# IR Pulsed Laser Ablation of Carbon Materials in High Vacuum
## 4 Discussion  
The last considerations are important, because they indicate that the ablation yield increases when the target has low density and low C binding energy, such as in vegetable carbon and active carbon, and decreases for materials with high density and high C binding energy, such as glassy carbon and HOPG. For diamond, for example, it is expected that the used laser ablation in a vacuum should induce a very low ablation yield concerning those reported in Table 2 due to its high binding energy (347 kJ/mol) and high density (3.52 g/cm\({}^{3}\)). The energy released by laser in different allotropic states of carbon produces photothermal and photo-chemical effects with strong increases in local temperature, chemical bond breaking, radical formation, coherent phonon conduction, and electron and X-ray generation [36]. Inverse Bremsstrahlung produces the ionization of carbon up to the fourth state, and the presence of a high electric field in the non-equilibrium plasma accelerates the ions up to about 200 eV per charge state, with an amount higher in the less compact targets, such as the vegetable and active carbon and the soft graphite. Also, the porosity of the target not permitting a fast energy conduction enhances the local temperature and the emission yield, as observed in the case of the high porose vegetable carbon. More compact targets instead allow for rapid energy and temperature disposal due to greater phonon diffusion, lower energy absorption due to minimal reflectivity and lower absorbance, and a more moderate and spatially defined ablation depth [37]. Finally, by calculating the ablation yield in terms of emitted carbon atoms per pulse, it is possible to evaluate the atomic plasma density at 1 us expansion time using the plasma volume acquired by the CCD images. Such plasma densities are reported in the last column of Table 2. They indicate that the maximum plasma density is obtained for the vegetable carbon (1.4 \(\times\) 10\({}^{18}\) ats/cm\({}^{3}\)) and the minimum for the glassy carbon target (6.7 \(\times\) 10\({}^{15}\) ats/cm\({}^{3}\)). Our measurements were performed using a 3 ns pulse duration. Of course, the change of this pulse duration changes the obtainable results. Long pulses involve more material to be ablated, because more energy is released not only to the direct irradiated material but also to the near one; however, this produces a minor laser intensity, and generally, the plasma parameters, such as the temperature, the density, and the charge states, decrease \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Carbon Target** & **Ablation Yield** & **Density** & **Binding Energy** & **Plasma Density** \\  & **(\(\upg\)/Pulse)** & & **(kJ/Mole)** & **at 1 μs (\(\times\)10\({}^{17}\) ats/cm\({}^{3}\))** \\ \hline HOPG & 2.81 & 2.26 & 285 & 0.25 \\ Graphite & 14.41 & 2.23 & [PERSON] and 285 & 1.5 \\ B Pencil graphite & 37.38 & 2.10 & [PERSON] and 285 & 2.9 \\ Glassy carbon & 0.46 & 1.46 & C=C, 614 and C-C, 347 & 0.067 \\ Active carbon & 28.17 & 1.45 & [PERSON] & 1.6 \\ Vegetable carbon & 96.14 & 0.63 & [PERSON] & 1.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Measured ablation yield in the different targets, density, and literature binding energies of the C atoms. with the intensity. On the contrary, shorter pulses limit the volume of irradiated materials, and atoms receive more energy and are emitted with higher kinetic energy; in this case, the intensity increases, and the plasma shows high temperature and density and higher charge states. In this last case, it is expected that carbon nanostructures can be synthesized. The presented study finds applications in different fields, from the innovative laser ion sources (LIS), useful to inject ions in accelerators, to the large application of carbon nanostructures, C-dots, and fullerenes, which can be prepared by laser ablation in vacuum, gases, and liquids, which can be applied to optoelectronics, microelectronics, biology, medicine, and material sciences, as largely reported in the literature [18; 19; 20; 21; 22].