Shock front behavior during pulsed laser ablation of graphite

Pulsed laser ablation of pyrolytic graphite with a 4-J  /  cm2 KrF laser in backgrounds of air, argon, nitrogen, and helium at pressures up to 10 Torr was performed to study the plume dynamics. Optical emission imaging with a 2-ns gated intensified charged-coupled device camera was used to determine shock front positions and plume trajectories for characterization by free expansion, Sedov–Taylor (ST) blast, and drag models. The plume expands with initial Mach numbers of M  =  55, decreasing to M  ∼  20 as the emission becomes too weak to detect. The plumes begin with a planar shock front and thickness of a few mean free paths, but evolve to higher dimensionality, n, depending on pressure and mass of the background gas. The ST energy released in the sudden ablation is typically 33% the laser pulse energy. Blast energy and plume dimensionality are correlated with stopping distances, which are typically greater than 103 mean free paths. Upper bounds for the mass ablated (1 to 2  μg  /  pulse) and hole depth (∼88 to 183 nm) are inferred from the shock kinetic energy relative to laser pulse energy. The inferred hole depth ranges from 0.32 to 0.67 percent of the thermal diffusion length as the pressure increases from 1 to 10 Torr for these conditions where the fluence is just above twice the ablation threshold.