Femtosecond streaking of electron diffraction patterns to study structural dynamics in crystalline matter

A table-top femtosecond, non-relativistic, electron diffraction setup is combined with a low-jitter, photo-triggered streak camera to follow the optically induced structural dynamics in complex solids. A temporal resolution of 550 fs is experimentally demonstrated, while the route to streaking with sub-250 fs temporal resolution is outlined. The streaking technique allows for parallel capturing of temporal information as opposed to the serial data acquisition in a conventional scanning femtosecond electron diffraction. Moreover, its temporal resolution is not corrupted by increasing the number of electrons per pulse. Thus, compared to the conventional scanning approach, a substantial increase in signal-to-noise ratio (SNR) can be achieved. These benefits are demonstrated by studying a photo-induced charge density wave phase transition in 4H(b)-TaSe2 using both methods. Within the same data acquisition time a three-fold increase in SNR is achieved when compared to the scanning method, with ways for a further improvement outlined. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4798518

Femtosecond streaking of electron diffraction patterns to study structural dynamics in crystalline matter

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Human γS-Crystallin–Copper Binding Helps Buffer against Aggregation Caused by Oxidative Damage

Divalent metal cations can play a role in protein aggregation diseases, including cataract. Here we compare the aggregation of human γS-crystallin, a key structural protein of the eye lens, via mutagenesis, UV light damage, and the addition of metal ions. All three aggregation pathways result in globular, amorphous structures that do not elongate into fibers. We also investigate the molecular mechanism underlying copper (II)-induced aggregation. This work was motivated by the observation that zinc (II)-induced aggregation of γS-crystallin is driven by intermolecular bridging of solvent-accessible cysteine residues, while in contrast, copper (II)-induced aggregation of this protein is exacerbated by the removal of solvent-accessible cysteines via mutation. Here we find that copper (II)-induced aggregation results from a complex mechanism involving multiple interactions with the protein. The initial protein-metal interactions result in the reduction of Cu(II) to Cu(I) with concomitant oxidation of γS-crystallin. In addition to the intermolecular disulfides that represent a starting point for aggregation, intramolecular disulfides also occur the cysteine loop, a region of the N-terminal domain that was previously found to mediate the early stages of cataract formation. This previously unobserved ability of γS-crystallin to transfer disulfides intramolecularly suggests that it may serve as an oxidation sink for the lens after glutathione levels have become depleted during aging. γS-crystallin thus serves as the last line of defense against oxidation in the eye lens, a result that underscores the chemical functionality of this protein, which is generally considered to play a purely structural role

Ultrafast dynamics of charge density waves in 4<I>H<SUB>b</SUB></I>-TaSe<SUB>2</SUB> probed by femtosecond electron diffraction

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Photoinduced Dynamics in the Charge Density Wave Compound 4HB-TaSe2

We report on ultrafast photoindued charge density wave (CDW) dynamics in the transition-metal dichalcogenide 4Hb-TaSe2, studied with ultrafast electron diffraction. Fluence dependence of the lattice superstructure suppression show a phase transition from the commensurate to the incommensurate phase of 4Hb-TaSe2. Unusually long recovery times of perturbed charge density waves indicate th importance of a coupling between the two dimensional CDWs

Ultrafast Dynamics of Charge Density Waves in 4H(b)-TaSe2 Probed by Femtosecond Electron Diffraction

The dynamics of the photoinduced commensurate-to-incommensurate charge density wave (CDW) phase transition in 4H(b)-TaSe2 are investigated by femtosecond electron diffraction. In the perturbative regime, the CDW re-forms on a 150-ps time scale, which is two orders of magnitude slower than in other transition-metal dichalcogenides. We attribute this to a weak coupling between the CDW carrying T layers and thus demonstrate the importance of three-dimensionality for the existence of CDWs. With increasing optical excitation, the phase transition is achieved, showing a second-order character, in contrast to the first-order behavior in thermal equilibrium

A compact streak camera for 150 fs time resolved measurement of bright pulses in ultrafast electron diffraction

We have developed a compact streak camera suitable for measuring the duration of highly charged subrelativistic femtosecond electron bunches with an energy bandwidth in the order of 0.1%, as frequently used in ultrafast electron diffraction UED experiments for the investigation of ultrafast structural dynamics. The device operates in accumulation mode with 50 fs shot-to-shot timing jitter, and at a 30 keV electron energy, the full width at half maximum temporal resolution is 150 fs. Measured durations of pulses from our UED gun agree well with the predictions from the detailed charged particle trajectory simulations