The femtosecond laser material interaction takes placeon a very short time scale. The transfer of energy fromthe pul pulse to the material cannot be described usingse thermal concepts as these are based on a time scalewhich is much longer than that during femtosecondpulse interaction. Therefore the femtosecond pulsemay be considered as an intense electromagnetic wave.When a ultra short laser pulse impinges asemiconductor, the atoms are ionized by two processesmainly multiphoton ionization and impact ionization.These processes are responsible for exciting theelectrons from the valence band to the conductionband. We als also study how collisional absorption ofo photons affects the ionization rate and bring out itssignificance. As the atoms are ionized the cohesiveenergy which binds them together becomes very small,the intermolecular bonds between nuclei are weakenedand th the positively charged nuclei repel each other. Ase the substrate is ionized by each laser pulse, thecohesive energy decreases and the material ablationresults due to Coulomb explosion which is thehypothesis for this study. As they are repelled, theelectr electrons that were excited from the valence band toons the conduction band show a tendency to recombinewith the holes created in the valence band. Thisprocess is called recombination. For very high carrierdensities generated by ultra short pulses the primaryre recombination mechanism is called Augercombination recombination. The recombination mechanism causesthe repulsion between atoms to reduce and our purposeis to try and understand how much of an effect therecombination mechanism has on the ablation rate andhence the expansion of the ablation zone. In this paperwe present a model for the ionization processes andhence solve for the ionization rate at the end of thepulse. As the material expands the electrons are assumed to move with it and hence this process can beaptly described by the 1 ptly 1-D convection equation withrecombination. This convection equation is solvedalong with the Euler equations and the ionization rateis used as the initial condition to predict expansionvelocity, pressure and the temperature in the ablationzone.
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