Direct Measurements of the Ionization Potential Depression in a Dense Plasma

We have used the Linac Coherent Light Source to generate solid-density aluminum plasmas at temperatures of up to 180 eV. By varying the photon energy of the x rays that both create and probe the plasma, and observing the $K\mathrm{\text{−}}\alpha$ fluorescence, we can directly measure the position of the $K$ edge of the highly charged ions within the system. The results are found to disagree with the predictions of the extensively used Stewart-Pyatt model, but are consistent with the earlier model of Ecker and Kröll, which predicts significantly greater depression of the ionization potential.

Figure 1

Experimentally recorded $K\mathrm{\text{−}}\alpha$ emission spectra from hot solid-density aluminum. SCFLY simulations with different IPD models are compared with the experimental data for a subset of the x-ray laser photon energies. From the bottom, the spectra corresponding to 1580, 1600, 1630, 1650, 1720 and 1830 eV pump photon energies are shown. The spectra have been artificially displaced in intensity for clarity, and a bremsstrahlung component (at the maximum temperature provided by the simulations) has been added to the calculated spectra. The grey shading indicates the difference between the calculations using different IPD models.

Figure 2

Left—The grey region shows the continuum for different charge states, as determined from the experimental spectra in Fig. 1, with the dark grey region corresponding to the observed range of the $K$ edge. The values given by the SP and EK calculations correspond to the edges calculated at the time of maximum emission for each associated $K\mathrm{\text{−}}\alpha$ line. The calculated energy to pump a $K\mathrm{\text{−}}L$ and $K\mathrm{\text{−}}M$ transition (for the same number of $L$ shell electrons in the final state) is also indicated. Right—IPD values; the darker colored zones of the histogram correspond to the IPD variation detected, for the experimental bars, and to the total IPD variation during the system evolution, for the simulated ones.

Figure 3

Observation of the $K$-shell fluorescence corresponding to initial states with double core holes, along with SCFLY simulations using the SP and EK models. From the bottom, the spectra correspond to 1720, 1750, 1780, 1805 eV pump photon energies.