Using Diffuse Scattering to Observe X-Ray-Driven Nonthermal Melting

TL;DR

This study uses high-intensity X-ray pulses at an XFEL facility to directly observe ultrafast, nonthermal melting of silicon through diffuse scattering, revealing a rapid transition to a liquid state within 100 femtoseconds.

Contribution

It introduces a novel X-ray scattering method to monitor nonthermal melting in real-time without interference from crystalline diffraction signals.

Findings

Rapid diffuse scattering increase indicates loss of lattice order.

Transition to liquid state occurs within 100 femtoseconds.

Results align with first principles simulations, faster than inertial predictions.

Abstract

We present results from the SPring-8 Angstrom Compact free electron LAser (SACLA) XFEL facility, using a high intensity ($\sim\!10^{20}\,$W/cm$^2$) X-ray pump X-ray probe scheme to observe changes in the ionic structure of silicon induced by X-ray heating of the electrons. By avoiding Laue spots in the scattering signal from a single crystalline sample, we observe a rapid rise in diffuse scattering, which we attribute to a loss of lattice order and a transition to a liquid state within 100 fs of irradiation, a timescale which agrees well with first principles simulations, but is faster than that predicted by purely inertial behavior. This method is capable of observing liquid scattering without masking or filtering of signal from the ambient solid, allowing the liquid structure to be measured throughout and beyond the phase change.