## 3 Results and Discussion
### Lips  
The plasma plume optical emission spectra upon laser ablation of the target in DI water for short time delays (up to \(\sim\)120 ns) are dominated by a continuum background, which is known to mainly originate from decelerated free electrons and the electron-ion recombination processes [34]. At later times, the background emission is extinguished (Figure 6a).  
The visual intensity of the plume and the intensity of the light emitted from any excited plasma plume species that are able to reach the detector was very weak even when using higher pulse energies because of the scattering and/or absorption of the plume emission light from the water and because of the absence of the plasma plume heating effect by the laser pulse due to the use of a fs laser pulse for ablation. There is a hint of the appearance in the spectra of emission due to the (0,0) fundamental vibrational transition \(\Delta\
u\) = 0 of the Swan band at 516.47 nm from radical diatomic carbon molecules C\({}_{2}\) (d \({}^{3}\Pi_{\mathrm{g}}\)-a \({}^{3}\Pi_{\mathrm{u}}\)) (Figure 6b). For comparison, in the same graph, spectra are also shown upon target ablation  
Figure 6: LIBS spectra measured for different time delays upon ablation (50 uJ/6 kHz) of the target in DI water in the whole spectral region (400–900 nm) (**a**) and in the region of 511–518 nm where the (0,0) transition of the C\({}_{2}\) Swan band appears at 516.47 nm (**b**). Spectra taken upon ablation of the target in air are also shown in (**b**) for comparison.  
in ambient air (Supplementary Materials and corresponding references [35; 36; 37; 38; 39; 40; 41; 42]). These carbon diatomic species are created in the plasma plume under the high-pressure conditions because of the water confinement of the plume, and they contribute to the formation of the C-NMs. However, based on the mechanism of polyyne formation (explained in the Introduction of the paper), the possibility that they may also contribute to the formation of polyynes that are subsequently destroyed by the laser beam is not excluded; the final result is that, at the end of ablation time duration, polyynes are not present in the solution and only C-NMs are (Figure 1b).  
Other carbon-related lines that are distinguished in the spectra are a line at 808.37 nm (Figure 7a) that corresponds to the 2s\({}^{2}\)2p3p \({}^{3}\)P\({}_{2}\)\(\leftarrow\) 2s\({}^{2}\)2p5s \({}^{3}\)P\({}_{1}\) transition of neutral carbon atoms (CI or C) [21] and a line at 868.25 nm (Figure 7b) that corresponds to the 2s\({}^{2}\)4s \({}^{2}\)5\({}_{1/2}\)\(\leftarrow\) 2p\({}^{3/2}\)P\({}_{3/2}\) transition of CII ions (the weak line at 869.66 nm that is distinguished in the spectra corresponding to 110, 120, and 130 ns corresponds to the 2s\({}^{2}\)4s \({}^{2}\)5\({}_{1/2}\)\(\leftarrow\) 2p\({}^{3}\)\({}^{2}\)P\({}_{1/2}\) transition of the CII ions) [21]. Notably, the peaks that appear on either side of the peak at 868.25 nm in the spectrum corresponding to 110 ns (at 867.52, 868.04, 868.39, and 868.67 nm) (Figure 7b) are due to nitrogen ions or neutrals [21]. These lines decrease in intensity for longer time delays and are finally extinguished from the spectra at \(\sim\)160 ns. The almost absent C\({}_{2}\) Swan band from the spectra measured in DI water may be due to the possibility that so much oxygen is produced by the water decomposition at the plume-liquid interface that its reaction with the carbon plume species largely inhibits the formation of C\({}_{2}\) dimers [C\({}_{2}\) + O \(\rightarrow\) CO + C].  
Other lines that are distinguished in the spectra measured in DI water (Figure 6a) are: H\({}_{\alpha}\) (656.26 nm), NI (742.46 nm), NI (744.32 nm), NI (746.96 nm), OI (777.41 nm), OI (795.08 nm), NI (818.79 nm), NI (821.71 nm), NI (824.4 nm), OI (844.73 nm), NI (863.06 nm), NI (871.95 nm), and NII (881.93 nm) [21]. The oxygen and hydrogen lines mainly originate from the water decomposition, whereas the nitrogen lines originate from the atmospheric nitrogen that exists dissolved in water.