamin; Howard, Andrew J.; Johnsson, Per; Lee, Jason W.L.; Manschwetus, Bastian; Mason, Robert; Passow, Christopher; Peschel, Jasper; Rivas, Daniel E.; Rörig, Aljoscha; Rouzée, Arnaud; Vallance, Claire; Ziaee, Farzaneh; Rolles, Daniel; Burt, MichaelThe predissociation dynamics of the 6s (B2E) Rydberg state of gas-phase CH3I were investigated by time-resolved Coulomb-explosion imaging using extreme ultraviolet (XUV) free-electron laser pulses. Inner-shell ionization at the iodine 4d edge was utilized to provide a site-specific probe of the ensuing dynamics. The combination of a velocity-map imaging (VMI) spectrometer coupled with the pixel imaging mass spectrometry (PImMS) camera permitted three-dimensional ionic fragment momenta to be recorded simultaneously for a wide range of iodine charge states. In accord with previous studies, initial excitation at 201.2 nm results in internal conversion and subsequent dissociation on the lower-lying A-state surface on a picosecond time scale. Examination of the time-dependent yield of low kinetic energy iodine fragments yields mechanistic insights into the predissociation and subsequent charge transfer following multiple ionization of the iodine products. The effect of charge transfer was observed through differing delay-dependencies of the various iodine charge states, from which critical internuclear distances for charge transfer could be inferred and compared to a classical over-the-barrier model. Time-dependent photofragment angular anisotropy parameters were extracted from the central slice of the Newton sphere, without Abel inversion, and highlight the effect of rotation of the parent molecule before dissociation, as observed in previous © 2020 The Author(s). Published by IOP Publishing Ltd Printed in the UK
- ItemAnisotropic photoemission time delays close to a Fano resonance([London] : Nature Publishing Group UK, 2018) Cirelli, Claudio; Marante, Carlos; Heuser, Sebastian; Petersson, C.L.M.; Galán, Álvaro Jiménez; Argenti, Luca; Zhong, Shiyang; Busto, David; Isinger, Marcus; Nandi, Saikat; Maclot, Sylvain; Rading, Linnea; Johnsson, Per; Gisselbrecht, Mathieu; Lucchini, Matteo; Gallmann, Lukas; Dahlström, J. Marcus; Lindroth, Eva; L’Huillier, Anne; Martín, Fernando; Keller, UrsulaElectron correlation and multielectron effects are fundamental interactions that govern many physical and chemical processes in atomic, molecular and solid state systems. The process of autoionization, induced by resonant excitation of electrons into discrete states present in the spectral continuum of atomic and molecular targets, is mediated by electron correlation. Here we investigate the attosecond photoemission dynamics in argon in the 20-40 eV spectral range, in the vicinity of the 3s -1 np autoionizing resonances. We present measurements of the differential photoionization cross section and extract energy and angle-dependent atomic time delays with an attosecond interferometric method. With the support of a theoretical model, we are able to attribute a large part of the measured time delay anisotropy to the presence of autoionizing resonances, which not only distort the phase of the emitted photoelectron wave packet but also introduce an angular dependence.
- ItemApplication of Matched-Filter Concepts to Unbiased Selection of Data in Pump-Probe Experiments with Free Electron Lasers(Basel : MDPI, 2017-06-16) Callegari, Carlo; Takanashi, Tsukasa; Fukuzawa, Hironobu; Motomura, Koji; Iablonskyi, Denys; Kumagai, Yoshiaki; Mondal, Subhendu; Tachibana, Tetsuya; Nagaya, Kiyonobu; Nishiyama, Toshiyuki; Matsunami, Kenji; Johnsson, Per; Piseri, Paolo; Sansone, Giuseppe; Dubrouil, Antoine; Reduzzi, Maurizio; Carpeggiani, Paolo; Vozzi, Caterina; Devetta, Michele; Faccialà, Davide; Calegari, Francesca; Castrovilli, Mattea; Coreno, Marcello; Alagia, Michele; Schütte, Bernd; Berrah, Nora; Plekan, Oksana; Finetti, Paola; Ferrari, Eugenio; Prince, Kevin; Ueda, KiyoshiPump-probe experiments are commonly used at Free Electron Lasers (FEL) to elucidate the femtosecond dynamics of atoms, molecules, clusters, liquids and solids. Maximizing the signal-to-noise ratio of the measurements is often a primary need of the experiment, and the aggregation of repeated, rapid, scans of the pump-probe delay is preferable to a single long-lasting scan. The limited availability of beamtime makes it impractical to repeat measurements indiscriminately, and the large, rapid flow of single-shot data that need to be processed and aggregated into a dataset, makes it difficult to assess the quality of a measurement in real time. In post-analysis it is then necessary to devise unbiased criteria to select or reject datasets, and to assign the weight with which they enter the analysis. One such case was the measurement of the lifetime of Intermolecular Coulombic Decay in the weakly-bound neon dimer. We report on the method we used to accomplish this goal for the pump-probe delay scans that constitute the core of the measurement; namely we report on the use of simple auto- and cross-correlation techniques based on the general concept of “matched filter”. We are able to unambiguously assess the signal-to-noise ratio (SNR) of each scan, which then becomes the weight with which a scan enters the average of multiple scans. We also observe a clear gap in the values of SNR, and we discard all the scans below a SNR of 0.45. We are able to generate an average delay scan profile, suitable for further analysis: in our previous work we used it for comparison with theory. Here we argue that the method is sufficiently simple and devoid of human action to be applicable not only in post-analysis, but also for the real-time assessment of the quality of a dataset.
- ItemSingleshot polychromatic coherent diffractive imaging with a high-order harmonic source(Washington, DC : Soc., 2020) Malm, Erik; Wikmark, Hampus; Pfau, Bastian; Villanueva-Perez, Pablo; Rudawski, Piotr; Peschel, Jasper; Maclot, Sylvain; Schneider, Michael; Eisebitt, Stefan; Mikkelsen, Anders; L’Huillier, Anne; Johnsson, PerSingleshot polychromatic coherent diffractive imaging is performed with a high-intensity high-order harmonic generation source. The coherence properties are analyzed and several reconstructions show the shot-to-shot fluctuations of the incident beam wavefront. The method is based on a multi-step approach. First, the spectrum is extracted from double-slit diffraction data. The spectrum is used as input to extract the monochromatic sample diffraction pattern, then phase retrieval is performed on the quasi-monochromatic data to obtain the sample’s exit surface wave. Reconstructions based on guided error reduction (ER) and alternating direction method of multipliers (ADMM) are compared. ADMM allows additional penalty terms to be included in the cost functional to promote sparsity within the reconstruction. © 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.
- ItemJitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser([Bad Honnef] : Dt. Physikalische Ges., 2017-04-10) Savelyev, Evgeny; Boll, Rebecca; Bomme, Cédric; Schirmel, Nora; Redlin, Harald; Erk, Benjamin; Düsterer, Stefan; Müller, Erland; Höppner, Hauke; Toleikis, Sven; Müller, Jost; Kristin Czwalinna, Marie; Treusch, Rolf; Kierspel, Thomas; Mullins, Terence; Trippel, Sebastian; Wiese, Joss; Küpper, Jochen; Brauβe, Felix; Krecinic, Faruk; Rouzée, Arnaud; Rudawski, Piotr; Johnsson, Per; Amini, Kasra; Lauer, Alexandra; Burt, Michael; Brouard, Mark; Christensen, Lauge; Thøgersen, Jan; Stapelfeldt, Henrik; Berrah, Nora; Müller, Maria; Ulmer, Anatoli; Techert, Simone; Rudenko, Artem; Rolles, DanielIn pump-probe experiments employing a free-electron laser (FEL) in combination with a synchronized optical femtosecond laser, the arrival-time jitter between the FEL pulse and the optical laser pulse often severely limits the temporal resolution that can be achieved. Here, we present a pump-probe experiment on the UV-induced dissociation of 2,6-difluoroiodobenzene (C6H3F2I) molecules performed at the FLASH FEL that takes advantage of recent upgrades of the FLASH timing and synchronization system to obtain high-quality data that are not limited by the FEL arrival-time jitter. We discuss in detail the necessary data analysis steps and describe the origin of the time-dependent effects in the yields and kinetic energies of the fragment ions that we observe in the experiment.
- ItemPhotodissociation 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, RebeccaWe 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.