Evidence for phonon hardening in laser excited gold using x-ray diffraction at a hard x-ray free electron laser

Studies of laser-heated materials on femtosecond timescales have shown that the interatomic potential can be perturbed at sufficiently high laser intensities. For gold, it has been postulated to undergo a strong stiffening leading to an increase of the phonon energies, known as phonon hardening. Despite efforts to investigate this behavior, only measurements at low absorbed energy density have been performed, for which the interpretation of the experimental data remains ambiguous. By using in situ single-shot x-ray diffraction at a hard x-ray free electron laser, the evolution of diffraction line intensities of laser-excited Au to a higher energy density provides evidence for phonon-hardening.

Methods
The X-ray diffraction data were collected at the LCLS using an ePix10k detector. The detector was calibrated using a CeO2 calibrant. The orientation of the detector was then determined using the open source software Dioptas. The data were then corrected for polarization (the LCLS beam was vertically polarized). The edges of each detector tile were masked. The details about the extraction of the normalized intensity is found in the Materials and Methods of the manuscript.

For timing between the optical laser pulse and the X-ray pulse, we used the time tool system available at MEC. The extraction of the relative timing, determination of the absolute timing, and timing uncertainties can be found in the Materials and Methods of the manuscript.