Thermal and nonthermal aspects of nanosecond pulsed laser ablation

It is known that the translational energy of ablated species is often much greater than the thermal energy at the target surface estimated by mathematical modelling of heat transfer in the target. Possible explanations of this effect include atomic collisions in the gas phase, laser radiation-vapour coupling, and direct photodesorption. To understand under what conditions the above mechanisms are important, we apply the thermal-model of laser ablation that consists of the heat transfer equation in the target and the Euler equations in the gas phase coupled by the boundary conditions of strong evaporation/condensation. Numerical results are compared with the recent nanosecond ablation experiments on Al and Au at 193 nm, 5.3 J/cm² and Au at 266 nm, 3.5 J/cm² [I. Horn, M. Guillong, D. Gunther, Appl. Surf. Sci. 182, 91 (2001)] and graphite at 1.06 μm, 12 J/cm² [F. Kokai, K. Takahashi, K. Shimizu, M. Yudasaka, S. lijima, Appl. Phys. A 69, S223 (1999)]. While the above experiments are described by the proposed model, laser radiation-vapour coupling seems to be important in graphite ablation at 193 and 248 nm, 6.7 and 17.7 J/cm² [A.A. Puretzky, D.B. Geohegan, G.E. Jellison Jr., M.M. McGibbon, Appl. Surf. Sci. 96-98, 859 (1996)].