In the ultrashort regime laser material interaction can no longer be described by thermal concepts. The femtosecond laser should be considered as an intense electromagnetic wave. In the case of semiconductors, when the electromagnetic wave interacts with the electrons, the electrons are excited from the valence band to the conduction band through multi-photon and impact. These highly energized electrons are now free carriers similar to those in metals and rapidly equilibrate with themselves, on the order of femtoseconds. The excited electrons collide with the lattice and may also recombine with the holes that were created when the electrons were lifted to the conduction band. When the electrons collide with the lattice energy is transferred to the lattice. At this time the electrons and the lattice are not in thermal equilibrium. Since the electrons that are providing the cohesive energy have been lifted to the conduction band, the intermolecular bonds between the nuclei are weakened and the positively charged nuclei repel each other. Consequently, the repulsive force between nuclei causes the material to expand. As the substrate is ionized by each laser pulse, the cohesive force decreases and ultimately leads to Coulomb explosion. It is hypothesized in this paper that the Coulomb explosion is the mechanism responsible for material ablation. We have developed a model for the multi-photon ionization and impact ionization. This model can predict the ionization at the end of the femtosecond laser pulse. The ionization at the end of the pulse is the initial condition for the modeling of the Coulomb explosion. Predictions of ionization rate and expansion velocities will be presented and discussed.
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