## 3 Results  
The first set of our measurements was aimed at the analysis of plasma emission from the ablated species. The plasma emission spectra in the 30-100 nm wavelength range for the six targets are presented in Figure 2.  
Figure 2: Spectral emission from different carbon-containing plasmas in the case of nanosecond laser ablation. The positions of harmonics (H9–H23) from 806 nm laser source are shown in red. Plasma emission was recorded at heating pulse parameters (pulse energy and fluence) larger than the optimal ablation characteristics for harmonics generation. The fluence of heating pulses during these studies was equal to 15 J cm\({}^{-2}\). We did not average the plasma spectra from these samples but instead used a single shot collection. The emission lines were determined using the NIST database [21]. The positions of the 806 nm radiation harmonics from the bulk carbon starting from 9 th (H9) to 23 rd (H23) orders are shown as the red curves. The harmonic profiles shown in this figure were included for demonstration of the closeness with some ionic transitions. For the bulk C plasma produced by HP with the energy of E\({}_{\text{HP}}\) = 3 mJ, we can see the presence of intense CI and CII lines corresponding to the transitions in neutral and singly charged ions of carbon in the spectral region between 55 and 100 nm, as well as the presence of CIII lines below 55 nm. The density of free electrons in bulk C plasma was estimated to be -5 \(\times\) 10\({}^{16}\) cm\({}^{-3}\), which is similar to the estimates at the same laser parameters presented in [22]. The particle density was estimated in [23] to be in the range \(\sim\)10\({}^{18}\) -10\({}^{19}\) cm\({}^{-3}\) for comparable laser fluencies. The contribution of continuum emission decreased the contrast of emission lines. In the case of the targets comprising nanostructured materials, the shift of emission to larger presence of the lines attributed to CII and CIII transitions was observed, except for CNF and DN. A pattern similar to the above-described spectrum was observed in the case of CNF (E\({}_{\text{HP}}\) = 3 mJ) and GR (E\({}_{\text{HP}}\) = 3 mJ). However, contrary to bulk C plasma, the continuum emission was localized in the shorter wavelength region below 65 nm. In the case of C\({}_{60}\) (E\({}_{\text{HP}}\) = 2 mJ) and MW CNT (E\({}_{\text{HP}}\) = 2 mJ), the relative intensities of CIII lines significantly increased, while additional strong CIV transitions appeared in the plasma spectra. A similar pattern was observed in the case of all nanostructure-containing plasmas, except CNF. In the case of C\({}_{60}\), this behavior confirms the earlier reported observations that, under similar experimental conditions, the plasma from fullerenes reaches much higher ionization states than in the case of bulk graphite ablation [24]. This statement is valid for all nanostructured materials, while at the same time, we observed a decrease in the relative intensity of CI (85.9 nm) line corresponding to the transition in neutral carbon. In DN (E\({}_{\text{HP}}\) = 4 mJ) case, the plasma spectrum contained less emission lines and a smaller continuum despite the stronger excitation of the target. The different applied HP energies for different samples are explained by the different optimal conditions for plasma formation suitable for HHG. The term \"optimal conditions\" refers here to the maximal harmonic yield from each ablated sample, which depends on the fluence of HP on the target surface at similar geometry of the focusing conditions. Additionally, the pressed tablets were destroyed at higher fluencies of HP due to their fragility. For example, the harmonic yield from GR and DN LIPs was more than twice as small as from other targets due to weaker ablation at which the pressed tablets remain intact. Correspondingly, we could not increase the HP fluence on those fragile surfaces. As shown below, the ablation corresponded to the formation of low-ionized plasma using suitable fluence of HPs provided the conditions for efficient HHG.