Time-resolved x-ray absorption spectroscopy of warm dense matter with betatron x-ray radiation*

Betatron x-ray radiation, produced by relativistic electrons oscillating in a laser-wakefield accelerator, has been used to perform time-resolved x-ray absorption spectroscopy. We measured the evolution of the X-ray Absorption Near Edge Structure (XANES) signal, near the oxygen K-edge (535 eV), of an SiO₂ sample excited with optical laser light. Experiments were performed at the LCLS, Matter under Extreme Conditions end station, with a 1 J, 40 fs laser system. The laser was split to produce betatron x-rays in a gas cell with an electron density of 1.5 x 10¹⁹ cm^-3, and to irradiate the SiO₂ sample with intensities around 10¹⁵ W/cm². The delay between the two pulses was varied to measure the evolution of the XANES signal. Our experiment produced electron beams with energies up to 200 MeV and betatron x-rays with critical energies of a few keV. By using an ellipsoidal mirror to refocus the x-rays onto the SiO₂ sample, as well as an imaging x-ray spectrometer with a variable line spacing grating, we were able to obtain a sub-picosecond resolution.