Volume 1, Issue 3, May 2014
- Experimental Methodologies
Measurement of transient atomic displacements in thin films with picosecond and femtometer resolution1(2014); http://dx.doi.org/10.1063/1.4875347View Description Hide Description
We report measurements of the transient structural response of weakly photo-excited thin films of BiFeO3, Pb(Zr,Ti)O3, and Bi and time-scales for interfacial thermal transport. Utilizing picosecond x-ray diffraction at a 1.28 MHz repetition rate with time resolution extending down to 15 ps, transient changes in the diffraction angle are recorded. These changes are associated with photo-induced lattice strains within nanolayer thin films, resolved at the part-per-million level, corresponding to a shift in the scattering angle three orders of magnitude smaller than the rocking curve width and changes in the interlayer lattice spacing of fractions of a femtometer. The combination of high brightness, repetition rate, and stability of the synchrotron, in conjunction with high time resolution, represents a novel means to probe atomic-scale, near-equilibrium dynamics.
1(2014); http://dx.doi.org/10.1063/1.4882074View Description Hide Description
The study of structural dynamics of complex macromolecular crystals using electrons requires bunches of sufficient coherence and charge. We present diffraction patterns from graphite, obtained with bunches from an ultracold electron source, based on femtosecond near-threshold photoionization of a laser-cooled atomic gas. By varying the photoionization wavelength, we change the effective source temperature from 300 K to 10 K, resulting in a concomitant change in the width of the diffraction peaks, which is consistent with independently measured source parameters. This constitutes a direct measurement of the beam coherence of this ultracold source and confirms its suitability for protein crystal diffraction.
1(2014); http://dx.doi.org/10.1063/1.4884937View Description Hide Description
Ultrafast electron diffraction allows the tracking of atomic motion in real time, but space charge effects within dense electron packets are a problem for temporal resolution. Here, we report on time-resolved pump-probe diffraction using femtosecond single-electron pulses that are free from intra-pulse Coulomb interactions over the entire trajectory from the source to the detector. Sufficient average electron current is achieved at repetition rates of hundreds of kHz. Thermal load on the sample is avoided by minimizing the pump-probe area and by maximizing heat diffusion. Time-resolved diffraction from fibrous graphite polycrystals reveals coherent acoustic phonons in a nanometer-thick grain ensemble with a signal-to-noise level comparable to conventional multi-electron experiments. These results demonstrate the feasibility of pump-probe diffraction in the single-electron regime, where simulations indicate compressibility of the pulses down to few-femtosecond and attosecond duration.
- Biological Systems
1(2014); http://dx.doi.org/10.1063/1.4883975View Description Hide Description
The platinum anti-cancer agents cisplatin and carboplatin bind to the histidine 15 residue in the model protein hen egg white lysozyme. By using temperatures either side of the protein glass transition state (∼180 K), several platinum binding modes are seen and show that not all these platinum modes are stable. In particular, the mean square displacement vibration amplitudes of the cisplatin and of the histidine to which it is bound are analysed in detail. As well as the multiple platinum peaks, the electron density for the His-15 side chain is weak to absent at 150 K and 200 K, which points to the imidazole ring of the His side chain sampling multiple positions. Most interestingly, the His-15 imidazole becomes more ordered at room temperature.
- Liquids and Solutions
Probing ion-specific effects on aqueous acetate solutions: Ion pairing versus water structure modifications1(2014); http://dx.doi.org/10.1063/1.4884600View Description Hide Description
The effect of monovalent cations (Li+, K+, NH4 +, Na +) on the water structure in aqueous chloride and acetate solutions was characterized by oxygen K-edge X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy, and resonant inelastic X-ray scattering (RIXS) of a liquid microjet. We show ion- and counterion dependent effects on the emission spectra of the oxygen K-edge, which we attribute to modifications of the hydrogen bond network of water. For acetates, ion pairing with carboxylates was also probed selectively by XAS and RIXS. We correlate our experimental results to speciation data and to the salting-out properties of the cations.