Explore the vast range of titles available.

Upon exposure to ultra-intense, hard X-ray pulses, polyatomic molecules containing one heavy atom reach a much higher degree of ionization than do individual heavy atoms, contrary to previous assumptions. Ultrafast molecular response to intense X-rays X-ray free-electron lasers offer many new applications such as the ability to structurally probe fast biological processes. This requires the use of hard and intense X-ray pulses, but the behaviour of matter under such conditions has not been fully explored. Artem Rudenko et al . show that when exposing small polyatomic molecules that contain one heavy atom to hard X-ray pulses with ultra-high intensities, the response is qualitatively different from what is seen in experiments carried out under less extreme conditions. The observed ionization of the molecule is considerably enhanced compared to that of an individual heavy atom under the same conditions, owing to ultrafast charge transfer within the molecule that replenishes the electrons removed from the heavy atom, enabling further ionization. Being able to account for this effect will aid further use of X-ray free-electron lasers for studying biological systems. X-ray free-electron lasers enable the investigation of the structure and dynamics of diverse systems, including atoms, molecules, nanocrystals and single bioparticles, under extreme conditions 1 , 2 , 3 , 4 , 5 , 6 , 7 . Many imaging applications that target biological systems and complex materials use hard X-ray pulses with extremely high peak intensities (exceeding 10 20 watts per square centimetre) 3 , 5 . However, fundamental investigations have focused mainly on the individual response of atoms and small molecules using soft X-rays with much lower intensities 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 . Studies with intense X-ray pulses have shown that irradiated atoms reach a very high degree of ionization, owing to multiphoton absorption 8 , 12 , 13 , 18 , which in a heteronuclear molecular system occurs predominantly locally on a heavy atom (provided that the absorption cross-section of the heavy atom is considerably larger than those of its neighbours) and is followed by efficient redistribution of the induced charge 14 , 15 , 16 , 17 , 19 , 20 . In serial femtosecond crystallography of biological objects—an application of X-ray free-electron lasers that greatly enhances our ability to determine protein structure 2 , 3 —the ionization of heavy atoms increases the local radiation damage that is seen in the diffraction patterns of these objects 21 , 22 and has been suggested as a way of phasing the diffraction data 23 , 24 . On the basis of experiments using either soft or less-intense hard X-rays 14 , 15 , 16 , 17 , 18 , 19 , 25 , it is thought that the induced charge and associated radiation damage of atoms in polyatomic molecules can be inferred from the charge that is induced in an isolated atom under otherwise comparable irradiation conditions. Here we show that the femtosecond response of small polyatomic molecules that contain one heavy atom to ultra-intense (with intensities approaching 10 20 watts per square centimetre), hard (with photon energies of 8.3 kiloelectronvolts) X-ray pulses is qualitatively different: our experimental and modelling results establish that, under these conditions, the ionization of a molecule is considerably enhanced compared to that of an individual heavy atom with the same absorption cross-section. This enhancement is driven by ultrafast charge transfer within the molecule, which refills the core holes that are created in the heavy atom, providing further targets for inner-shell ionization and resulting in the emission of more than 50 electrons during the X-ray pulse. Our results demonstrate that efficient modelling of X-ray-driven processes in complex systems at ultrahigh intensities is feasible.

Sub-picosecond magnetisation manipulation via femtosecond optical pumping has attracted wide attention ever since its original discovery in 1996. However, the spatial evolution of the magnetisation is not yet well understood, in part due to the difficulty in experimentally probing such rapid dynamics. Here, we find evidence of a universal rapid magnetic order recovery in ferrimagnets with perpendicular magnetic anisotropy via nonlinear magnon processes. We identify magnon localisation and coalescence processes, whereby localised magnetic textures nucleate and subsequently interact and grow in accordance with a power law formalism. A hydrodynamic representation of the numerical simulations indicates that the appearance of noncollinear magnetisation via optical pumping establishes exchange-mediated spin currents with an equivalent 100% spin polarised charge current density of 10 7  A cm −2 . Such large spin currents precipitate rapid recovery of magnetic order after optical pumping. The magnon processes discussed here provide new insights for the stabilization of desired meta-stable states. The understanding of the magnetisation evolution upon femtosecond optical pumping remains elusive. The authors perform resonant X-ray magnetic scattering measurements and multiscale simulations that reveal rapid magnetic order recovery in ferrimagnets via nonlinear magnon processes.

Some X-ray free-electron laser facilities are pushing towards sub-10 fs pulses, making it desirable to reduce errors in X-ray/optical delay measurements to the 1 fs level. Researchers have now demonstrated X-ray measurements with a temporal resolution shorter than 1 fs, opening up new possibilities for time-resolved X-ray experiments. Today's brightest coherent X-ray sources, X-ray free-electron lasers, produce ultrafast X-ray pulses for which full-width at half-maximum durations as short as 3 fs have been measured 1 . There has been a marked increase in the popularity of such short pulses now that optical timing techniques have begun to report an X-ray/optical delay below ∼10 fs r.m.s. errors. As a result, sub-10 fs optical pulses have been implemented at the Linac Coherent Light Source (LCLS) X-ray beamlines, thus warranting a push to reduce the error in X-ray/optical delay measurements to the 1 fs level. Here, we report a unique two-dimensional spectrogram measurement of the relative X-ray/optical delay. This easily scalable relative delay measurement already surpasses previous techniques by an order of magnitude with its sub-1 fs temporal resolution and opens up the prospect of time-resolved X-ray measurements to the attosecond community.

Intense X-ray free-electron lasers (XFELs) can rapidly excite matter, leaving it in inherently unstable states that decay on femtosecond timescales. The relaxation occurs primarily via Auger emission, so excited-state observations are constrained by Auger decay. In situ measurement of this process is therefore crucial, yet it has thus far remained elusive in XFELs owing to inherent timing and phase jitter, which can be orders of magnitude larger than the timescale of Auger decay. Here we develop an approach termed ‘self-referenced attosecond streaking’ that provides subfemtosecond resolution in spite of jitter, enabling time-domain measurement of the delay between photoemission and Auger emission in atomic neon excited by intense, femtosecond pulses from an XFEL. Using a fully quantum-mechanical description that treats the ionization, core-hole formation and Auger emission as a single process, the observed delay yields an Auger decay lifetime of 2.2−0.3+0.2 fs for the KLL decay channel.Self-referenced attosecond streaking enables in situ measurements of Auger emission in atomic neon excited by femtosecond pulses from an X-ray free-electron laser with subfemtosecond time resolution and despite the jitter inherent to X-ray free-electron lasers.

In this study, nano fibrillated kenaf cellulose (NFKC) derived from kenaf fiber after varying chemico-mechanical treatments were introduced into poly lactic acid (PLA) as reinforcements to improve the mechanical and morphological properties of the biocomposites. The new strategy was aiming to realize the synergistic effects of chemical treatment and mechanical fibrillation process parameters (blending speed and time) for yielding nano fibers and its reinforcement effects on the properties of biocomposites. The yield percentage of the NFKC was determined using centrifugal method and the NFKC fibers with PLA pellet were hot pressed to form NFKC-PLA composites. The distribution and dispersion morphologies of NFKC in NFKC-PLA composites were observed by using optical microscope (OM) and scanning electron microscope (SEM). The reinforcing effect on the mechanical properties of NFKC-PLA composite was investigated by tensile strength test. Average length and diameter of fibrillated fibers were decreased with the concurrent increase of blending speed and time. The maximum increase in tensile strength of 59.32% and elongation of 100% were observed for NFKC-PLA composite with NFKC yielded at a blending speed and time of 15000 rpm and 15 minutes as compared to pure PLA. The tensile properties indicated that the strength and modulus were improved with increased nanofiber contents.

Synopsis The time resolved photoionization of C 1s in uracil following excitation of the neutral molecule by 260 nm pulses has been studied at LCLS.

We report x-ray free-electron lasers enable the investigation of the structure and dynamics of diverse systems, including atoms, molecules, nanocrystals and single bioparticles, under extreme conditions. Many imaging applications that target biological systems and complex materials use hard X-ray pulses with extremely high peak intensities (exceeding 1020 watts per square centimetre). However, fundamental investigations have focused mainly on the individual response of atoms and small molecules using soft X-rays with much lower intensities. Studies with intense X-ray pulses have shown that irradiated atoms reach a very high degree of ionization, owing to multiphoton absorption, which in a heteronuclear molecular system occurs predominantly locally on a heavy atom (provided that the absorption cross-section of the heavy atom is considerably larger than those of its neighbours) and is followed by efficient redistribution of the induced charge. In serial femtosecond crystallography of biological objects—an application of X-ray free-electron lasers that greatly enhances our ability to determine protein structure—the ionization of heavy atoms increases the local radiation damage that is seen in the diffraction patterns of these objects and has been suggested as a way of phasing the diffraction data. On the basis of experiments using either soft or less-intense hard X-rays, it is thought that the induced charge and associated radiation damage of atoms in polyatomic molecules can be inferred from the charge that is induced in an isolated atom under otherwise comparable irradiation conditions. Here we show that the femtosecond response of small polyatomic molecules that contain one heavy atom to ultra-intense (with intensities approaching 1020 watts per square centimetre), hard (with photon energies of 8.3 kiloelectronvolts) X-ray pulses is qualitatively different: our experimental and modelling results establish that, under these conditions, the ionization of a molecule is considerably enhanced compared to that of an individual heavy atom with the same absorption cross-section. This enhancement is driven by ultrafast charge transfer within the molecule, which refills the core holes that are created in the heavy atom, providing further targets for inner-shell ionization and resulting in the emission of more than 50 electrons during the X-ray pulse. Fnally, our results demonstrate that efficient modelling of X-ray-driven processes in complex systems at ultrahigh intensities is feasible.

Background While involuntary motor dysfunction is most commonly used as the hallmark of manifest Huntington's disease (HD), cognitive functions are known to decline decades before the presence of motor symptoms. Aims We used a spatial working memory task (n-back) to investigate functional brain changes in presymptomatic HD (pre-HD) and early symptomatic HD (symp-HD), compared with controls, via fMRI. Methods 35 pre-HD (UHDRS <5), 23 symp-HD (UHDRS ≥5) and 32 controls participated. During baseline (0-back), participants reported the location of the current stimulus, while the 1-back and 2-back conditions required report of stimulus location presented 1 or 2 screens back, respectively. Data were analysed with FSL's FEAT. FLAME was used to conduct group analyses. Z statistic images were thresholded at Z >2.3 and a corrected cluster significance of p<0.05. Results No group differences across conditions for behavioural data. fMRI data revealed a common network of activity when comparing 1-back to 0-back conditions with groups displaying significant increases in middle frontal gyrus, insula, middle temporal gyri, precentral gyri, parietal lobe and cerebellum. Between group differences revealed significant increases in BOLD signal in the caudate and putamen, left insula and left superior temporal gyrus in controls compared with pre-HD. Compared with symp-HD, pre-HD showed significant increases in the DL-PFC and cerebellum. We subsequently performed time series analyses to assess per cent BOLD signal change over time during task performance. Differential patterns of brain activation over time were observed across groups in DL-PFC, thalamus, anterior cingulate, hippocampus, insula and caudate. Conclusions Pre-HD, symp-HD and controls show differential patterns of both functional BOLD activation and per cent BOLD signal changes during task performance. Variable activation patterns indicate crucial time points during the neurodegenerative process of HD involving onset or worsening of more than one pathological process.