Ultrafast melting of Warm Dense Cu studied by x-ray spectroscopy

We present a study of ultrafast heating of Warm Dense Cu diagnosed by the means of x-ray absorption spectroscopy carried out at the Lund Laser Center and Draco laser at HZDR. A thin Cu foil was directly heated to few eV temperature by a 30-fs laser pulse and probed with variable delay by a laser-driven betatron radiation. This betatron radiation, emitted by laser wake field accelerated electrons, is an unique x-ray source with its ultrashort duration and broadband spectrum, therefore it is ideally suited for this type of measurements. The sample was studied via the X-ray Absorption Near Edge Spectroscopy (XANES) in region above the Cu K-edge at 8.9 keV. This method provides temporally-resolved information about both the ionic structure of the matter and its temperature during the process of ultrafast heating and melting of the material. We deployed a newly developed spectrometer with a highly reflective HOPG crystal in a geometry well suited for detection of low intensity signals with high spectral and angular resolution.