## 3 Results and Discussion
### UV-Vis Spectrophotometry
Moreover, the higher fluence applied in the case of the 343 nm ablation compared with the 800 nm ablation for almost the same pulse energy does not necessarily ensure a higher ablation rate of the material; this is because the higher fluence is obtained by the decrease in the focus beam diameter onto the material surface (spot size), which results in a decrease in the material volume that can absorb high enough energy from the beam to reach the threshold for ablation. In fact, ablation studies using ns laser beams have shown that the polyyne concentration in the produced solution becomes lower at higher fluences that are determined by a smaller spot size (radius \(\leq\) 1.5 mm) and low pulse energy, whereas a higher concentration (by at least twice) is obtained for lower fluences but is determined by a larger spot size and higher energy [4]. These arguments can explain the absence of any polyyne peaks in the spectra that correspond to the 343 nm ablation compared with the 800 nm ablation; meanwhile, the presence of the weak peak in the spectra (and also from the longer chain (C\({}_{8}\)H\({}_{2}\)) polyyne) that corresponds to the 1 kHz interval ablation compared with the continuous 1 kHz ablation for the same total time duration can be explained by assuming that any polyynes that have not been destroyed by the laser beam during the first ablation interval (33 min) have already been diffused outside of the ablating laser beam path in the liquid volume, thus remaining undestroyed by the laser beam during the second ablation interval and/or by the formation of any new polyynes [4; 24]. In all spectra, there is also a band centered at \(\sim\)262 nm, which is known to be related to C-NMs that are formed in the solution together or apart from the polyynes [2; 3] and which originates from the \(\pi\)-\(\pi^{*}\) transition of the carbon skeleton. In the case of a ps laser ablation (700 ps/1064 nm/7 or 10 kHz/0.50-0.95 J.cm\({}^{-2}\)) of graphite in water, such a band appeared in the spectra at \(\sim\)260-300 nm, and it was attributed to a resonance transition of possibly existing C-related bonds onto the surfaces of CNPs that are formed in the solution [12]. In the case of a ns laser ablation (5 ns/532 nm/10 Hz) of graphite in water, a similar band at \(\sim\)260 nm in the spectra was attributed to reduced graphene oxide (rGO) layers that are detached from the graphite during the laser ablation process and float on the solution surface (air-water interface) [10]. The band is only observed for ablation at high fluences (above 6 J/cm\({}^{2}\)) together with peaks that are only from the longer chain C\({}_{8}\)H\({}_{2}\) polyynes that also have reduced intensity, and this is consistent with the present observation about the presence in the spectrum that corresponds to the interval ablation (Figure 1b) of only the long-chain C\({}_{8}\)H\({}_{2}\) polyyne (together with the 262 nm band); for lower fluences, the band was absent, and peaks from the long- and short-chain polyynes were observed [10]. Upon fs laser ablation (35 fs/800 nm/1 kHz) of HOPG in water at a high fluence of 25 J/cm\({}^{2}\), this band was observed at \(\sim\)270 nm (but no peaks from polyynes were present in the spectra) and was attributed to GQDs and PG [11]. In the case of an 800 nm ablation, the increase in the intensity of the peaks in the spectra with fluence (Figure 1a) indicates an increase in the synthesis yield of polyynes as well as C-NMs. In addition to the peaks from the polyynes and the band from C-NMs in the low wavelength region, the spectra are also characterized by a background absorption in the whole spectral region up to the longest measured wavelength of 1100 nm. This background absorption is attributed to the formation of various residual hydrocarbons (impurities) that have a form that is different than that of the polyynes, and it typically appears during polyyne synthesis by laser ablation of graphite in water using ns or ps laser sources [5; 7; 12].