Measurements of the momentum-dependence of plasmonic excitations in matter around 1 Mbar using an X-ray free electron laser

We present measurements of the plasmon shift in shock-compressed matter as a function of momentum transfer beyond the Fermi wavevector using an X-ray Free Electron Laser. We eliminate the elastically scattered signal retaining only the inelastic plasmon signal. Our plasmon dispersion agrees with both the random phase approximation (RPA) and static Local Field Corrections (sLFC) for an electron gas at both zero and finite temperature. Further, we find the inclusion of electron-ion collisions through the Born-Mermin Approximation (BMA) to have no effect. Whilst we cannot distinguish between RPA and sLFC within our error bars, our data suggest that dynamic effects should be included for LFC and provide a route forward for higher resolution future measurements.