Compressive effects in melting of palladium thin films studied by ultrafast x-ray diffraction

With the advent of x-ray free electron lasers (XFELs), ultrafast laser excitation leading to nonequilibrium states has become an important way to investigate thermal processes. Here, we use ultrafast x-ray diffraction in an XFEL pump-probe experiment to examine the lattice dynamics of 300 nm polycrystalline palladium (Pd) thin films. In our experiment, at higher laser fluence levels, we directly observe a compression effect launched from the surface skin layer heated by the optical pulse which propagates across the film over the first tens of picoseconds. After that, a lattice expansion becomes dominant, followed by a quasi-steady state lasting into the nanosecond timescale. For the lowest fluence, the compression component is not detected, indicating that the dynamics is highly dependent on the laser power. Our results shed light on the dynamics behind ultrafast processes in metallic Pd, and they can be extended to different crystalline structures.