## 3 Results and Discussion
### UV-Vis Spectrophotometry  
UV-vis absorption spectra from the solutions produced by 800 nm/90 fs/1 kHz laser ablation for different fluences (pulse energies) of 4 (40), 11 (110), and 20 (200) J/cm\({}^{2}\) (uJ) (30 min ablation time) and those produced by 343 nm/190 fs/1 or 6 kHz/98 or 80 uJ laser ablation are shown in Figure 1a,b. In the case of the 343 nm ablation, due to the shorter laser beam wavelength, the focus beam diameter on the target surface (\(\sim\)7 um) becomes much smaller than in the case of 800 nm ablation; this results in the application of a much higher fluence, even for a relatively small pulse energy (250 and 200 J/cm\({}^{2}\) for the above pulse energies, respectively). The implication of this fact on the production yield of polyynes will be discussed later. Considering the 800 nm ablation, absorption peaks appear for all fluences in the UV region of the spectra, which correspond to the allowed electric dipole transitions of hydrogen terminated polymers from the ground state to the excited state (\({}^{1}\Sigma_{\text{u}}{}^{+}\leftarrow\text{X}\,^{1}\Sigma_{\text{g}}{}^{+}\)). Particularly, peaks due to C\({}_{8}\)H\({}_{2}\) are observed at 225.5, 215.5, and 206 nm, respectively, and C\({}_{6}\)H\({}_{2}\) peaks are observed at 199 nm [22]. Absorption peaks from the longer chain polyyne C\({}_{8}\)H\({}_{2}\) are observed at higher wavelengths compared with those from the shorter chain polyyne C\({}_{6}\)H\({}_{2}\) due to the decrease in the HOMO-LUMO gap of the polyyne with the increase in its length [23,24]. Contrary to the 800 nm ablation, no polyyne peaks are observed in the spectra corresponding to the 343 nm ablation that was carried out at 80 uJ/6 kHz and 98 uJ/1 kHz continuously for 33 and 66 min, respectively (Figure 1b), indicating that polyynes are not formed under these conditions of ablation. However, a weak peak from polyynes, particularly from the longer chain C\({}_{8}\)H\({}_{2}\) polyyne at 225.5 nm, is distinguished in the spectrum of the solution that was produced at 98 uJ/1 kHz, not under a continuous 66 min ablation, but carried out instead in two intervals of 33 min each with a pause of ablation for \(\sim\)5 min between them. These findings can be understood when considering the interaction of the ablating laser beam with the synthesized polyynes in the solution that is determined by its wavelength and pulse energy in relation to the ablation rate of the material that in turn is determined by the pulse energy and the focus beam diameter on its surface (the fluence), which leads to the polyyne formation [4,24]. The balance between these two  
Figure 1: UV-vis absorption spectra of the produced solutions corresponding to the 800 (**a**) and 343 nm (**b**) ablation. competing processes determines the polyyne synthesis yield and thus the concentration in the final produced solution. Density functional theory (DFT) calculations have shown that the band gap of a polyyne decreases with its number of carbon atoms [23; 25; 26]. The lower the wavelength of the ablating laser beam, the stronger its interaction with the polyynes for the same pulse energy. In fact, from the calculated band gap value of the polyynes, the interaction of the IR-wavelength (800 nm) beam with the polyynes (particularly for the shorter chain C\({}_{6}\)H\({}_{2}\) and C\({}_{8}\)H\({}_{2}\)) is expected to be minimum, whereas that of the UV wavelength (343 nm) is expected to be strong. This interaction of the polyynes with the beam may result in their destruction. It is also likely that the higher pulse repetition rate (6 kHz) ablation will result in the destruction of the polyynes at a higher rate than at the lower pulse repetition rate (1 kHz) ablation for the same pulse energy and ablation time duration. The rate of destruction will be even higher in a smaller solution volume (a slight increase was observed for the increase of the solution volume from 1.5 to 2.5 mL in the case of a ns ablation [24]).