Observation Laser-induced Transient Melting of Gold Thin Films*

Time-resolved optical laser pump and X-ray/or electron probe techniques opens up an opportunity to look at the dynamical melting of solid state materials which is still one of the grand challenges in ultrafast science. Recent Ultrafast Electron and X-ray Diffraction studies of gold thin films found distinct changes of the melted state with reduced crystalline peak intensity and the rise of a new disorder state after laser excitation. The results were interpreted using two-temperature model and showed the formation of a new state, however, further insights into the local structure of the new disordered state are still lacking.

Here we report recent XRD results measured on gold thin films evaporated on silicon nitride membranes shortly after 400 nm laser excitation, conducted at the new Pohang Accelerator Lab-X-ray Free-electron Laser Facility (PAL-XFEL), Pohang, South Korea. Preliminary analysis shows the initial (111) peak intensity drops and shifts inwards due to thermal expansion, oscillating because of the generation of acoustic waves. At higher fluence, an additional peak appears on the low-Q side associated with melting of the film.