In the past decade femtosecond laser ablation has found a wide range of applications (e.g. in micromachining, pulsed laser deposition), however, the underlying physical mechanisms and their dynamics are not fully understood. Indeed, ultrafast laser technology opens the way to track the dynamics of absorption and redistribution of the excitation energy on the femtosecond time scale. Ablation induced by fs laser pulses occurs on time scales significantly shorter than one would expect for thermal processes involving transitions through non-equilibrium states of matter. The results suggest that the ablation dynamics is a complex phenomenon, consisting of both ultrafast nonthermal and slow thermal components. The nonthermal component is associated with emission of fast electrons and transient breakdown of surface electrical neutrality. Such dynamics can lead to the softening and the instability of chemical bonds and finally to the ejection of charged particles in a process known as Impulsive Coulomb Explosion (ICE).
Our new results demonstrate that ICE is the main mechanism for ion emission in femtosecond laser ablation of graphite at low laser fluences. In order to reveal the dynamics of the process, we performed time resolved measurements, monitoring the velocities of the emitted products (both ions and electrons). The data suggest that ICE is a strongly non-equilibrium process occurring on time scales much shorter than the electron-lattice relaxation. The combination of mass spectrometric data and ex situ Atomic Force Microscopy (AFM) imaging indicates nanoscopic removal of intact monolayers. The results imply a significant degree of charge localization in the surface graphite layers.
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