Femtosecond Electron Diffraction

The visualization of atomic motions has long been used as a gedanken experiment to develop a conceptual basis for various phenomena.  One can look to the typical transition states or proposals of reactive intermediates in organic chemistry as classis examples of this thought experiment.  Similar examples can be found in biology in pondering how protein structure affects transition states at active sites, unwinding of DNA etc.  Within physics, there are numerous examples from Coulomb explosions to atomic displacements involved in phase transition and phonon propagation.  This classis thought experiment has long been considered out of the realm of experiment.  With the recent development of femtosecond electron pulse sources with sufficient number density to execute nearly single-shot structure determinations, this experiment has finally been realized.  Atomic-level views of melting have been obtained under strongly driven conditions for thin films of aluminum and gold.  The melting process can be described as a thermally activated barrier crossing.

In these experiments, an intense femtosecond laser pulse was used to deposit sufficient energy for superheating. The initially generated hot electron distribution must re-equilibrate with the cold lattice, a process that can be approximately described by the two-temperature model. The timescale for this equilibration is determined by the material-specific electron-phonon coupling constant. In the case of Al, optical measurements detected dielectric properties characteristic of liquid Al within 500 fs after high fluence excitation. By using FED, we were able to watch the atomic details responsible for the collapse of the ordered lattice on time scales faster than thermalization. The observed dynamics were consistent with a thermally propagated phase transition. In a 20 nanometer thick freestanding film, this process takes place in ~3.5 picoseconds, at which point the system takes on the atomic pair correlation function corresponding to the liquid state. Subsequent studies of the slower melting of Au have helped further elucidate the mechanism for the melt zone propagation. The detailed mechanism of the melting process is being addressed by comparing different metals to clarify the roles of heterogeneous and homogeneous nucleation in the high-fluence regime and testing predictions from classical molecular dynamics simulations