Multistep transition of diamond to warm dense matter state revealed by femtosecond X-ray diffraction

TL;DR

This study uses femtosecond X-ray diffraction simulations to reveal the multistep transition of diamond into warm dense matter, highlighting nonthermal damage, transient graphite-like states, and the importance of accurate modeling.

Contribution

It introduces a hybrid simulation approach to capture the ultrafast transition of diamond to warm dense matter, including transient states and comparison with experimental data.

Findings

Disordering occurs within 100 fs after irradiation.

(220) diffraction peak vanishes faster than (111).

Transient graphite-like state appears during transition.

Abstract

Diamond bulk irradiated with a free-electron laser pulse of 6100 eV photon energy, 5 fs duration, at the $\sim 19-25$ eV/atom absorbed doses, is studied theoretically on its way to warm dense matter state. Simulations with our hybrid code XTANT show disordering on sub-100 fs timescale, with the diffraction peak (220) vanishing faster than the peak (111). The warm dense matter formation proceeds as a nonthermal damage of diamond with the band gap collapse triggering atomic disordering. Short-living graphite-like state is identified during a few femtoseconds between the disappearance of (220) peak and the disappearance of (111) peak. The results obtained are compared with the data from the recent experiment at SACLA, showing qualitative agreement. Challenges remaining for the accurate modeling of the transition of solids to warm dense matter state and proposals for supplementary measurements are discussed in detail.