# IR Pulsed Laser Ablation of Carbon Materials in High Vacuum
## 2 Materials and Methods  
An Nd:YAG pulsed laser (Litron TRLi 850, Warwickshire, UK) operating at a 1064 nm wavelength, \(\mathrm{E_{L}}\) = 100 mJ pulse energy, 3 ns pulse duration, from a single pulse up to 10 Hz repetition rate, with a 1 mm\({}^{2}\) spot size and 3.3 \(\times\) 10\({}^{9}\) W/cm\({}^{2}\) intensity was employed. The laser was focused by a 50 cm focal length lens placed in the air. The laser beam crossed a thin quartz window that separated the air from the vacuum of the scattering chamber. Here, at 10\({}^{-6}\) mbar, the laser hit a target placed in the center of a cylinder chamber evacuated by a molecular turbo pump. The holder contained different targets with a surface area of about 1.5 cm\({}^{2}\), which could be moved in proper positions and angles concerning the laser entrance direction by manual external control. Irradiation was generally performed at 45\({}^{\circ}\) or at 0\({}^{\circ}\) concerning the laser beam direction.  
The employed targets consisted of a carbon glassy sheet with high purity and a 1 mm thickness: a high-quality HOPG sheet with a 1 mm thickness and graphite with a 1 mm thickness were purchased by Goodfellow [23]. Graphite from a type B pencil (soft graphite, Faber-Castell, Stein, Germany) with 1 mm thickness and parallelepiped shape, active carbon tablet by Vivi&Diet [24] with 4 mm thickness, vegetal carbon (charcoal) from a coconut husk by Il-braciere-Eurospin [25] were obtained. All materials contain pure C atoms, but in the case of active carbon and vegetable carbon, many functional groups of oxygen and water can be present [6].  
The density of such targets is different, ranging between 2.26 g/cm\({}^{3}\) in the case of HOPG [23] and 0.63 g/cm\({}^{3}\) in the case of vegetable carbon, as measured in our laboratory.  
A ring ion collector (ICR) was used as a Faraday cup to monitor the ion detection emitted from the laser-generated plasma [26]. The ICR, furnished with an electron suppression grid polarized to \(-\)50 V, was used in a time-of-flight (TOF) configuration, connecting its output to a fast memory oscilloscope (Tektronix TDS 5104B, 1 GHz, 5 GS/s, Beaverton, OR, USA). The ion detection was triggered by the laser shot pulse. The ICR was placed at a distance of 93 cm from the target surface and a 45\({}^{\circ}\) angle concerning the laser beam direction. ICR permits this to be used to evaluate the maximum ion kinetic energy, the total ion yield, and the photon emission yield produced by the fast plasma pulse.  
An ion energy analyzer (IEA), based on electrostatic ion deflection to 90\({}^{\circ}\) and ion detection using a high sensitivity windowless electron multiplier (WEM), was employed to measure the ion energy charge states and energy distributions as a function of the deflecting electric potential. The WEM detector was placed after the ICR, at 150 cm from the irradiated target. Its output was also connected to the fast storage oscilloscope and was triggered by the laser shot. The IEA worked as a filter for detecting the energy-to-charge, E/z, of the deflected ions, as reported in the literature [26].  
The IEA allowed for measuring the ion kinetic energy from TOF measurements; in addition, it gave the ion charge states and the ion energy distributions as a function of their charge state. A scheme of the experimental setup is shown in Figure 1.  
Figure 1: Sketch of the experimental setup.  
The laser-generated plasma was monitored using a visible CCD camera with a minimum exposition time of 1 us (PixeFly PCO Imaging 200 SX, Gottingen Germany), which was triggered by the laser shot. The CCD camera was placed at a 90\({}^{\circ}\) angle concerning the normal direction to the target surface, together with its optics that permit focalization on the plasma plume. The laser craters and the removed mass from the laser ablation were measured using optical microscopy coupled to the surface profiler (Tencor P-10, KLA, Milpitas, CA, USA), which permits a profile of the crater diameter along its diameter with a depth resolution of up to 10 nm.