## 3 Results and Discussion
### Selection of Single Harmonics  
The optimization experiments for a single harmonic are shown in Figure 7. The applied optimization strategy was as follows: the signal in a certain spectral region (marked with a colored bar in Figure 7a,c) has been maximized, after normalization to the signal summed up over the full displayed spectral region. So the fitness is maximized if the signal is relatively enhanced in a defined spectral region also at the expense of a lower absolute signal level. Note that the described optimization of a fraction of the signal is not appropriate for the realization of a high-flux source, requiring the optimization of the fractional signal and the total signal. As we are interested in comparison of different broadening media, we limit ourselves here to relative optimization of selected spectral features.  
Using argon for spectral broadening the results for selective optimization of three different spectral regions are shown in Figure 7a, and in Figure 7b we depict the evolution of the fitness. The most  
Figure 7: Optimization of single harmonics for (**a**) argon broadened and (**c**) SF\({}_{6}\) broadened shaped pulses. The colored areas show the chosen spectral area for the optimization and the respective optimization result is shown as a line plot. The accompanying development of the fitness is shown in (**b**) and (**d**) in the same color as the optimized spectra. In case of using SF\({}_{6}\) the spectral fraction of 25 th to 29 th harmonic can be optimized over 50%.  
remarkable observations are: (i) for selection of the 29 th harmonic (blue marked range) the fitness only slightly increases and the low energy harmonics still substantially contribute to the observed spectrum; (ii) for the 27 th harmonic (red marked range) the spectral fraction can be increased up to 65% of the spectrum. However, the spectral selection is achieved at the cost of a reduced total count rate; (iii) Optimization for the 25 th harmonic (black) shows an increase of the spectral fraction of the harmonic by a factor of 2. The absolute yield of the 25 th harmonic is very similar to that of the optimization for the 29 th harmonic but lower and higher orders of harmonics are suppressed.  
Using SF\({}_{6}\) for spectral broadening only two different evolutions of the optimization can be observed as shown in Figure 7c,d. For high energy harmonics (blue, red and black marked regions) a single harmonic can be increased above 50% of the spectrum. For lower order harmonics (green and purple marked regions) even after optimization the spectrum contains all harmonics and the spectral fraction of a single harmonic is below 30%. This different behavior using SF\({}_{6}\) instead of argon for spectral broadening can be attributed to a slightly changed phase profile due to the SRS contribution, _i.e._, will enhance the dynamic range for the phase only pulse-shaping, because spectral components, which are not needed for putting together the ideal pulse in time domain, can be suppressed by stretching them in the time domain. On the other hand, the role of the missing spectral parts in case of argon can be hardly enhanced by a phase shaper.  
Summing up, the selection of single harmonics works are best for the 27 th harmonic (red area in Figure 7a,c) independent of the medium for spectral broadening. In both cases this harmonic achieved the highest spectral fraction (above 60%). Using SF\({}_{6}\) for spectral broadening the spectral window for selective optimization is wider and allows us to enhance also 29 th and 25 th harmonic substantially and the corresponding development of the fitness is very similar.