Excimer laser ablation of molten metals as followed by ultrafast photography

Molten Sn and Bi are ablated in vacuum by an ArF excimer laser. Pictures of the surface and the ablated material are taken by ultrafast photography, w ith temporal resolution of 1 ns using delayed dye laser pulses. The series of snapshots covering the 0 ns-200 mu s time domain contain information on the ablated plume, the development of waves on the target surface, and the initial phase of droplet formation. The velocity of the front of the ablate d plume is approximately 6 km/s for both Sn and Bi at 5.5 J/cm(2). While on the molten Sn surface only wave generation is observed with practically no droplet emission. the Bi surface emits a remarkable amount of material in the form of droplets originating from liquid jets. The speed of these dropl ets is two orders of magnitude smaller than that of the plume front. The re laxation of the whole perturbed melt pool lasts second(s) after ablation. B y decreasing the fluence below 2.5 J/cm(2) the Bi droplet formation can als o be suppressed. (C) 1999 Elsevier Science B.V. All rights reserved.