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Structural Dynamics (Melville, N.Y.)

C Gutt, T Sant, D Ksenzov, F Capotondi, E Pedersoli, L Raimondi, I P Nikolov, M Kiskinova, S Jaiswal, G Jakob, M Kläui, H Zabel, U Pietsch
We report the results of resonant magnetic XUV reflectivity experiments performed at the XUV free-electron laser FERMI. Circularly polarized XUV light with the photon energy tuned to the Fe M2,3 edge is used to measure resonant magnetic reflectivities and the corresponding Q-resolved asymmetry of a Permalloy/Ta/Permalloy trilayer film. The asymmetry exhibits ultrafast changes on 240 fs time scales upon pumping with ultrashort IR laser pulses. Depending on the value of the wavevector transfer Qz , we observe both decreasing and increasing values of the asymmetry parameter, which is attributed to ultrafast changes in the vertical spin and charge density profiles of the trilayer film...
September 2017: Structural Dynamics (Melville, N.Y.)
Daniel S Badali, R J Dwayne Miller
In conjunction with ultrafast diffraction, streak cameras offer an unprecedented opportunity for recording an entire molecular movie with a single probe pulse. This is an attractive alternative to conventional pump-probe experiments and opens the door to studying irreversible dynamics. However, due to the "smearing" of the diffraction pattern across the detector, the streaking technique has thus far been limited to simple mono-crystalline samples and extreme care has been taken to avoid overlapping diffraction spots...
September 2017: Structural Dynamics (Melville, N.Y.)
Donghyun Ka, Suji Hong, Ugeene Jeong, Migyeong Jeong, Nayoung Suh, Jeong-Yong Suh, Euiyoung Bae
Clustered regularly interspaced short palindromic repeats (CRISPRs) and CRISPR-associated (Cas) proteins constitute a microbial, adaptive immune system countering invading nucleic acids. Cas2 is a universal Cas protein found in all types of CRISPR-Cas systems, and its role is implicated in new spacer acquisition into CRISPR loci. In subtype I-C CRISPR-Cas systems, Cas2 proteins are metal-dependent double-stranded DNA (dsDNA) nucleases, and a pH-dependent conformational transition has been proposed as a prerequisite for catalytic action...
September 2017: Structural Dynamics (Melville, N.Y.)
Sena Hashimoto, Atsushi Yabushita, Izumi Iwakura
Transient process spectroscopy has previously been thought to be applicable only to the analysis of intra-molecular processes. Two metal ion bridges used in the present work have allowed us to visualize real-time variations of the molecular vibration frequencies during photo-disproportionation inside bimolecule aggregates, which directly shows transient inter-molecular reactions.
September 2017: Structural Dynamics (Melville, N.Y.)
Markus Kowalewski, Kochise Bennett, Shaul Mukamel
We examine time-resolved X-ray diffraction from molecules in the gas phase which undergo nonadiabatic avoided-crossing dynamics involving strongly coupled electrons and nuclei. Several contributions to the signal are identified, representing (in decreasing strength) elastic scattering, contributions of the electronic coherences created by nonadiabatic couplings in the avoided crossing regime, and inelastic scattering. The former probes the charge density and delivers direct information on the evolving molecular geometry...
September 2017: Structural Dynamics (Melville, N.Y.)
M Kozina, T van Driel, M Chollet, T Sato, J M Glownia, S Wandel, M Radovic, U Staub, M C Hoffmann
We use ultrafast X-ray pulses to characterize the lattice response of SrTiO3 when driven by strong terahertz fields. We observe transient changes in the diffraction intensity with a delayed onset with respect to the driving field. Fourier analysis reveals two frequency components corresponding to the two lowest energy zone-center optical modes in SrTiO3. The lower frequency mode exhibits clear softening as the temperature is decreased while the higher frequency mode shows slight temperature dependence.
September 2017: Structural Dynamics (Melville, N.Y.)
Yuki Obara, Hironori Ito, Terumasa Ito, Naoya Kurahashi, Stephan Thürmer, Hiroki Tanaka, Tetsuo Katayama, Tadashi Togashi, Shigeki Owada, Yo-Ichi Yamamoto, Shutaro Karashima, Junichi Nishitani, Makina Yabashi, Toshinori Suzuki, Kazuhiko Misawa
The charge-carrier dynamics of anatase TiO2 nanoparticles in an aqueous solution were studied by femtosecond time-resolved X-ray absorption spectroscopy using an X-ray free electron laser in combination with a synchronized ultraviolet femtosecond laser (268 nm). Using an arrival time monitor for the X-ray pulses, we obtained a temporal resolution of 170 fs. The transient X-ray absorption spectra revealed an ultrafast Ti K-edge shift and a subsequent growth of a pre-edge structure. The edge shift occurred in ca...
July 2017: Structural Dynamics (Melville, N.Y.)
Francesco Pennacchio, Giovanni M Vanacore, Giulia F Mancini, Malte Oppermann, Rajeswari Jayaraman, Pietro Musumeci, Peter Baum, Fabrizio Carbone
Ultrafast electron diffraction is a powerful technique to investigate out-of-equilibrium atomic dynamics in solids with high temporal resolution. When diffraction is performed in reflection geometry, the main limitation is the mismatch in group velocity between the overlapping pump light and the electron probe pulses, which affects the overall temporal resolution of the experiment. A solution already available in the literature involved pulse front tilt of the pump beam at the sample, providing a sub-picosecond time resolution...
July 2017: Structural Dynamics (Melville, N.Y.)
Azhr A Raheem, Martin Wilke, Mario Borgwardt, Nicholas Engel, Sergey I Bokarev, Gilbert Grell, Saadullah G Aziz, Oliver Kühn, Igor Yu Kiyan, Christoph Merschjann, Emad F Aziz
The kinetics of ultrafast photoinduced structural changes in linkage isomers is investigated using Na2[Fe(CN)5NO] as a model complex. The buildup of the metastable side-on configuration of the NO ligand, as well as the electronic energy levels of ground, excited, and metastable states, has been revealed by means of time-resolved extreme UV (XUV) photoelectron spectroscopy in aqueous solution, aided by theoretical calculations. Evidence of a short-lived intermediate state in the isomerization process and its nature are discussed, finding that the complete isomerization process occurs in less than 240 fs after photoexcitation...
July 2017: Structural Dynamics (Melville, N.Y.)
Kasper S Kjær, Wenkai Zhang, Roberto Alonso-Mori, Uwe Bergmann, Matthieu Chollet, Ryan G Hadt, Robert W Hartsock, Tobias Harlang, Thomas Kroll, Katharina Kubiček, Henrik T Lemke, Huiyang W Liang, Yizhu Liu, Martin M Nielsen, Joseph S Robinson, Edward I Solomon, Dimosthenis Sokaras, Tim B van Driel, Tsu-Chien Weng, Diling Zhu, Petter Persson, Kenneth Wärnmark, Villy Sundström, Kelly J Gaffney
We have used femtosecond resolution UV-visible and Kβ x-ray emission spectroscopy to characterize the electronic excited state dynamics of [Fe(bpy)2(CN)2], where bpy=2,2'-bipyridine, initiated by metal-to-ligand charge transfer (MLCT) excitation. The excited-state absorption in the transient UV-visible spectra, associated with the 2,2'-bipyridine radical anion, provides a robust marker for the MLCT excited state, while the transient Kβ x-ray emission spectra provide a clear measure of intermediate and high spin metal-centered excited states...
July 2017: Structural Dynamics (Melville, N.Y.)
Michael Zürch, Hung-Tzu Chang, Peter M Kraus, Scott K Cushing, Lauren J Borja, Andrey Gandman, Christopher J Kaplan, Myoung Hwan Oh, James S Prell, David Prendergast, Chaitanya D Pemmaraju, Daniel M Neumark, Stephen R Leone
Semiconductor alloys containing silicon and germanium are of growing importance for compact and highly efficient photonic devices due to their favorable properties for direct integration into silicon platforms and wide tunability of optical parameters. Here, we report the simultaneous direct and energy-resolved probing of ultrafast electron and hole dynamics in a silicon-germanium alloy with the stoichiometry Si0.25Ge0.75 by extreme ultraviolet transient absorption spectroscopy. Probing the photoinduced dynamics of charge carriers at the germanium M4,5-edge (∼30 eV) allows the germanium atoms to be used as reporter atoms for carrier dynamics in the alloy...
July 2017: Structural Dynamics (Melville, N.Y.)
Sang Tae Park, Renske M van der Veen
In this article, we present a continuum mechanics based approach for modeling thermally induced single-nanoparticle phase transitions studied in ultrafast electron microscopy. By using coupled differential equations describing heat transfer and the kinetics of the phase transition, we determine the major factors governing the time scales and efficiencies of thermal switching in individual spin-crossover nanoparticles, such as the thermal properties of the (graphite) substrate, the particle thickness, and the interfacial thermal contact conductance between the substrate and the nanoparticle...
July 2017: Structural Dynamics (Melville, N.Y.)
John C H Spence
Toward the end of his career, Zewail developed strong interest in fast electron spectroscopy and imaging, a field to which he made important contributions toward his aim of making molecular movies free of radiation damage. We therefore compare here the atomistic mechanisms leading to destruction of protein samples in diffract-and-destroy experiments for the cases of high-energy electron beam irradiation and X-ray laser pulses. The damage processes and their time-scales are compared and relevant elastic, inelastic, and photoelectron cross sections are given...
July 2017: Structural Dynamics (Melville, N.Y.)
P Musumeci, D Cesar, J Maxson
In this paper, we study by numerical simulations a time-resolved MeV electron scattering mode where two consecutive electron pulses are used to capture the evolution of a material sample on 10 ps time scales. The two electron pulses are generated by illuminating a photocathode in a radiofrequency photogun by two short laser pulses with adjustable delay. A streak camera/deflecting cavity is used after the sample to project the two electron bunches on two well separated regions of the detector screen. By using sufficiently short pulses, the 2D spatial information from each snapshot can be preserved...
July 2017: Structural Dynamics (Melville, N.Y.)
J G H Franssen, O J Luiten
The temporal resolution of sub-relativistic ultrafast electron diffraction (UED) is generally limited by the radio frequency (RF) phase and amplitude jitter of the RF lenses that are used to compress the electron pulses. We theoretically show how to circumvent this limitation by using a combination of several RF compression cavities. We show that if powered by the same RF source and with a proper choice of RF field strengths, RF phases, and distances between the cavities, the combined arrival time jitter due to RF phase jitter of the cavities is cancelled at the compression point...
July 2017: Structural Dynamics (Melville, N.Y.)
Gero Storeck, Simon Vogelgesang, Murat Sivis, Sascha Schäfer, Claus Ropers
We present the design and fabrication of a micrometer-scale electron gun for the implementation of ultrafast low-energy electron diffraction from surfaces. A multi-step process involving photolithography and focused-ion-beam nanostructuring is used to assemble and electrically contact the photoelectron gun, which consists of a nanotip photocathode in a Schottky geometry and an einzel lens for beam collimation. We characterize the low-energy electron pulses by a transient electric field effect and achieve pulse durations of 1...
July 2017: Structural Dynamics (Melville, N.Y.)
Young Min Lee, Young Jae Kim, Ye-Jin Kim, Oh-Hoon Kwon
In the past decade, we have witnessed the rapid growth of the field of ultrafast electron microscopy (UEM), which provides intuitive means to watch atomic and molecular motions of matter. Yet, because of the limited current of the pulsed electron beam resulting from space-charge effects, observations have been mainly made to periodic motions of the crystalline structure of hundreds of nanometers or higher by stroboscopic imaging at high repetition rates. Here, we develop an advanced UEM with robust capabilities for circumventing the present limitations by integrating a direct electron detection camera for the first time which allows for imaging at low repetition rates...
July 2017: Structural Dynamics (Melville, N.Y.)
Omid Zandi, Kyle J Wilkin, Yanwei Xiong, Martin Centurion
We have constructed an experimental setup for gas phase electron diffraction with femtosecond resolution and a high average beam current. While gas electron diffraction has been successful at determining molecular structures, it has been a challenge to reach femtosecond resolution while maintaining sufficient beam current to retrieve structures with high spatial resolution. The main challenges are the Coulomb force that leads to broadening of the electron pulses and the temporal blurring that results from the velocity mismatch between the laser and electron pulses as they traverse the sample...
July 2017: Structural Dynamics (Melville, N.Y.)
Benjamin E Van Kuiken, Matthew R Ross, Matthew L Strader, Amy A Cordones, Hana Cho, Jae Hyuk Lee, Robert W Schoenlein, Munira Khalil
Picosecond X-ray absorption (XA) spectroscopy at the S K-edge (∼2.4 keV) is demonstrated and used to monitor excited state dynamics in a small organosulfur molecule (2-Thiopyridone, 2TP) following optical excitation. Multiple studies have reported that the thione (2TP) is converted into the thiol (2-Mercaptopyridine, 2MP) following photoexcitation. However, the timescale and photochemical pathway of this reaction remain uncertain. In this work, time-resolved XA spectroscopy at the S K-edge is used to monitor the formation and decay of two transient species following 400 nm excitation of 2TP dissolved in acetonitrile...
July 2017: Structural Dynamics (Melville, N.Y.)
L Le Guyader, T Chase, A H Reid, R K Li, D Svetin, X Shen, T Vecchione, X J Wang, D Mihailovic, H A Dürr
Transitions between different charge density wave (CDW) states in quasi-two-dimensional materials may be accompanied also by changes in the inter-layer stacking of the CDW. Using MeV ultrafast electron diffraction, the out-of-plane stacking order dynamics in the quasi-two-dimensional dichalcogenide 1T-TaS2 is investigated for the first time. From the intensity of the CDW satellites aligned around the commensurate l = 1/6 characteristic stacking order, it is found out that this phase disappears with a 0...
July 2017: Structural Dynamics (Melville, N.Y.)
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