Normalized single-shot X-ray absorption spectroscopy at a free-electron laser (original) (raw)
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Proceedings of the …, 2012
The ultrabright femtosecond X-ray pulses provided by X-ray freeelectron lasers open capabilities for studying the structure and dynamics of a wide variety of systems beyond what is possible with synchrotron sources. Recently, this "probe-before-destroy" approach has been demonstrated for atomic structure determination by serial X-ray diffraction of microcrystals. There has been the question whether a similar approach can be extended to probe the local electronic structure by X-ray spectroscopy. To address this, we have carried out femtosecond X-ray emission spectroscopy (XES) at the Linac Coherent Light Source using redox-active Mn complexes. XES probes the charge and spin states as well as the ligand environment, critical for understanding the functional role of redox-active metal sites. Kβ 1,3 XES spectra of Mn II and Mn 2 III,IV complexes at room temperature were collected using a wavelength dispersive spectrometer and femtosecond X-ray pulses with an individual dose of up to >100 MGy. The spectra were found in agreement with undamaged spectra collected at low dose using synchrotron radiation. Our results demonstrate that the intact electronic structure of redox active transition metal compounds in different oxidation states can be characterized with this shotby-shot method. This opens the door for studying the chemical dynamics of metal catalytic sites by following reactions under functional conditions. The technique can be combined with X-ray diffraction to simultaneously obtain the geometric structure of the overall protein and the local chemistry of active metal sites and is expected to prove valuable for understanding the mechanism of important metalloproteins, such as photosystem II. energy-dispersive XES | Kβ emission lines | femtosecond x-ray spectroscopy T he first X-ray free-electron laser (XFEL) operating in the hard X-ray regime (1), the Linac Coherent Light Source (LCLS), produces ∼5to 400-fs X-ray pulses with up to ∼10 12 photons per pulse at 6-10 keV at a repetition rate of 120 Hz. Each of these X-ray pulses is intense enough to expel multiple electrons from the target, which can lead to a Coulomb explosion that destroys the sample. In a shot-by-shot experiment, data are collected from each pulse before the destruction of the sample, and the sample is replenished after each pulse. The feasibility of this "probebefore-destroy" approach for X-ray crystallography experiments using XFEL pulses was first demonstrated by Chapman et al. with various systems and has subsequently been corroborated by others at the LCLS (2-6).
Few-femtosecond time-resolved measurements of X-ray free-electron lasers
Nature Communications, 2014
X-ray free-electron lasers, with pulse durations ranging from a few to several hundred femtoseconds, are uniquely suited for studying atomic, molecular, chemical and biological systems. Characterizing the temporal profiles of these femtosecond X-ray pulses that vary from shot to shot is not only challenging but also important for data interpretation. Here we report the time-resolved measurements of X-ray free-electron lasers by using an X-band radiofrequency transverse deflector at the Linac Coherent Light Source. We demonstrate this method to be a simple, non-invasive technique with a large dynamic range for single-shot electron and X-ray temporal characterization. A resolution of less than 1 fs root mean square has been achieved for soft X-ray pulses. The lasing evolution along the undulator has been studied with the electron trapping being observed as the X-ray peak power approaches 100 GW.
Ultrafast X-ray pulse characterization at free-electron lasers
Nature Photonics, 2012
The ability to fully characterize ultrashort, ultra-intense X-ray pulses at free-electron lasers (FELs) will be crucial in experiments ranging from single-molecule imaging to extreme-timescale X-ray science. This issue is especially important at current-generation FELs, which are primarily based on self-amplified spontaneous emission and radiate with parameters that fluctuate strongly from pulse to pulse. Using single-cycle terahertz pulses from an optical laser, we have extended the streaking techniques of attosecond metrology to measure the temporal profile of individual FEL pulses with 5 fs full-width at half-maximum accuracy, as well as their arrival on a time base synchronized to the external laser to within 6 fs r.m.s. Optical laser-driven terahertz streaking can be utilized at any X-ray photon energy and is non-invasive, allowing it to be incorporated into any pump-probe experiment, eventually characterizing pulses before and after interaction with most sample environments.
Scientific reports, 2014
The rapidly growing ultrafast science with X-ray lasers unveils atomic scale processes with unprecedented time resolution bringing the so called "molecular movie" within reach. X-ray absorption spectroscopy is one of the most powerful x-ray techniques providing both local atomic order and electronic structure when coupled with ad-hoc theory. Collecting absorption spectra within few x-ray pulses is possible only in a dispersive setup. We demonstrate ultrafast time-resolved measurements of the LIII-edge x-ray absorption near-edge spectra of irreversibly laser excited Molybdenum using an average of only few x-ray pulses with a signal to noise ratio limited only by the saturation level of the detector. The simplicity of the experimental set-up makes this technique versatile and applicable for a wide range of pump-probe experiments, particularly in the case of non-reversible processes.
Nature Photonics, 2014
Short-wavelength free-electron lasers (FELs) are now well established as essential and unrivalled sources of ultrabright coherent X-ray radiation. One of the key characteristics of these intense Xray pulses is their expected few-femtosecond duration. These properties pave the way for the highly anticipated investigations of a plethora of phenomena on the atomic scale in space and time, ranging from single-atom processes such as inner-shell multiple ionization all the way to biological systems, such as three-dimensional studies of viral genomes, diffraction tomography of whole cells and the dynamics of photosynthetic reactions. A prerequisite for any kind of timeresolved measurement is the precise characterization of the X-ray pulse duration. However, so far no measurement has succeeded in directly determining the temporal structure or even the duration of ultrashort FEL pulses in the few-femtosecond range. By deploying the so-called 'Streaking Spectroscopy' technique at the Linac Coherent Light Source free-electron laser we have been able to demonstrate a non-invasive scheme for temporal characterization of X-ray pulses with sub-femtosecond resolution. This method is independent of photon energy, decoupled from machine parameters and provides an upper bound on the X-ray pulse duration. Thus, we measured the duration of the shortest X-ray pulses available today to be on average not longer than 4.4 femtoseconds. In addition, an analysis of the pulse substructure indicates that the duration of a small percentage of the FEL pulses consisting of individual high-intensity spikes is of the order of only hundreds of attoseconds.