The RATIO method for time‐resolved Laue crystallography

A RATIO method for analysis of intensity changes in time‐resolved pump–probe Laue diffraction experiments is described. The method eliminates the need for scaling the data with a wavelength curve representing the spectral distribution of the source and removes the effect of possible anisotropic absorption. It does not require relative scaling of series of frames and removes errors due to all but very short term fluctuations in the synchrotron beam.     

Quick X‐ray reflectivity using monochromatic synchrotron radiation for time‐resolved applications

A new technique for the parallel collection of X‐ray reflectivity (XRR) data, compatible with monochromatic synchrotron radiation and flat substrates, is described and applied to the in situ observation of thin‐film growth. The method employs a polycapillary X‐ray optic to produce a converging fan of radiation, incident onto a sample surface, and an area detector to simultaneously collect the XRR signal over an angular range matching that of the incident fan. Factors determining the range and instrumental resolution of the technique in reciprocal space, in addition to the signal‐to‐background ratio, are described in detail. This particular implementation records ～5° in 2gθ and resolves Kiessig fringes from samples with layer thicknesses ranging from 3 to 76 nm. The value of this approach is illustrated by showing in situ XRR data obtained with 100 ms time resolution during the growth of epitaxial La_0.7Sr_0.3MnO₃ on SrTiO₃ by pulsed laser deposition at the Cornell High Energy Synchrotron Source (CHESS). Compared with prior methods for parallel XRR data collection, this is the first method that is both sample‐independent and compatible with the highly collimated, monochromatic radiation typical of third‐generation synchrotron sources. Further, this technique can be readily adapted for use with laboratory‐based sources.     

Recent advances in ultrafast time‐resolved chirality measurements: perspective and outlook

Observing chirality changes as they occur is an important topic of research. It provides information that deepens the understanding of biomolecular configuration and conformation under environmental changes. Also, knowing the specific steps in chiral synthesis would simplify the production of specific chiral enantiomers that have a specific function. To gain better insight to the initial steps of conformational and configurational changes, the time‐resolution of chiral spectroscopy is continually pushed toward a shorter time‐scale. Recent advances have produced measurements of chirality changes with a femtosecond time‐resolution. These measurements are hindered by the inherently weak chirality signal, which can be overshadowed by different optical artefacts. This minireview will look at the so far successful techniques which measure chirality changes with femtosecond time‐resolution and discuss the advantages and disadvantages of these techniques. A short outlook will also look at new techniques that could improve the ability to measure chirality changes on an ultrafast time‐scale.     

BioCARS: a synchrotron resource for time‐resolved X‐ray science

BioCARS, a NIH‐supported national user facility for macromolecular time‐resolved X‐ray crystallography at the Advanced Photon Source (APS), has recently completed commissioning of an upgraded undulator‐based beamline optimized for single‐shot laser‐pump X‐ray‐probe measurements with time resolution as short as 100 ps. The source consists of two in‐line undulators with periods of 23 and 27 mm that together provide high‐flux pink‐beam capability at 12 keV as well as first‐harmonic coverage from 6.8 to 19 keV. A high‐heat‐load chopper reduces the average power load on downstream components, thereby preserving the surface figure of a Kirkpatrick–Baez mirror system capable of focusing the X‐ray beam to a spot size of 90 µm horizontal by 20 µm vertical. A high‐speed chopper isolates single X‐ray pulses at 1 kHz in both hybrid and 24‐bunch modes of the APS storage ring. In hybrid mode each isolated X‐ray pulse delivers up to ～4 × 10¹⁰ photons to the sample, thereby achieving a time‐averaged flux approaching that of fourth‐generation X‐FEL sources. A new high‐power picosecond laser system delivers pulses tunable over the wavelength range 450–2000 nm. These pulses are synchronized to the storage‐ring RF clock with long‐term stability better than 10 ps RMS. Monochromatic experimental capability with Biosafety Level 3 certification has been retained.     

Non‐invasive depth profiling by space and time‐resolved Raman spectroscopy

Time‐resolved Raman spectroscopy, spatially offset Raman spectroscopy and time‐resolved spatially offset Raman spectroscopy (TR‐SORS) have proven their capability for the non‐invasive profiling of deep layers of a sample. Recent studies have indicated that TR‐SORS exhibits an enhanced selectivity toward the deep layers of a sample. However, the enhanced depth profiling efficiency of TR‐SORS, in comparison with time‐resolved Raman spectroscopy and spatially offset Raman spectroscopy, is yet to be assessed and explained in accordance to the synergistic effects of spatial and temporal resolutions. This study provides a critical investigation of the depth profiling efficiency of the three deep Raman techniques. The study compares the efficiency of the various deep Raman spectroscopy techniques for the stand‐off detection of explosive precursors hidden in highly fluorescing packaging. The study explains for the first time the synergistic effects of spatial and temporal resolutions in the deep Raman techniques and their impact on the acquired spectral data. Copyright © 2013 John Wiley & Sons, Ltd.     

Adaption of a diffractometer for time‐resolved X‐ray resonant magnetic scattering

A new set‐up is presented to measure element‐selective magnetization dynamics using the ALICE chamber [Grabis et al. (2003), Rev. Sci. Instrum. 74 , 4048–4051] at the BESSY II synchrotron at the Helmholtz‐Zentrum Berlin. A magnetic‐field pulse serves as excitation, and the magnetization precession is probed by element‐selective X‐ray resonant magnetic scattering. With the use of single‐bunch‐generated X‐rays a temporal resolution well below 100 ps is reached. The ALICE diffractometer environment enables investigations of thin films, described here, multilayers and laterally structured samples in reflection or diffuse scattering geometry. The combination of the time‐resolved set‐up with a cryostat in the ALICE chamber will allow temperature‐dependent studies of precessional magnetization dynamics and of damping constants to be conducted over a large temperature range and for a large variety of systems in reflection geometry.     

Time‐resolved in situ tomography for the analysis of evolving metal‐foam granulates

An experimental setup has been developed that allows for capturing up to 25 tomograms s^?1 using the white X‐ray beam at the experimental station EDDI of BESSY II, Berlin, Germany. The key points are the use of a newly developed, precise and fast rotation stage, a very efficient scintillator and a fast CMOS camera. As a first application, the foaming of aluminium alloy granules at 923 K was investigated in situ. Formation and growth of bubbles in the liquid material were observed and found to be influenced by the limited thermal conductivity in the bulk granules. Changes that took place between two tomographic frames separated in time by 39 ms could be detected and analysed quantitatively.     

Femtosecond time‐resolved spectroscopy on biological photoreceptor chromophores

This paper reviews our results on femtosecond time‐resolved spectroscopy (transient absorption, transient‐grating and fluorescence spectroscopy) to study the photophysics and photochemistry of the two very important biological photoreceptor chromophores phycocyanobilin (PCB) and protochlorophyllide a (PChla). The compound PCB serves as a model chromophore for the photoreceptor phytochrome. By means of transient‐grating spectroscopy where the excitation wavelength was varied ove r the spectral region of the S₀→ S₁‐absorption the ultrafast processes were studied upon excitation with varying excess energy delivered to the system. On the basis of the results obtained, both the rate of the photoreaction in PCB and the rate of the decay of different excited‐state species via different decay channels depend on the excitation wavelength. Furthermore, transient absorption experiments illuminating the excited‐state dynamics of PChla, a porphyrin‐like compound and, as substrate of the NADPH/protochlorophyllide oxidoreductase (POR), a precursor of the chlorophyll biosynthesis are presented. In addition to pump‐energy‐dependent measurements performed with PChla dissolved in methanol, the excited‐state dynamics of PChla was interrogated in different solvents that were chosen to mimic different environmental conditions. In addition to the femtosecond time‐resolved absorption experiments the picosecond time‐resolved fluorescence of the system was studied. The transient absorption and tim e‐resolved fluorescence data allow suggesting a detailed model for the excited‐state relaxation of PChla describing the excited‐state processes in terms of a branching of the initially excited state population into a reactive and nonreactive path. Thus, the excited‐state potential energy surface exhibits two distinct S₁ and S_x minima separated from the Franck–Condon region along two most likely orthogonal reaction coordinates. Finally, the model derived is related to models suggested to acco unt for the reduction of PChla to chlorophyllide a within the natural enzymatic environment of POR.     

The characteristics of laser‐driven shock wave investigated by time‐resolved Raman spectroscopy

An accurate and simple method, Raman peak‐shift simulation, is proposed to determine the characteristics of a laser‐driven shock wave. Using the principle of the Raman peaks shifting at high pressure and the pressure distribution in the gauge layer, the profile of the Raman peak can be numerically simulated. Combined with time‐resolved Raman spectroscopy, some main characteristics of the shock wave were determined. In the experiment, polycrystalline anthracene was used as the pressure gauge. The pump–probe technique was used to obtain the time‐resolved Raman spectra of anthracene under shock loading. The velocity of the shock wave, the peak pressure and the rise time of the shock front were determined by simulating the experimental spectra numerically. The result shows that the method of Raman peak‐shift simulation is effective in obtaining the characteristics of a laser‐driven shock wave. Copyright © 2010 John Wiley & Sons, Ltd.     

Sub‐picosecond Raman spectrometer for time‐resolved studies of structural dynamics in heme proteins

We describe a pump–probe Raman spectrometer based on a femtosecond Ti:sapphire laser, an optical parametric generator and two optical parametric amplifiers for time‐resolved studies, with emphasis on the structural dynamics in heme proteins. The system provides a 100‐fs pump pulse tunable in the range 500–600 nm and a transform‐limited sub‐picosecond probe pulse tunable in the range 390–450 nm. The spectrometer has spectral (25 cm⁻¹) and temporal (∼0.7 ps) resolutions which constitute an effective compromise for identifying transient heme protein species and for following their structural evolution by spontaneous Raman scattering in the time range 0.5 ps to 2 ns. This apparatus was applied to time‐resolved studies of a broad range of heme proteins, monitoring the primary dynamics of photoinduced heme coordination state and structural changes, its interaction with protein side‐chains and diatomic gaseous ligands, as well as heme vibrational cooling. The treatment of transient Raman spectra is described in detail, and the advantages and shortcomings of spontaneous resonance Raman spectroscopy for ultrafast heme proteins studies are discussed. We demonstrate the efficiency of the constructed spectrometer by measuring Raman spectra in the sub‐picosecond and picosecond time ranges for the oxygen‐storage heme protein myoglobin and for the oxygen‐sensor heme protein FixLH in interaction with the diatomic gaseous ligands CO, NO, and O₂. Copyright © 2010 John Wiley & Sons, Ltd.     

On the design of ultrafast shutters for time‐resolved synchrotron experiments

A comprehensive treatment of the limitations and possibilities for single‐pulse selection in synchrotron operating modes with ～150 ns bunch separation, as occurs in the standard operating mode at the Advanced Photon Source, is presented. It is shown that the strength of available materials and allowable kinetic energy build‐up limit single‐bunch selection for this separation to sample sizes of ～100 µm, and that for minimization of kinetic energy build‐up it is preferable to increase the r.p.m. within physically acceptable limits rather than increase the disc radius to obtain a desirable peripheral speed. A slight modification of the equal‐bunch spacing standard fill patterns is proposed that allows use of samples as large as 500 µm. The corresponding peripheral speed of the chopper wheel is ～600 m s^?1, which is within the limits of high‐strength titanium alloys. For smaller samples, peripheral speeds are proportionally lower. Versatility can be achieved with interchangeable chopper wheels and the use of different orientations of the rotation axis relative to the X‐ray beam, which opens the possibility of larger, rather than one‐of‐a‐kind, production runs.     

X‐ray analog pixel array detector for single synchrotron bunch time‐resolved imaging

Dynamic X‐ray studies can reach temporal resolutions limited by only the X‐ray pulse duration if the detector is fast enough to segregate synchrotron pulses. An analog integrating pixel array detector with in‐pixel storage and temporal resolution of around 150 ns, sufficient to isolate pulses, is presented. Analog integration minimizes count‐rate limitations and in‐pixel storage captures successive pulses. Fundamental tests of noise and linearity as well as high‐speed laser measurements are shown. The detector resolved individual bunch trains at the Cornell High Energy Synchrotron Source at levels of up to 3.7 × 10³ X‐rays per pixel per train. When applied to turn‐by‐turn X‐ray beam characterization, single‐shot intensity measurements were made with a repeatability of 0.4% and horizontal oscillations of the positron cloud were detected.     

Picosecond time‐resolved laser pump/X‐ray probe experiments using a gated single‐photon‐counting area detector

The recent developments in X‐ray detectors have opened new possibilities in the area of time‐resolved pump/probe X‐ray experiments; this article presents the novel use of a PILATUS detector to achieve X‐ray pulse duration limited time‐resolution at the Advanced Photon Source (APS), USA. The capability of the gated PILATUS detector to selectively detect the signal from a given X‐ray pulse in 24 bunch mode at the APS storage ring is demonstrated. A test experiment performed on polycrystalline organic thin films of α‐perylene illustrates the possibility of reaching an X‐ray pulse duration limited time‐resolution of 60 ps using the gated PILATUS detector. This is the first demonstration of X‐ray pulse duration limited data recorded using an area detector without the use of a mechanical chopper array at the beamline.     

Effect of chlorine substitution on triplet state structure of thioxanthone: A time‐resolved resonance Raman study

Substitution plays an important role in determining the triplet state reactivity. In this paper, we have studied the effect of chlorine substitution on the triplet state structure and the reactivity of thioxanthone (TX). We have employed time‐resolved resonance Raman technique to understand the structure of the lowest triplet excited state of 2‐chlorothioxanthone (CTX). The experimental findings have been corroborated with the computational results using density functional theory. Akin to the parent compound (TX), coexistence of two lowest triplet states has been observed in case of CTX, which has been substantiated using resonant probe wavelength dependence study. The relative contribution of ³n–π* to ³π–π* to the equilibrated triplet state has been found to be more for CTX compared to TX suggesting increase in the triplet state reactivity after the substitution. The above observation has been further supported by the flash photolysis experiments. Copyright © 2013 John Wiley & Sons, Ltd.     

Time‐resolved Raman studies on Al2O3: Cr3+: lifetime measurements of the excited‐state transition Ē → 2Ā

A time‐resolved intensified charge coupled device‐based Raman microspectrometer system dedicated to the study of solid samples is described, offering good optical, temporal and spatial resolution. The advantages of this approach are demonstrated on Al₂O₃:Cr³⁺, obtaining for the first time the temporal evolution of the excited state transition Ē → 2Ā. Moreover, the time dependence of the luminescence due to the chromium ion was also determined by the same Raman device. Copyright © 2010 John Wiley & Sons, Ltd.     

Fabrication and testing of a newly designed slit system for depth‐resolved X‐ray diffraction measurements

A new system of slits called `spiderweb slits' have been developed for depth‐resolved powder or polycrystalline X‐ray diffraction measurements. The slits act on diffracted X‐rays to select a particular gauge volume of sample, while absorbing diffracted X‐rays from outside of this volume. Although the slit geometry is to some extent similar to that of previously developed conical slits or spiral slits, this new design has advantages over the previous ones in use for complex heterogeneous materials and in situ and operando diffraction measurements. For example, the slits can measure a majority of any diffraction cone for any polycrystalline material, over a continuous range of diffraction angles, and work for X‐ray energies of tens to hundreds of kiloelectronvolts. The design is generated and optimized using ray‐tracing simulations, and fabricated through laser micromachining. The first prototype was successfully tested at the X17A beamline at the National Synchrotron Light Source, and shows similar performance to simulations, demonstrating gauge volume selection for standard powders, for all diffraction peaks over angles of 2–10°. A similar, but improved, design will be implemented at the X‐ray Powder Diffraction beamline at the National Synchrotron Light Source II.     

Novel high‐temperature reactors for in situ studies of three‐way catalysts using turbo‐XAS

Two novel high‐temperature reactors for in situ X‐ray absorption spectroscopy (XAS) measurements in fluorescence are presented, each of them being optimized for a particular purpose. The powerful combination of these reactors with the turbo‐XAS technique used in a dispersive‐XAS beamline permits the study of commercial three‐way catalysts under realistic gas composition and temporal conditions.     

Time‐resolved spectroscopic and density functional theory investigation of the photochemistry of suprofen

The photochemistry of suprofen (SPF) was investigated by femtosecond transient absorption (fs‐TA), resonance Raman (RR) and nanosecond time‐resolved resonance Raman (ns‐TR³) spectroscopic methods to gain additional information so as to better elucidate the possible photochemical reaction mechanism of suprofen in several different solvents. In neat acetonitrile (MeCN), the fs‐TA and ns‐TR³ experimental data indicated that the lowest lying excited singlet state S₁ (nπ*) underwent an efficient intersystem crossing process (ISC) to the excited triplet state T₃ (ππ*), followed by an internal conversion (IC) process to T₁ (ππ*). In the aqueous solution, a triplet biradical species (³ETK‐1) was obtained as the product of a decarboxylation process from triplet suprofen anion (³SPF⁻) and the reaction rate of the decarboxylation process was determined by the concentration of H₂O. A protonation process for ³ETK‐1 leads to formation of a neutral species (³ETK‐3) that was directly observed by ns‐TR³ spectra, then this ³ETK‐3 species decayed via ISC process to generate final product. Copyright © 2014 John Wiley & Sons, Ltd.     

Implementation of a combined SAXS/WAXS/QEXAFS set‐up for time‐resolved in situ experiments

It has previously been shown that there are many benefits to be obtained in combining several techniques in one in situ set‐up to study chemical processes in action. Many of these combined set‐ups make use of two techniques, but in some cases it is possible and useful to combine even more. A set‐up has recently been developed that combines three X‐ray‐based techniques, small‐ and wide‐angle X‐ray scattering (SAXS/WAXS) and quick‐scanning EXAFS (QEXAFS), for the study of dynamical chemical processes. The set‐up is able to probe the same part of the sample during the synthesis process and is thus able to follow changes at the nanometre to micrometre scale during, for example, materials self‐assembly, with a time resolution of the order of a few minutes. The practicality of this kind of experiment has been illustrated by studying zeotype crystallization processes and revealed important new insights into the interplay of the various stages of ZnAPO‐34 formation. The flexibility of this set‐up for studying other processes and for incorporating other additional non‐X‐ray‐based experimental techniques has also been explored and demonstrated for studying the stability/activity of iron molybdate catalysts for the anaerobic decomposition of methanol.     

Influence of the chloro substituent position on the triplet reactivity of benzophenone derivatives: a time‐resolved resonance Raman and density functional theory study

A nanosecond time‐resolved resonance Raman (ns‐TR³) spectroscopic investigation of the photoreduction reactions and ability of several chloro‐substituted benzophenone (Cl‐BP) triplets is described. The TR³ results show that the 3‐chlorobenzophenone (3‐Cl‐BP), 4‐chlorobenzophenone (4‐Cl‐BP) and 4,4′‐dichlorobenzophenone (4,4′‐dichloro‐BP) triplets exhibit similar hydrogen abstraction ability with the parent BP triplet. In 2‐propanol, the 3‐Cl‐, 4‐Cl‐ and 4,4′‐dichloro‐diphenylketyl (DPK) radicals were observed and they appear to react with dimethylketyl radicals at the para‐position to form a light absorption transient species. These transient species were characterized with TR³ spectra, and identified with the help of results from density functional theory calculations. In an acetontitrile/water (MeCN:H₂O) 1:1 mixed solvent, these DPK radicals were also observed but with slower formation rates. However, the 2‐Cl‐DPK radical was observed to form with a lower yield and a significantly slower formation rate than the other chloro‐substituted benzophenones examined here in 2‐propanol under the same experimental conditions. These results reveal that the 2‐chloro substituent reduces the hydrogen abstraction ability of the substituted BP triplet, which was not as expected based on the assumption that the electron‐withdrawing group could increase its photoreduction ability. This unusual ortho effect of the chlorine substitution is briefly discussed. Copyright © 2011 John Wiley & Sons, Ltd.