Probing the atomic dynamics of ultrafast melting with femtosecond electron diffraction

TL;DR

This study uses femtosecond electron diffraction and simulations to investigate the atomic-level dynamics of ultrafast melting in copper, revealing the process's temporal evolution and energy transfer mechanisms.

Contribution

It provides detailed experimental and simulation insights into the structural evolution and energy transfer during ultrafast melting, a process less understood than conventional melting.

Findings

Melting initiates at the surface below the melting point and then rapidly throughout the volume.

Weak electron-lattice energy transfer rate observed during ultrafast melting.

No evidence of rapid lattice collapse beyond superheating limits.

Abstract

Melting is an everyday phase transition that is determined by thermodynamic parameters like temperature and pressure. In contrast, ultrafast melting is governed by the microscopic response to a rapid energy input and, thus, can reveal the strength and dynamics of atomic bonds as well as the energy flow rate to the lattice. Accurately describing these processes remains challenging and requires detailed insights into transient states encountered. Here, we present data from femtosecond electron diffraction measurements that capture the structural evolution of copper during the ultrafast solid to liquid phase transformations. At absorbed energy densities 2 to 4 times the melting threshold, melting begins at the surface slightly below the nominal melting point followed by rapid homogeneous melting throughout the volume. Molecular dynamics simulations reproduce these observations and reveal a weak electron lattice energy transfer rate for the given experimental conditions. Both simulations and experiments show no indications of rapid lattice collapse when its temperature surpasses proposed limits of superheating, providing evidence that inherent dynamics limits the speed of disordering in ultrafast melting of metals.