# IR Pulsed Laser Ablation of Carbon Materials in High Vacuum
## 3 Results  
First, attention was given to the measurements of the mass density, which are different for the different targets used. Using a caliber and a microbalance, the densities of the six used carbon targets, listed in Table 1, were measured. It is possible to observe that the targets have a decreasing density from the higher HOPG (2.26 g/cm\({}^{3}\)) [23] up to the low vegetable carbon (0.63 g/cm\({}^{3}\)), in agreement with the literature [23; 27]. Figure 2 shows the six ICR-TOF spectra obtained by irradiating, under the same conditions, the different carbon targets. The spectra are similar for the different irradiated targets and show the initial photopeak due to photon detection from laser-generated plasma, permitting the position of the zero-time scale to be determined, followed in time by the carbon ion peak detection emitted from the plasma. The photopeak intensity remains the same, at about 0.43 V, in all irradiated targets, indicating that the plasma emission of visible, UV, and soft X-rays maintains the same intensity under the used laser conditions. The carbon ion peak, containing different contributions from the different ion charge states, indicates that the TOF initial position is the same (5.71 us) in all targets except in the graphite target (5.51 us). Considering the flight distance of 0.93 m, such a peak position indicates that the maximum kinetic energy of the C ions corresponds to 1.66 keV, except for the graphite target, where it increases to 1.78 keV. The carbon ion peak intensity, instead, depends on the type of target, but no strong differences were observed. The maximum C peak intensity was obtained for graphite and corresponds to 0.36 V, while the minimum value of 0.22 V was obtained for glassy carbon and active carbon targets. No regular trend in the revealed carbon ion yield as a function of the target density was found. IEA measurements were obtained using deflection potentials ranging between zero and \(\pm\)70 V with a 5 V step variation between each measure. Some IEA spectra relative to the graphite target are shown in the six plots in Figure 3. The spectra indicate that there were only four charge states detected in the plasma--C1+, C2+, C3+, and C4+--while the charge states C5+ and C6+ were not detected. This is expected because the electron binding energy in the carbon K-shell is significantly higher than in the L-shells. \begin{table}
\begin{tabular}{c c} \hline \hline
**Carbon Target** & **Density** \\ \hline HOPG & 2.26 \\ Graphite & 2.23 \\ B pencil graphite & 2.10 \\ Glassy carbon & 1.46 \\ Active carbon & 1.45 \\ Vegetable carbon & 0.63 \\ \hline \end{tabular}
\end{table} 
Table 1: Measured mass density of the six used carbon targets. Figure 2: ICR-TOF spectra relative to the six laser irradiated targets: HOPG (**a**), graphite (**b**), pencil-graphite (**c**), glassy carbon (**d**), active carbon (**e**), and vegetable carbon (**f**). From the NIST database, the ionization potential of C4+ is 64.5 eV, while that of C5+ is 392 eV, indicating that the maximum electron energy of the plasma is higher than 65 eV and lower than 392 eV. It is plausible to assume that the maximum electron energy is about 150 eV (assumption by considering the average value and the high C4+ yield concerning C3+), from which we can calculate the average value of the Boltzmann electron energy distribution, which is about one-third of the maximum value, i.e., \(<\)E\({}_{\rm e}\)\(>\)\(\approx\) 50 eV.