## 3 Results and Discussion
The mean values for the relative K yield are given in Table 1. It is worth mentioning that the observed polarization dependence for the K\({}_{\alpha}\) emission had the same origin as the bremsstrahlung emission discussed above, since both types of radiation are related to the generation of hot electrons. Finally, the X-ray CCD camera used for the line emission detection in the quartz crystal spectrometer (CCD1 in Figure 1a) also detected quite intense background bremsstrahlung radiation homogeneously covering the chip. To investigate the origin of this strong background radiation, we performed several tests. (1) We blocked the quartz crystal, so that no emission could be generated or reflected from it. No change in the measured signal was observed in this case. (2) We placed a 2 mm Al foil at the output Kapton window of the experimental vacuum chamber. The background radiation disappeared completely. Since the foil is highly transparent (the transmission efficiency is \(>\)70%) for X-ray quanta with energies above 30 keV, we concluded that the observed emission was generated by fast electrons emitted through the Kapton window. The possibility for direct hits on the CCD chip by electrons was excluded, because forward accelerated electrons were shielded by the 2.5 cm thick Al wall of the vacuum chamber (see Figure 1a). Thus, we suggest that the background emission with the spectrum shown in Figure 7 was a bremsstrahlung emission generated by energetic electrons that radiatively lost their energy when interacting with air. Using data for the radiative stopping power of electrons in air [43], we estimated that the spectra in Figure 7 could be emitted by electrons with kinetic energies above 100 keV. Therefore, Figure 7a suggests that, in the case of p-polarization, which was the most favorable for the flat target, ordered nanowire arrays enable nearly two orders of magnitude and disordered arrays nearly one order of magnitude enhancement in the amount of high-energy electrons. This enhancement can be explained by the much higher efficiency of the laser energy absorption and much larger volume of the plasma (volumetric heating effect) for nanowire morphology in comparison to a flat surface. Again, the polarization dependence of the spectra in Figure 7 shows that the efficiency of fast electron generation was essentially insensitive in \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Flat** & **Ordered NW** & **Disordered NW** \\ \hline p-polarization & 1 & 1.19 \(\pm\) 0.04 & 1.31 \(\pm\) 0.05 \\ s-polarization & 1 & 2.79 \(\pm\) 0.11 & 3.18 \(\pm\) 0.08 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The mean values of the integrated Zn K\({}_{\alpha}\) emission for the investigated targets obtained with p- and s-polarized laser pulse. The values are normalized on the integrated Zn K\({}_{\alpha}\) flux from the flat sample. Figure 6: The K-shell emission and K\({}_{\alpha}\) doublet (insets) from ZnO targets (flat (black line), ordered NW (red line), and disordered NW (blue line)) detected with LiF (220) and quartz (40-4) bent crystal spectrometer, respectively. The spectra were obtained from interaction with a p-polarized (**a**) and s-polarized (**b**) laser beam. the disordered NW arrays, was weakly sensitive in ordered arrays, and was strongly sensitive to the laser polarization for the flat surface.