Intermediate Stages of Ultrafast Melting of Gold Thin Films*

Understanding the structure of materials with complementary non-destructive X-ray techniques becomes essential to design better and more efficient materials. This talk will describe the results of an ultrafast single-shot melting experiment carried out at the PAL-XFEL, combined with synchrotron-based X-ray nanodiffraction to examine the domain formed. Polycrystalline thin films of gold were irradiated with femtosecond optical pulses in the fluence range sufficient to melt the film. Heterogeneous two-phase melting was observed in the form of a split Au (111) diffraction powder ring. The colder, high-Q, peak showed oscillations as a function of pump-probe delay time, consistent with acoustic waves generated in the film. The new peak on the low-Q side was attributed to a compressed, hotter region of gold that absorbs the latent heat during the melting and increases with the melt-front moving in time. To understand the grain size effects of melting, samples were characterized using X-ray nanodiffraction technique and preliminary result will be presented.