Explosive phase transformation in excimer laser ablation

This work investigated phase change mechanisms during excimer laser ablation of nickel specimens. Time-resolved measurements were carried out to determine optical properties and the velocity of the laser-ablated plume, the ablation rate per pulse, light scattering from the laser-ablated particles and the size of the laser-ablated particles, in the laser fluence range between 2.5 J cm(-2) and 10.5 J cm(-2) (or 100 MW cm(-2) and 400 MW cm(-2) for a laser pulse of 26 ns). It was found that normal surface evaporation occurred when the laser fluence was below 5.2 J cm(-2). At a laser fluence of about 5.2 J cm(-2) or higher, the temperature at the target surface approached the critical point. The surface experienced an explosive-type vaporization process, ejecting large size droplets from the molten pool. Further increase of the laser fluence up to 9.0 J cm(-2) did not significantly change the surface temperature and the velocity and transmission of the laser-ablated plume. Explosive phase transformation was determined to be the main material removal mechanism when the laser fluence was higher than 5.2 J cm(-2). (C) 1998 Elsevier Science B.V.