Investigation of phonon hardening in laser-excited gold using in-situ single-shot X-ray diffraction at the LCLS.

The use of ultra-short, ultra-bright X-ray pulses generated at X-ray Free Electron Laser (XFEL) light sources coupled with high-intensity femtosecond optical pulses has enabled the investigation of ultrafast phenomena taking place in warm dense matter. In such experiments, the optical laser pulse transfers its energy primarily to the electronic system which reacts almost instantaneously. The subsequent response of this far-from-equilibrium state of matter strongly depends on the excitation conditions. Density functional theory simulations performed on laser excited Au have suggested that the lattice undergoes a near-instantaneous hardening of its interatomic potential when electrons are heated to ~6 eV, and known as phonon hardening [1, 2]. This behavior is expected to have a significant impact as phonons contribute to intrinsic thermodynamic quantities such as constant-volume specific heat, entropy, and would also affect the melting behavior.



Previous studies have claimed evidence of the stiffening of the interatomic potential using ultrafast electron [3]. However, recent modelling, which do not assume phonon hardening, were also able to reproduce these experimental [2]. As a result, the experimental observation of phonon hardening in laser excited Au is still debated. Here, we investigate phonon hardening describe through X-ray diffraction at a hard XFEL to measure the temporal evolution of several diffraction lines at a higher absorbed energy density than previously reported.

[1] Recoules, V., et. al. (2006). Physical Review Letters, 96(5), 1–4.

[2] Smirnov, N. A. (2020. Physical Review B, 101(9), 1–11.

[3] Ernstorfer, R., et. al. (2009) Science, 323(5917), 1033–1037.