X-ray spectroscopic studies of solid-density plasmas created by an X-ray Free Electron Laser

The advent of Free-Electron-Lasers (FELs) operating in the x-ray regime, such as LCLS, have revolutionised our ability to create and diagnose solid-density plasmas. These remarkable sources have peak spectral brightnesses a billion times greater than those of any synchrotron device, and provide the ability to irradiate matter with x-rays at intensities that were previously confined to the optical domain. When solid-density matter is irradiated on sub-100-fsec timescales at high x-ray intensities the electrons are heated to temperatures of several hundred eV. Core holes induced in the system via photoionisation by the FEL rapidly fill and, depending on the FEL photon energy and intensity, radiation is only emitted during the FEL pulse itself: the photon energies are too great for thermal emission. In this case a time-integrated spectrum can reveal remarkable information about the state of the solid density plasma on time-scales shorter than that required for an atom to move further than a few lattice spacings: the heating is truly isochoric. We report here on how spectroscopic studies of such plasmas allow an accurate measurement of the ionisation energy of the charge states produced. We find that the ionisation potential is significantly depressed beyond that predicted by simple models used in many atomics-kinetics calculations, but is in good agreement with calculations based on density-functional-theory. Further detailed study of the x-ray spectra emitted by such targets allows us to glean information on the femtosecond collisional ionisation rates, to observe saturable absorption in the x-ray regime, and to make opacity measurements of solid density plasmas very close to LTE conditions.