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 excim er laser in air. The fluence was varied in the range 1-25 J/cm(2). Every la ser shot hit a pristine graphite surface. The emerging shock wave was recor ded by a nanosecond-resolution photographic arrangement. The velocity of th e shock wave as a function of time and laser fluence was measured. The amou nt of energy that generates the shock wave was determined and found to be a bout 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 t hat, even if high-energy (10-100 eV) ions, atoms, or clusters leave the sur face, a layer several 10 nm thick has to be removed during this short perio d. The temperature of the shock front is similar to 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/cm(2), and the effective absorption coefficient is similar to 1.5 x 10(5) cm(-1).