Time Structure Of Synchrotron Radiation Research Articles

Nanoparticles and nanowires: synchrotron spectroscopy studies

This paper reviews the research in nanomaterials conducted in our laboratory in the last decade using conventional and synchrotron radiation techniques. While preparative and conventional characterisation techniques are described, emphasis is placed on the analysis of nanomaterials using synchrotron radiation. Materials of primary interests are metal nanoparticles and semiconductor nanowires and naonribbons. Synchrotron techniques based on absorption spectroscopy such as X-ray absorption fine structures (XAFS), which includes X-ray absorption near edge structures (XANES) and extended X-ray absorption fine structures (EXFAS), and de-excitation spectroscopy, including X-ray excited optical luminescence (XEOL), time-resolved X-ray excited optical luminescence (TRXEOL) and X-ray emission spectroscopy (XES) are described. We show that the tunability, brightness, polarisation and time structure of synchrotron radiation are providing unprecedented capabilities for nanomaterials analysis. Synchrotron studies of prototype systems such as gold nanoparticles, 1-D nanowires of group IV materials, C, Si and Ge as well as nanodiamond, and compound semiconductors, ZnS, CdS, ZnO and related materials are used to illustrate the power and unique capabilities of synchrotron spectroscopy in the characterisation of local structure, electronic structure and optical properties of nanomaterials.

Software defined photon counting system for time resolved x-ray experiments

The time structure of synchrotron radiation allows time resolved experiments with sub-100 ps temporal resolution using a pump-probe approach. However, the relaxation time of the samples may require a lower repetition rate of the pump pulse compared to the full repetition rate of the x-ray pulses from the synchrotron. The use of only the x-ray pulse immediately following the pump pulse is not efficient and often requires special operation modes where only a few buckets of the storage ring are filled. We designed a novel software defined photon counting system that allows to implement a variety of pump-probe schemes at the full repetition rate. The high number of photon counters allows to detect the response of the sample at multiple time delays simultaneously, thus improving the efficiency of the experiment. The system has been successfully applied to time resolved scanning transmission x-ray microscopy. However, this technique is applicable more generally.

PEEM with high time resolution—imaging of transient processes and novel concepts of chromatic and spherical aberration correction

AbstractThe potential of time‐resolved photoemission electron microscopy (PEEM) for imaging ultrafast processes and for aberration correction in full‐field imaging is discussed. In particular, we focus on stroboscopic imaging of precessional magnetic excitations via XMCD‐PEEM exploiting the time structure of synchrotron radiation (magnetic field pulse pump–X‐ray probe). In a special bunch‐compression mode at BESSY, a time resolution of about 15 ps has been obtained. Further, we discuss an all‐optical pump–probe technique using femtosecond laser excitation. A highly promising alternative to stroboscopic imaging is an approach using time‐resolved image detection. As a second application of time‐resolved PEEM we discuss potential ways of aberration correction. These approaches go back to the old ideas of Scherzer in the light of state‐of‐the‐art equipment. The excellent time structure of synchrotron radiation or pulsed lasers along with advanced methods of time‐resolved image detection and fast electronic pulsers opens ways for driving the resolution limit of a PEEM into the range of a few nanometers. Copyright © 2006 John Wiley & Sons, Ltd.

Progress in the application of synchrotron radiation in chemistry and biology

It is well known that the use of synchrotron radiation has revolutionised structural biology and is responsible for the exponential growth of known macromolecular structures in the Protein Data Bank. So onemaywonder if continued progress is possible. The recent developments of beamlines for macromolecular crystallography at the ESRF have had the purpose to make them highly automated. This has enabled the users to screen a multiplum of samples to find the best suited crystal for data collection. The use of crystallization robots tends to yield smaller crystals, which creates an increasing demand for beamlines with small beams to be used for the diffraction experiments. These developmentsmake it possible to study larger andmore complex biological systems, bringing the structural biology closer to the studies of soft condensed matter. We can also observe an increasing interest to complement the diffraction studies with complementarymethods like small angle scattering and imaging. The use of nanometer sized X-ray beams is also in demand for investigations of chemical systems. The use of nanosized beams opens possibilities for an unprecedented spatial resolution that open new scientific possibilities inmaterials science, soft matter chemistry and the characterization of complex chemical systems. The possibility of conducting the experiments at the synchrotron on samples under different thermodynamic conditions has stimulated an avenue of studies of chemical systems under extreme conditions. There is a similar interest in employing the time structure of synchrotron radiation in time resolved studies. The trends in the application of synchrotron radiation in chemistry and biology will be described and illustrated by recent scientific achievements [1].

Charge, spin and momentum densities: different faces of this same nature

It is well known that the use of synchrotron radiation has revolutionised structural biology and is responsible for the exponential growth of known macromolecular structures in the Protein Data Bank. So onemaywonder if continued progress is possible. The recent developments of beamlines for macromolecular crystallography at the ESRF have had the purpose to make them highly automated. This has enabled the users to screen a multiplum of samples to find the best suited crystal for data collection. The use of crystallization robots tends to yield smaller crystals, which creates an increasing demand for beamlines with small beams to be used for the diffraction experiments. These developmentsmake it possible to study larger andmore complex biological systems, bringing the structural biology closer to the studies of soft condensed matter. We can also observe an increasing interest to complement the diffraction studies with complementarymethods like small angle scattering and imaging. The use of nanometer sized X-ray beams is also in demand for investigations of chemical systems. The use of nanosized beams opens possibilities for an unprecedented spatial resolution that open new scientific possibilities inmaterials science, soft matter chemistry and the characterization of complex chemical systems. The possibility of conducting the experiments at the synchrotron on samples under different thermodynamic conditions has stimulated an avenue of studies of chemical systems under extreme conditions. There is a similar interest in employing the time structure of synchrotron radiation in time resolved studies. The trends in the application of synchrotron radiation in chemistry and biology will be described and illustrated by recent scientific achievements [1].

Implementing sub-ns time resolution into magnetic x-ray microscopies

X-ray microscopies using polarized x-rays allow for imaging magnetic microstructures down to a 20 nm lateral resolution with chemical sensitivity. Utilizing the pulsed time structure of synchrotron radiation a sub-ns time resolution can be implemented by performing stroboscopic pump-and-probe experiments. The temporal evolution of the magnetization in a 50 nm thin circular permalloy element with a diameter of 2 μm could be imaged with magnetic transmission x-ray microscopy. The magnetic pump pulse to the sample was generated electronically with a rise time of less than 100 ps up to 150 Oe by a microcoil. The probe pulse is the flash of the circularly polarized x-ray beam with a pulse width of about 70 ps at a frequency of 3 MHz. Images were recorded at varying delay times between pump and probe pulse up to 2000 ps. Fast detection schemes utilizing avalanche photo diodes are capable to pick individual bunches from the storage ring.

Time and layer resolved magnetic domain imagig of FeNi/Cu/Co trilayers using x-ray photoelectron emission microscopy (invited)

We have performed magnetic domain imaging with spatial, temporal, and layer resolution using x-ray photoelectron emission microscopy. The element selectivity of x-ray magnetic circular dichroism allows the magnetization dynamics of the different magnetic layers in spin-valve-like FeNi/Cu/Co trilayers to be studied separately, using the time structure of synchrotron radiation. The unique possibilities of this technique have been used to study the influence of the intrinsic magnetic properties of the different layers on the magnetization dynamics and the interlayer magnetic coupling.

Time-of-flight photoemission electron microscopy – a new way to chemical surface analysis

The time structure of synchrotron radiation at BESSY I (Berlin) was utilised to operate a photoemission electron microscope in the time-of-flight mode. The electrons that are emitted from the sample surface with different energies are dispersed in a drift tube subsequent to the imaging optics. Two ways of fast image detection have been explored, a fast gated intensified CCD camera (800 ps gate time) and a special counting electronics in combination with a 3D ( x, y, t)-resolving delay line detector ( time resolution <500 ps). The latter device has a lateral resolution of about 50 μm in the image plane being equivalent to 1000 pixels along the image diagonal. An energy resolution of 400 meV has been achieved. The future potential of time-resolving photoemission microscopy is discussed.

Element-selective nanosecond magnetization dynamics in magnetic heterostructures.

We have developed a new original technique to study the magnetization reversal dynamics of thin films with element selectivity in the nanosecond time scale. X-ray magnetic circular dichroism measurements in pump-probe mode are carried out taking advantage of the time structure of synchrotron radiation. The dynamics of the magnetization reversal of each of the layers of complex heterostructures (like spin valves or tunnel junctions) can be probed independently. The interlayer coupling in the studied systems has been shown to play a key role in the determination of the magnetization reversal of each individual layer.

Far-infrared pump-probe measurement of an organic semiconductor β'-(BEDT-TTF)2ICl2using synchrotron radiation source

We have constructed a far-infrared pump-probe measurement system employing a time structure of synchrotron radiation (SR) source at UVSOR in the Institute for Molecular Science (IMS). The system consists of the SR source (operating at 90.115 MHz), a Michelson-type rapid scan interferometer, a silicon-composite bolometer detector, and a mode-locked Nd:YAG laser that is synchronized to the SR pulses. The system achieved full synchronization between the laser and the SR pulses without reducing the number of electron bunches in the storage ring, and can trace phenomena with pump/probe delays between 1ns (the SR pulse width) and 11ns (the interbunch separation). We chose in this study an organic semiconductor β'-(BEDT-TTF)2ICl2 as a candidate for pump-probe sample. In the β'-type crystal BEDT-TTF molecule are stacked along the b-axis, forming strongly dimerized chains. The E∣b optical conductivity derived from the polarized reflectance showed a strong absorption at 456 cm-1 (Ag mode) as a result of electron-molecular-vibration (EMV) coupling in the dimeric structure. A definite SR delay-time dependence of order nanoseconds was found in the frequency range 470-480 cm-1, possibly related to this Ag mode. This suggests that the EMV dynamics might extend to nanosecond time scale.

Extraction of single bunches of synchrotron radiation from storage rings with an X-ray chopper based on a rotating mirror

An ultrafast shutter has been developed for alteration of the time structure of synchrotron radiation from storage rings in the hard X-ray regime. In test applications on the wiggler beamline BW6 at DORIS, single bunches were extracted from the incident pulsed synchrotron radiation with minimum bunch-to-bunch distances of 482 ns. Even substantially shorter time windows may be defined in the case of tight collimation in the incident beam, e.g. on low-emittance sources. The shutter system is based on a new chopper concept involving a rotating X-ray mirror which totally reflects the incident radiation onto the sample through a remote slit. Rather low rotational velocities are sufficient to reach extremely short full open times. An additional shutter consisting of a slowly rotating disk prevents frame overlap and controls the repetition rate. A coincidence timing circuit checks the synchronization with the synchrotron bunch clock and provides trigger signals, e.g. for external excitation of a sample. The chopper system may be used, for example, in nanosecond time-resolved Laue diffraction experiments.

Time-resolved structures of macromolecules at the ESRF: Single-pulse Laue diffraction, stroboscopic data collection and femtosecond flash photolysis

We review the time structure of synchrotron radiation and its use for fast time-resolved diffraction experiments in macromolecular photocycles using flash photolysis to initiate the reaction. The source parameters and optics for ID09 at ESRF are presented together with the phase-locked chopper and femtosecond laser. The chopper can set up a 900 Hz pulse train of 100 ps pulses from the hybrid bunch-mode and, in conjunction with a femtosecond laser, it can be used for stroboscopic data collection with both monochromatic and polychromatic beams. Single-pulse Laue data from cutinase, a 22 kD lipolic enzyme, are presented which show that the quality of single-pulse Laue patterns are sufficient to refine the excited state(s) in a reaction pathway from a known ground state. The flash photolysis technique is discussed and an example is given for heme proteins. The radiation damage from a laser pulse in the femto and picosecond range can be reduced by triggering at a wavelength where the interaction is strong. We propose the use of microcrystals in the range 25–50 μm for efficient photolysis with femto and picosecond pulses. The performance of circular storage rings is compared with the predicted performance of an X-ray free electron laser (XFEL). The combination of micro beams, a gain of 10 5 photons per pulse and an ultrashort pulse length of 100 fs is likely to improve pulsed diffraction data very substantially. It may be used to image coherent nuclear motion at atomic resolution in ultrafast uni-molecular reactions.

Apparatus for the measurement of the electronic excited-state structure of single crystals using X-ray diffraction

Instrumentation for measuring the X-ray diffraction pattern of optically excited crystals is described. The experiment uses a high-power (~1 W) laser and a single-crystal diffractometer equipped with a helium cryostat (T &lt; 70 K). The laser beam is modulated by a mechanical chopper and the diffraction signal gated in synchronization with the chopper phase. The modulation method is capable of observing small changes (down to about 0.01%) in the structure factors upon excitation of a fraction of the molecules in the crystal, given adequate counting statistics. The technique can be used for relatively long lived electronic excited states (τ ≃ 0.1–10 ms). The optical system is also suitable for time-resolved measurements using the time structure of synchrotron radiation.

Radiation damage to free adenosine triphosphate and guanine molecules by monochromatic x rays

A summary description of the equipment for time-of-flight (TOF) mass spectroscopy and its uses in the x-ray region for the study of radiation damage (photofragmentation) of biologically relevant molecules is given. The technique is illustrated with some recent x-ray (∼8.1 keV) TOF results obtained from free adenosine triphosphate and guanine molecules. The well defined time structure of synchrotron radiation has been employed in order to record the molecular decomposition in real time. It has, thereby, been possible to determine which bonds are most susceptible to bond breakage following irradiation of the molecules by x rays.

The dynamics of adiabatic photoreactions as studied by means of the time structure of synchrotron radiation

The decay of the dual fluorescence of the para-substituted dialkylanilines (esters and nitriles) as well as the fluorescence decay of several triphenylmethane (TPM) dyes is investigated. The esters relax several times faster than the nitriles, and in the TPM series, relaxation is accelerated by increasing the substituent acceptor strength. In both cases, non-exponential decay behaviour is observed. It is more pronounced for the compounds with stronger driving forces along the reaction pathways. Freezing of the relaxation process transforms the decay into a mono-exponential one. This is discussed in terms of two models, namely by a log-gaussian distribution of relaxation times, and by a non-radiative sink model.

Time Resolved Spectroscopy Using Synchrotron Radiation

The time structure of synchrotron radiation from a single bunch of high-energy electrons circulating in a storage ring is readily exploited for studies of fast kinetic and radiative processes involving short-lived electronically-excited species in bulk matter. The sample is excited repetitively by very short bursts of monochromatized synchrotron radiation once each orbital period, as the radiation pattern associated with the radially accelerating electron bunch sweeps past an entrance aperture. Time resolved measurements of the rise and fall of features in the afterglow fluorescence spectrum are analyzed to yield rates for the formation and decay of the radiating species.