3-D structure determination of CaMoO₄using HVEM

Abstract

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...