## 4 Discussion and Conclusions  
We have presented the results from a experiment at LCLS in which solid-density germanium plasmas were generated using four different photon energies. We have shown that careful consideration of the photon energy of the XFEL with respect to the position of the edges of the dominant ionization stages and bound-bound transitions in a heated plasma is necessary to achieve uniformity in the plasma conditions. The experimental spectra have been modeled using one-dimensional radiative transfer simulations, weighted accordingly with the measured profile of the laser spot. The synthetic spectra showed that the time-and-space integrated spectra are heavily weighted by the plasma conditions near the peak intensity of the laser pulse, and are therefore a reasonable diagnostic of the peak temperature of the plasma. This temperature can be extracted from the slope of the bound-free spectral emission which allowed us to compare the experimental results with the predictions from the simulations. We observed that the peak temperature of the plasma is lowest for a XFEL photon energy of 1400 eV, as this energy lies between bound-bound and bound-free absorption features in hot germanium, and therefore no hot layer is generated at the front of the target. The agreement between experiment and simulations provides further confidence that the generated conditions for this case are uniform across the target. Our measurements also show how for lower (1300 eV) and higher (1500 and 1600 eV) XFEL photon energies, the obtained peak temperatures are larger, owing to resonance with the L-M bound-bound transitions, and lying above the L-edge of the ions in the heated system respectively. Thus, the main features of the spectra, in terms of the temperatures extracted from them, are broadly in good agreement with the computational results. This work shows that generating uniform conditions in solid density plasmas created with a XFEL for high-Z materials requires careful consideration of the experimental parameters, as the complicated atomic structure of these elements greatly complicates the interaction between the XFEL and the plasma itself. Conceptualization, S.M.V., T.R.P. and J.S.W. ; Investigation, S.M.V., T.R.P., B.A.H., H.-K.C., T.B., V.V., M.-F.L., T.B.v.D. and J.S.W. ; Formal analysis, G.P.-C., S.R. and R.R. ; Software, G.P.-C., S.R., O.H., R.R. ; Funding acquisition, S.M.V. and J.S.W. ; Project administration, S.M.V. and J.S.W. ; writing--original draft preparation, G.P.-C.; writing--review and editing, S.M.V. and J.S.W. All authors have read and agreed to the published version of the manuscript. Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76 SF00515. The SXR Instrument is funded by a consortium whose membership includes the LCLS, Stanford University through the Stanford Institute for Materials Energy Sciences (SIMES), Lawrence Berkeley National Laboratory (LBNL), University of Hamburg through the BMBF priority program FSP 301, and the Center for Free Electron Laser Science (CFEL). G.P.-C., and [PERSON] gratefully acknowledge support from LLNL under grant number B617350. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07 NA27344. [PERSON] is a Royal Society University Research Fellow. [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] acknowledge support from the U.K. EPSRC under grant EP/P015794/1. [PERSON] acknowledges support from the Oxford University Centre for High Energy Density Physics (OxCHEDS). This document was prepared as an account of work sponsored by an agency of the United States government.