Femtosecond Electron Diffraction (FED) has been developed to directly observe structural transitions on the atomic scale in real time. The duration of the electron pulse dominates the time resolution of any current FED setup. Velocity mismatch between the electron probe and the laser pump pulse requires the use of transmission mode electron diffraction to achieve femtosecond temporal resolution. Hence, thin film samples with a thickness on the order of tens of nanometers have to be used. Intense heating of the sample due to the pump laser combined with the extreme surface-to-volume ratio makes most transitions non-reversible. A high number of electrons per pulse is thus required to achieve high signal-to-noise in one or few shots.We have developed an electron gun that provides high flux electron pulses at pulse durations down to 250 fs. This has been achieved by minimizing the space-charge induced broadening of the electron pulses. Traditional methods for characterization of high flux electron pulses like streak cameras fail below the picosecond range. We have proposed the use of the ponderomotive potential [1] of an intense laser field to sample the temporal profile of the electron pulse by selectively scattering parts of the electron beam. This method and other approaches providing a direct cross-correlation between light and electron pulses hold promise of measuring pulses as short as 100 fs.[1] Siwick B.J., Green A.A., Hebeisen C.T., Miller R.J.D., Characterization of Ultrashort Electron Pulses by Electron-Laser Pulse Cross-Correlation, in press. Femtosecond (fs) lasers are an ideal tool to excite materials on timescales even shorter than vibrational periods, typically ~100 fs. The ability to resolve all of the resulting structural dynamics depends on having a technique with fs temporal resolution and capable of providing high structural resolution. Femtosecond electron diffraction satisfies both criteria and offers an unprecedented view of the fastest possible structural dynamics [1].By using a fs laser, one is able to very quickly deposit energy into a material-in these experiments gold and nickel. This leads to superheating of the metal and thereby permits the study of the familiar phenomenon of melting, only in this case the process is strongly driven. Even under these conditions, however, the material properties mediate the material response. In our first work [1], aluminum melted in 3.5 picoseconds (ps); under equivalent conditions, gold melts in 12 ps and nickel more quickly than gold. The difference in timescales is a consequence of a material parameter-the electron-phonon coupling constant-that differs by an order of magnitude between the gold and nickel and determines how quickly the laser energy absorbed by the electrons is transferred to nuclear motion. The observed structural changes, however, are the same for the two metals and thus allow for a generalized description of the melting mechanism.[1] Siwick B.J., et al., Science, 2003Science, , 302, 1382 The three-dimensional (3-D) structure of...
Poster Sessions depending on the experimental condition [1][2][3][4]. The deficient line is black and the excess line white on photograph. The deficient-excess unindexed line is black on one side and white on the other side of the line. In the present work for the first time the contrast reversal along the unindexed line is obtained. The specimens were single crystalline silicon films prepared by chemical etching of bulky crystals. The transmission electron diffraction patterns were obtained in an EG-100M electron diffraction camera at an accelerating voltage of 100kV with the primary electron beam almost parallel to [111] axis. In the obtained Kikuchi patterns the unindexed line runs along the middle line of the Kikuchi band. The deficient unindexed line in the vicinity of the strong and spot reflections changes the contrast and transforms into excess line. The experimental conditions of unindexed line contrast reversal are founded. It is shown that the contrast is reversed when unindexed line passes through or in vicinity of an intense spot reflection. The contrast reversal of unindexed line is explained within the framework of the Kikuchi patterns formation mechanism with due regard for the double Kikuchi diffraction [5] The synthesis of new nanocrystalline structures demands new rapid methods of solving their crystal structures. Our goal is real-time structure solution at the electron microscope, based on automated acquisition of three-dimensional electron diffraction data with subsequent phasing of the data set and presentation of a unit-cell potential map that displays atomic positions and even species. To achieve this we must consider: 1) translation of the specimen during automated tilting; 2) automated recognition of zone-axis orientations; 3) multiple-scattering artifacts; 4) indexing methods; 5) absolute intensity scaling of the data; 6) scaling of data collected at different orientations; and 7) the phase problem. Initially, we have focused on issues 3) through 7) following manual acquisition of three-dimensional diffraction data from a known test crystal (the MgAl2O4 spinel structure). Data was collected by two techniques, both of which minimize multiple-scattering artifacts: precession electron diffraction (PED) and kinematic convergent beam electron diffraction (CBED) using an in-column Omega energy filter. After indexing and scaling, experimental structure-factor magnitudes were obtained from the patterns. These provide input to the charge-flipping algorithm [1], which works well with relatively poor-quality electron diffraction data or powder diffraction data [2], to solve the phase problem and obtain the correct crystal structure. Solutions for PED and kinematic CBED data are presented for comparison with each other and with simulations. Further development requires automated, scripted control of specimen tilt and data acquisition.[1] G. Oszlanyi and A. Suto. Acta Cryst., A60, 134 (2004).[2] J.S. Wu, J. Spence, M. O'Keeffe, and K. Leinenweber. Nat. Mater., 5, 647 (2006 A new metastable zirconi...
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