## 3 Results
### Experimental Results
Moreover, the measured variations in the absolute intensity of the bound-free emission from single-shots, also correspond to small changes in the temperature between different shots, as shown by the colored rectangles in Figure 7.  
As expected, the lowest temperature is found for the 1400 eV case, where the laser absorption within the plasma is minimal. Additionally, as the plasma is most uniform, the temperature variations observed are minimal as well. On the contrary, for the 1300 eV case, [PERSON] predicts variation of \(\sim\)50% in the temperature as the laser energy is modified, owing to the sensitivity of the peak temperature achieved in this case to the position of the bound-bound absorption features. Although such variation is not found in our experimental measurements, that should not be surprising, given that (as mentionedabove), the L-shell structure of mid- and high-Z materials is extremely complicated to model, and small deviations from the real position of the lines can give rise to considerable temperature variations. For photon energies other than 1400 eV, the large peak temperatures measured (60 to 160% greater than the peak temperature measured for 1400 eV) are owed to the hot layer that is generated at the front of the target, in agreement with the Cretin simulations. Additionally, note how for the 1500 and 1600 eV cases, the error in the temperature obtained from the average spectrum is extremely large. This can be understood by looking at Figure 5, where we show that for these cases, although the signal above \(\sim\)1490 eV is mostly caused by bound-free emission, the slope is heavily affected by the multiple absorption edges from the different charge states present in the plasma, which therefore affect the fit of the slope. Figure 5: Comparison between the experimental (solid, colored) and simulated spectra (dashed) for the four different XFEL photon energies. The dotted lines correspond to the bound-free contribution to the overall simulated spectra. The gray area indicates the photon energy above which the emission has been used to estimate the temperature. For the 1500 and 1600 eV cases, the bound-free emission presents several edges from the different charge states in the plasma, which makes getting a temperature from the slope more subject to error. In all cases the simulations agree with the data in the regions above the gray areas, whereas the bound-bound emission is poorly reproduced. The experimental spectra for 1300, 1400, 1500 and 1600 eV have been averaged over 102, 116, 156 and 122 shots respectively. Figure 6: Spectra obtained for the different shots with a XFEL photon energy of 1400 eV as a function of the beam energy. The small variations in the beam energy affect the spectral intensity, and effectively, the temperature of the plasma. Figure 7: Comparison between the peak temperature of the plasma predicted by Cretin (triangles) and the results obtained from the experimental spectra for the different XFEL photon energies of consideration. The error bars in the Cretin results correspond to the variation of the peak temperature of the plasma when the intensity of the laser is varied by \(\sim\)\(\pm\)15% as in the experiment. The circles correspond to the temperatures obtained from fitting the shot-averaged spectra, whereas the colored rectangles represent the range of temperatures obtained from individual shots by comparing their absolute spectral bound-free intensity with that of the average spectrum (shown in Figure 6), as given by \(I\propto e^{(-E/kT)}\).