Investigating the structural evolution of warm dense aluminum using single-shot mega-electron-volt ultrafast electron diffraction*

Warm Dense Matter (WDM) refers a regime where materials possess a simultaneously high density (ρ ~ 1 g/cm³) and temperature (T ~ 1 eV = 11605 K). Intense lasers pulses can drive thin metal foils to WDM conditions. Initially, the laser pulse heats the electrons, and subsequent equilibration between the electrons and the solid lattice yields a state of high temperature and density. While this method has been used to produce and study WDM, it has only recently been possible to study the structural evolution of this complex state of matter.

 

We present measurements using single-shot mega-electron-volt ultrafast electron diffraction (MeV-UED) to study the structural evolution of laser heated Warm Dense Aluminum (WD-Al). MeV-UED is an ideally suited to probe of the structure of thin films. We determined the timescale of melting of Al films and observed saturation in the melt time at high excitation conditions. To understand the melting dynamics, we developed a modified two-temperature model that considers latent heat and different nucleation mechanisms. This study provides crucial understanding of the evolution of WD-Al, and give vital insight necessary for interpreting other experiments.