## 3 Results and Discussion
In contrast, for disordered targets it seems that the polarization played no role, while at any orientation of polarization  
Figure 4: X-ray spectra from the flat (solid black line) and nanostructured (ordered NW (dashed red line) and disordered NW (dash-dotted blue line)) ZnO targets for p-polarization (**a**) and s-polarization (**b**) measured by CCD1 in single photon counting regime. The spectra were fitted with an exponential function (solid grey line), providing an estimation of the electron bulk temperature \(T_{e}\). there was always a component of the field normal to the local plasma gradient and responsible for the efficient generation of electrons causing bremsstrahlung. A rough estimation of the hard X-ray flux in the range 150-350 keV gave up to 10\({}^{6}\) photons/pulse in 4\(\pi\). Our observations are in agreement with recent studies using copper nanorods with similar length to our NWs (10 \(\upmu\)m). Despite the higher average density of the Cu target material [41], the reported X-ray flux was similar to that in our experiments. The K-shell emission spectra, measured with two crystal spectrometers for all investigated samples, are shown in Figure 6. The observed lines correspond to the K-L (K\({}_{\alpha}\)) and K-M (K\({}_{\beta}\)) transitions in cold material. Note that K\({}_{\alpha}\) and K\({}_{\beta}\) emission from ion states up to Zn\({}^{20+}\) (Ne-like Zn) strongly overlap and cannot be resolved with our spectrometers. Therefore, line spectra in Figure 6 could be a superposition of line emissions from transient charge states up to 20+ and suggest that no Zn ions with a charge state higher than 20+ were generated under the conditions of our experiments. The polarization dependence of the line spectra is similar to the dependence observed for the bremsstrahlung radiation: The disordered NW target showed the same efficiency for all polarization directions, the ordered NW target demonstrated a weak polarization dependence, and the flat target was strongly sensitive to the laser polarization. Comparing the dependence of the line emission and high-energy bremsstrahlung radiation on the target morphology (Figures 5 and 6), we conclude that, in contrast to high-energy photons, no significant enhancement in the characteristic emission (also known as \"cold\" emission) for NW arrays was observed. This result can be explained as follows. The yield of the continuum of bremsstrahlung emission in the range of quanta energy below 100 keV is essentially the same, or very similar, for polished and nanostructured targets (see Figures 4 and 5). Therefore, the density of electrons with similar kinetic energies should be comparable for all types of targets. However, these are the electrons contributing the most in the generation of the K\({}_{\alpha}\) emission in Zn [42]. Therefore, despite the assumed deeper penetration of the laser pulse into the NW targets and volumetric heating, the yield of the K\({}_{\alpha}\) emission was not enhanced because of reabsorption. Additionally, the hot electrons could go far into the target volume (12 \(\upmu\)m for 50 keV electrons) creating the K-shell holes. However, in the case of the NW targets, the substrate was made of Si, and thus this volume did not make any contribution to the Zn K\({}_{\alpha}\). Concluding the results on K\({}_{\alpha}\) emission, it is noteworthy that the ratios of the integrated K\({}_{\alpha}\) fluxes from the investigated ZnO targets, calculated from the data measured with two crystal spectrometers and directly with the CCD camera (CCD1),  
Figure 5: Hard X-ray spectra from the flat (black line), ordered (red line), and disordered (blue line) nanostructured ZnO targets for p-polarization (**a**) and s-polarization (**b**) measured by the Timepix detector. Relation of the hard X-ray flux is given for the marked (yellow) energy range. The spectra were fitted with an exponential function (solid grey line) providing an estimation of the electron “hot” temperatures \(T_{e}\). were in close agreement with each other.