# IR Pulsed Laser Ablation of Carbon Materials in High Vacuum
## 3 Results
Figure 6: IC-TOF spectra acquired using a laser ablation at 45\({}^{\circ}\) incidence angle of graphite target and at different angles of detection around the normal direction: 0\({}^{\circ}\) (a), 22\({}^{\circ}\) (b), 45\({}^{\circ}\) (c), and 60\({}^{\circ}\) (d). Figure 7d shows a comparison between the normalized C ion emission yield in graphite and vegetable carbon targets versus the angle emission for similar irradiation conditions. In agreement with the CCD images, the angular distribution is larger for C ions emitted from vegetable carbon targets concerning those emitted from the graphite one. This is expected, because the C-atom binding energy in vegetable carbon is weak, but rich in Van der Waals forces, which cause large particle emissions, while in graphite, the higher C binding energy of the hexagonal structure of its planes decreases the angular emission, while retaining low bond energy between the various planes [33]. A final investigation was performed for the evaluation of the ablation yield in terms of emitted C atoms per laser pulse in the different targets with different atomic structures, chemical bonding, energy binding, and mass densities. This study was executed using a surface profiler to measure the crater size produced by a single laser shot ablating a fresh target surface. Each crater was analyzed along its diameter with a scan length of about 2-3 mm, a depth resolution of 10 nm, a tip weight of 2 mg, and a sweep velocity of 10 \(\mu\)m/s. Figure 8 shows some crater profiles produced by a single laser shot at 100 mJ pulse energy focused on the target surface with an incidence angle of 0\({}^{\circ}\), at which the spot size on photographic calibration paper was 1 mm\({}^{2}\), corresponding to a spot diameter of 1130 \(\mu\)m. They are referred to as the targets of HOPG (a), graphite (b), pencil graphite (c), glassy carbon (d), active carbon (e), and vegetable carbon (f), respectively. Of course, the surface roughness of each target is different, and non-homogeneity may be present overall in the vegetable carbon. In any case, the comparison shows that the crater diameter, d, and its average depth, z, reported in each spectrum of Figure 8, are different for each target, because the crater size depends on the electrical and thermal conductivity and the  
Figure 7: Carbon ion angular emission distribution in graphite, for yield (**a**), maximum energy (**b**), photon intensity (**c**), and comparison between yield emission from graphite and vegetable carbon target (**d**). density and structure of the target. In the porous and low-density vegetable carbon, the crater depth assumes the maximum value, while the minimum value is measured in the crystalline HOPG and in the pyrolytic glassy carbon. Assuming an isotropic circular ablation and the material density values reported in Table 1, it is possible to calculate the amount of mass removed per laser shot to evaluate the laser ablation yield for the six different carbon targets. The evaluated ablation yields are reported in Table 2, summarizing the results obtained as average values between three crater profile measurements. It is possible to observe that the ablation yield depends on both the material density and the C binding energy. The  
Figure 8: Crater depth profiles of the six targets irradiated in the same experimental conditions: HOPG (**a**), graphite (**b**), pencil graphite (**c**), glassy carbon (**d**), active carbon (**e**), and vegetable carbon (**f**). ablation yield is inversely proportional to the density; comparing the denser HOPG with the less dense vegetable carbon, the ablation yield changes from 2.81 up to 96.14 ug/pulse. The ablation yield is inversely proportional to the C binding energy; in fact, it is high in the case of vegetable carbon, where the C binding is due only to Van der Waals attraction forces, while it is low in the case of glassy carbon, which contains a significant amount of the high binding C=C energy of the order of 614 kJ/mol [34].