Biomedical subjects

S Techert

Publications and source records attributed to S Techert.

9 recordsLinked to original sources

PubMed2006

Aspects of the photodimerization mechanism of 2,4-dichlorocinnamic acid studied by kinetic photocrystallography.

Photoinduced structural variations in single crystals of 2,4-dichloro-trans-cinnamic acid (C9H6Cl2O2, DiClCA) have been investigated using X-ray diffraction (photocrystallography) and optical spectroscopic methods. During UV irradiation, which initiates the irreversible dimerization reaction, a loss of the long-range order of the reactant single crystal was found, i.e., that the dimerization is a heterogeneous one. This unexpected result emphasizes the still-existing problem of predicting changes or of remaining periodicity during chemical reactions in the solid state. On the basis of the experimental results, we propose a qualitative kinetic reaction scheme for DiClCA heterogeneous dimerization reaction.

Cinnamates↗

PubMed2005

Femtosecond time-resolved powder diffraction experiments using hard X-ray free-electron lasers.

In the next decade the scientific community expects a strong impact in physics, chemistry, biology, material research and life sciences by the availability of high-brilliance X-ray radiation from free-electron laser (FEL) sources. In particular, in the field of ultrafast science these new sources will allow new types of experiments, enabling new phenomena to be discovered. Whereas today ultrafast X-ray diffraction experiments are strongly restricted by the limited X-ray flux of current sources of sub-picosecond X-ray pulses, FELs will provide short pulses of typically 10(12) photons with a duration of the order of 100 fs and monochromaticity of 10(-3). Here, the feasibility of time-resolved single-shot powder diffraction experiments using these intense pulses, and the requirements of these experiments, are discussed. The detector count rates are estimated for diffraction from a model compound in a wide q-regime under the special consideration of high resolving power. In the case of LCLS radiation parameters, single-shot experiments will be feasible although high-resolution powder diffraction will require a reduction of the intrinsic FEL radiation bandwidth.

Journal Article↗

PubMed2005

Atomic-scale visualization of inertial dynamics.

The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.

Journal Article↗

PubMed2005

Clocking femtosecond X rays.

Linear-accelerator-based sources will revolutionize ultrafast x-ray science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are hampered by the inability to precisely synchronize an external laser to the accelerator. At the Sub-Picosecond Pulse Source at the Stanford Linear-Accelerator Center we solved this problem by measuring the arrival time of each high energy electron bunch with electro-optic sampling. This measurement indirectly determined the arrival time of each x-ray pulse relative to an external pump laser pulse with a time resolution of better than 60 fs rms.

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PubMed2004

Experimental requirements for light-induced reactions in powders investigated by time-resolved X-ray diffraction.

A general outline of how to perform a light-excited time-resolved diffraction experiment by applying the optical pump/X-ray probe technique is given. Owing to the difference in penetration depths between the optical light (laser) pump and the X-ray probe, only specific or specially designed crystalline systems can be investigated, so special requirements have to be fulfilled concerning the sample and its compartments. A summary of the experimental conditions of optical pump/X-ray probe experiments is presented, emphasizing why the use of powder diffraction is a useful and necessary X-ray technique for this kind of experiment. The possibilities and bottlenecks of time-resolved X-ray diffraction on the picosecond time scale will be demonstrated in the powder diffraction studies of N,N-dimethylaminobenzonitrile and N,N-diisopropylaminobenzonitrile, where the photo-induced structural changes of these molecular organic systems have been studied as a function of time.

Artifacts↗

PubMed2004

Nonradiative relaxation of thiacarbocyanine dyes in binary mixtures.

It was found that preferential solvation of cyanine dyes in binary mixtures can strongly affect both their isomerization and aggregation; the comparison of absorption and fluorescence excitation spectra might be a useful tool for studying these nonradiative processes.

Journal Article↗

PubMed2003

First investigations of the kinetics of the topochemical reaction of p-formyl-trans-cinnamic acid by time-resolved X-ray diffraction.

Under UV irradiation p-formyl-trans-cinnamic acid (p-FCA) crystals in the beta-phase dimerise irreversibly to solid 4,4'-diformyl-beta-truxinic acid. The experimental conditions were chosen in such a way (non-aqueous environment and room temperature) that the product formed is amorphous. The kinetics of this bimolecular reaction, which has not yet been characterised, was investigated by picosecond time-resolved X-ray diffraction. From the experimental results a mechanism for this topochemical reaction is proposed including two observed time constants, one less than 100 ps and another of several seconds. The feasibility of investigating this class of substances by time-resolved X-ray diffraction from third generation synchrotron sources and future free-electron lasers is discussed.

Cinnamates↗

PubMed2001

Visualizing photochemical dynamics in solution through picosecond x-ray scattering.

A photoexcited state of molecular iodine in solution is observed using diffuse x-ray scattering at a synchrotron source. The measured changes in the diffuse scattering profile were consistent with earlier models of iodine's photodissociation and geminate recombination reaction, for which the recombined A/A(') state has a 0.4 A greater interatomic spacing than the resting state and has a lifetime of 500 ps in CH2Cl2. This technique should find application in the study of increasingly complicated photochemical systems which undergo structural rearrangements following rapid photolysis.

Journal Article↗

PubMed2001

Picosecond X-ray diffraction probed transient structural changes in organic solids.

In this Letter, we report on the experimental characterization of the geometry of short-lived electronically excited states in organic solids by time-resolved x-ray diffraction. Here, the structure factor of the organic crystal is measured as a function of time. Since this technique gives complete structural information, it is a very useful tool for learning more about atom motions on the excited-state energy surface-"beyond" the broad band typical of conventional spectroscopy. Although we used molecular crystals rather than free molecules, the compounds show detectable transient structural changes on the ps to ns time scale in our study.

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