## 4 Discussion
Returning to the model [27], harmonics generation is described as a process where the first step is the formation of the electron wave packet by tunneling ionization and the second step includes a classical mechanics description of released electron in laser field. The first step can be described by the Ammosov-Delone-Krainov (ADK) expression for ionization rates \(\Gamma_{ion}\) for different ionization potentials [30] and defines in two-step model the efficiency of conversion. While estimating ADK rates we assumed the laser intensity to be equal to what we measured in the focal plane of the focused driving beam (2 \(\times\) 10\({}^{14}\) W cm\({}^{-2}\)). Direct comparison of ionization rates by ADK formula gives \(\Gamma_{ion}\)[\(I_{p}\)(C) = 11.26 eV]/\(\Gamma_{ion}\)[\(I_{p}\)(C\({}^{+}\)) = 24.38 eV]-10\({}^{5}\) higher rate for tunnel ionization in the cases of the neutral and single charged carbon atom. By applying the same approach to evaluate the difference in ionization rates of fullerenes and carbon, one can define the following ratio \(\Gamma_{ion}\)[\(I_{p}\)(C\({}_{60}\)) = 7.6 eV]/\(\Gamma_{ion}\)[\(I_{p}\)(C) = 11.26 eV] \(\approx\) 5. Thus, C\({}_{60}\) molecules can be considered more efficient high harmonics emitters than ordinary carbon atoms or small-sized carbon molecules. Their presence in LIP leads to a stronger enhancement of the low-order part of HHG spectrum comprising the harmonics between 9 th and 17 th orders. We can conclude that, despite the lower density of C\({}_{60}\) nanoparticles in plasma compared to C atoms and ions originated from graphite ablation, higher HHG efficiency allows the observation of harmonics even at notably larger delays (>1000 ns) compared with the latter species (Figure 4a). Higher cutoff position in the case of ablated graphite is attributed to the presence of neutral carbon and small-sized C\({}_{n}\) (where \(n<\) 20) molecules. Note that the role of different small carbon clusters synthesized during ablation of graphite was analyzed in the studies related to HHG in carbon LIP [20]. The middle column of Figure 3 represents HHG spectra obtained from, to some extent, morphologically similar species: multiwalled carbon nanotubes (blue solid lines) and carbon nanofibers (red solid lines). MW CNT plasma dynamics showed the 400 ns optimal delay from the beginning of ablation corresponding to maximal harmonic yield. Simultaneously, the delay dependence (Figure 4a, blue line with solid circles) had the plateau-like structure between 200 and 400 ns. The harmonic yield in this time scale was equal or slightly higher than in the case of C\({}_{60}\) plasma. The enhanced harmonics from 9 th to 17 th orders were observed between 100 and 400 ns delays (Figure 3, middle column). At the same time, the cutoff (25 th harmonic) at the optimal delay (400 ns) was slightly higher compared to C\({}_{60}\) plasma (Figure 4b). We can assume that the enhanced low-order harmonics were attributed to the presence of small fragments of CNTs in LIP. At the same time, higher cutoff can be attributed to the coherent contribution to HHG from the carbon neutral atoms and small-sized carbon molecules. Notice that the ionization potential of single-walled CNT is in the range of 5-6 eV [31]. The HHG spectrum in the case of short (100 ns) delay indicates that small-sized components of such plasma can gain, to some extent, larger velocities, thus causing the enhanced harmonic spectra with larger cutoff order. This is what we see in the case of bulk C and MW CNT. At the same time, the large-sized components of LIP responsible for enhancing the lower orders of spectra are expected to appear at larger delays. Summarizingthe analysis of HHG in MW CNT, one can conclude that, at the used parameters of HP, MW CNT plasma also contains small carbon particles alongside the fragments of nanotubes. Meanwhile, the HHG spectrum from CNF was surprisingly similar to C\({}_{60}\).