Femtosecond electron diffraction: making the "molecular movie"

Abstract

The ability to watch atoms move in real time - to directly observe transition states - has been referred to as "making the molecular movie". Femtosecond electron diffraction is ideally suited for this purpose since it records the atomic structure of the sample with sub-Angstrom spatial resolution and femtosecond temporal resolution. Many-body simulations of ultrashort electron pulse propagation dynamics allowed the development of sources for femtosecond electron pulses with sufficient number density to perform near single shot structure determinations, a requirement for studies of irreversible processes. We have obtained atomic level views of melting of thin films of aluminum and gold under strongly driven conditions. The results are consistent with a thermally driven phase transition and the observed time scales reflect the different electron-phonon coupling constants for these metals. Recent technical advances in electron gun design have further improved the temporal resolution of femtosecond electron diffraction. New electron pulse characterization techniques use direct laser-electron interaction and electron-electron interaction to determine the temporal overlap of the pump and probe pulses as well as the time resolution of the system. These advances have made femtosecond electron diffraction capable of observing transition states in molecular systems. The camera for "making the molecular movie" is now in hand.