Initiation of an early-stage plasma during picosecond laser ablation of solids

Picosecond time-resolved images of plasma initiation were recorded during pulsed-laser ablation of metal targets in an air atmosphere. An early-stage plasma was observed to form before the release of a material vapor plume. Close to the target surface, interferometry measurements indicate that the early-stage plasma has an electron number density on the order of 10(20) cm(-3). The longitudinal expansion of the ionization front for this plasma has a velocity 10(9) cm/s, during the laser pulse. In contrast, a material-vapor plume forms approximately 200 ps after the laser pulse, and it moves away from the target at 10(6) cm/s. The experimental observations of the early-stage plasma were simulated by using a theoretical model based on a two-fluids description of laser plasmas. The results indicate that the initiation of the plasma is due to air breakdown assisted by electron emission from the target. (C) 2000 American Institute of Physics. [S0003-6951(00)01342-5].