# IR Pulsed Laser Ablation of Carbon Materials in High Vacuum
## 3 Results
At this average, the plasma electron energy corresponds to an average plasma temperature kT of about  
\[<\)kT\(>\) = 2 \(<\)E\({}_{\rm e}\)\(>\)/3 \(\approx\) 33 eV  
Figure 3: IEA-TOF spectra relative to graphite target analyzed to different E/z filter ratios: 100 eV/z (a), 200 eV/z (b), 400 eV/z (c), 600 eV/z (d), 800 eV/z (e), and 1000 eV/z (f).  
The IEA detailed analysis, in terms of detected ion charge state and kinetic energy, has permitted measurements of the carbon ion charge states and energy distributions, whose experimental data are reported in the plot of Figure 4.  
Figure 4 shows the experimental ion energy distributions of the four carbon charge states, according to other data reported in the literature [28]. Aside from measurement errors, the data should represent four Boltzmann energy distributions that are shifted towards higher energy, increasing the ion charge state, by the Coulomb-Boltzmann-Shifted (CBS) theory reported in the literature [29]. The average energy of each ion energy distribution is about 200 eV for C1+, 400 eV for C2+, 600 eV for C3+, and 800 eV for C4+, indicating that, in the used laser conditions, the plasma ion acceleration is about 200 eV per charge state. The measured high kinetic energy of emitted ions may be useful to produce well-adherent carbon thin film deposition on different substrates, to generate C nucleation, nanoaggregates, and C nanoparticles in liquids, as reported in the literature [4; 30; 31].  
The CCD camera imaging has permitted the recording of the visible light plasma emission along the normal direction to the target surface, mainly due to carbon atom deexcitations. The recorded images, acquired in 1 \(\upmu\)s exposition time and produced under all the same experimental conditions of laser irradiation, are reported for comparison in Figure 5 for the target irradiated HOPG (a), graphite (b), pencil graphite (c), glassy carbon (d), active carbon (e), and vegetal graphite (f), respectively. Taking into account that the target surface roughness is not the same for all the irradiated targets, that the laser ablation has little change between the fresh surface and the laser irradiated surface, that the target thickness has little change between one target and another, and that the laser pulse energy may have about 5% energy oscillation for each laser pulse, the CCD images may give an approximated evaluation of the volume geometry of the expanded plasma in a vacuum in 1 \(\upmu\)s time from the laser shot.  
Of course, the acquired images are not well representative of the mechanism of plasma production, because they do not reproduce the light emission as a function of time and because they can be influenced by accumulations of errors caused by small variations in the angle of incidence, laser pulse energy, the surface roughness and thickness of the target, the trigger laser pulse synchronization process on CCD cameras, and more.  
Figure 4: IEA measurements of C ion energy and charge state distributions.  
Now, we consider one plasma image, the first of Figure 5, for example, from which it is possible to observe that the expansion length along the direction normal to the target surface is about 20 mm in the case of the HOPG target (a); this corresponds to an adiabatic expansion velocity of about \(\
u_{k}\) = 20 mm/1 \(\upmu\)s = 2 \(\times\) 10\({}^{4}\) m/s. Thus, from this value, taking into consideration the carbon mass \(m\), the adiabatic coefficient \(\gamma\) of monoatomic atoms, and the electron charge \(e\), it is possible to calculate the average plasma temperature \(kT\) using the following equation:  
\[kT=\frac{m\,\
u_{k}^{2}}{\gamma\;e}=\frac{12\;amu\;\text{-4.}\;\frac{10^{6}m^{ 2}}{s^{2}})}{1.67\;e}=30\;\text{eV}, \tag{2}\]  
a value in good agreement with that above given in Equation (1) of about 33 eV, given by the observation of the carbon charge state distributions.