  A molecular-dynamics thermal annealing model is proposed to investigate the
mechanisms involved in picosecond pulsed laser ablation of crystalline silicon.
In accordance with the thermal annealing model, a detailed description of the
microscopic processes which result from the interaction of a 308 nm, 10 ps,
Gaussian pulse with a Si(100) substrate has been embedded into a
molecular-dynamics scheme. This was accomplished by explicitly accounting for
carrier-phonon scattering and carrier diffusion. Below the predicted threshold
fluence for ablation, $F_{th}=0.25 \text{J/cm}^{2}$, a surface breathing mode
indicates that the solid restores internal equilibrium by the generation of
pressure waves. Above $F_{th}$, our simulations reveal that matter removal is
triggered by subsurface superheating effects: intense heating of the material
leads to the thermal confinement of the laser-deposited energy. As a result,
the material is overheated up to a temperature corresponding to the critical
point of silicon and a strong pressure gradient builds up within the absorbing
volume. At the same time, diffusion of the carriers into the bulk leads to the
development of a steep temperature gradient beneath the surface. Matter removal
is subsequently driven by the relaxation of the pressure gradient: large pieces
--- several atomic layers thick --- of molten material are expelled from the
surface with initial axial velocities of $\sim 1000 \text{m/s}$, their ejection
following the nucleation of voids beneath the surface.
