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

Picosecond time-resolved images of plasma initiation were recorded during p ulsed-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. C lose to the target surface, interferometry measurements indicate that the e arly-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-vap or plume forms approximately 200 ps after the laser pulse, and it moves awa y 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-fl uids description of laser plasmas. The results indicate that the initiation of the plasma is due to air breakdown assisted by electron emission from t he target. (C) 2000 American Institute of Physics. [S0003-6951(00)01342-5].