Time-resolved shock-wave photography above 193-nm excimer laser-ablated graphite surface

Highly oriented pyrolytic graphite (HOPG) was ablated by a 193-nm ArF excimer laser in air. The fluence was varied in the range 1–25 J/cm2. Every laser shot hit a pristine graphite surface. The emerging shock wave was recorded by a nanosecond-resolution photographic arrangement. The velocity of the shock wave as a function of time and laser fluence was measured. The amount of energy that generates the shock wave was determined and found to be about 5–7% of the incident laser energy. The shock wave is already present 10–15 ns after the maximum of the incident laser pulse. These facts imply that, even if high-energy (10–100 eV) ions, atoms, or clusters leave the surface, a layer several 10 nm thick has to be removed during this short period. The temperature of the shock front is ∼2500–4000 K, as derived from the measured velocities. Measuring the ablation depth by atomic force microscopy as a function of fluence revealed that the single-shot ablation threshold is 1.4±0.2 J/cm2, and the effective absorption coefficient is ∼1.5×105 cm-1.