Time-resolved observation of gas-dynamic discontinuities arising during excimer laser ablation and their interpretation

Ablation of materials (Cu is presented in this report) in air at an ambient gas pressure of 1 bar with a KrF excimer laser (3-47 J cm-2) leads to gasdynamic processes above the target surface which affect the processing result, the efficiency of the treatment and the debris in the environment of the irradiated area. These laser-induced processes have been diagnosed using fast schlieren photography and shadowgraphy. Five discontinuities have been discerned and their propagation mechanisms have been detected. A physical interpretation of the discontinuities is given along the lines of existing theories and plausible reasoning. The locally most advanced discontinuity can be explained by the classical Sedov-Taylor blast wave theory, and conclusions on the energy content in the shock wave, the pressure distribution and the surface pressure evolution will be presented. The results show that, at high energy densities (3-47 J cm-2), about 80% of the available laser pulse energy is deposited in the shock wave. A reduction in the energy density leads to a decrease in the fraction of the energy that is deposited in the shock wave. Close behind the first discontinuity follows a second one that is interpreted as the ionization front. The contact front, which separates shocked air and ablated material vapour, has been observed within the laser pulse duration. The complex structure of the contact front is interpreted in terms of gas flow phenomena inside the two outer discontinuities.