Resonant K-alpha spectroscopy of a hot dense plasma created by the LCLS x-ray free electron laser

We present one of the first experimental studies of the interaction of high intensity x-ray free electron laser radiation with solid density matter. In the experiment performed at the LCLS, an intense 80 fs x-ray pulse at 10$^{17}$ Wcm$^{-2}$ with photon energies of 1480 $\sim $ 1560 eV is focused on a thin Al foil and K-alpha emission spectra are observed. Although x-ray photon energy is lower than the absorption edge, because of its high intensity the sample is surprisingly heated up to 100$\sim $200 eV in the pulse duration and a hot dense plasma is created. Observed x-ray spectra indicate this dense plasma resonantly interacts with the x-ray photons. The emission spectra are also simulated using the collisional-radiative code, SCFLY which provides information about the electron temperature and density, the charge state distribution and opacity. The comparison of experiment and simulation provides a detailed description of a dense plasma resonantly interacting with an intense x-ray pulse.