SCHLIEREN AND DYE-LASER RESONANCE-ABSORPTION PHOTOGRAPHIC INVESTIGATIONS OF KRF EXCIMER LASER-ABLATED ATOMS AND MOLECULES FROM POLYIMIDE, POLYETHYLENETEREPHTHALATE, AND ALUMINUM

Hydrodynamic phenomena from KrF excimer laser ablation (10(-3)-20 J/cm2) of polyimide, polyethyleneterephthalate, and aluminum are diagnosed by schlieren photography, shadowgraphy, and dye laser resonance absorption photography (DLRAP). Experiments were performed both in vacuum and gaseous environments (10(-5)-760 Torr air, nitrogen, and argon). In vacuum, ablation plumes are observed to expand like a reflected rarefaction wave. As the background gas pressure is increased, shock waves and reduced-density ablation plumes become visible. Below 10 Torr, the ablation plume follows closely behind the shock wave. Between 20 and 100 Torr, the plume recedes behind the shock wave. Below 10 Torr and above about 200 Torr, both the plume and the shock expand with the same temporal power law dependence. Agreement is found between these power law dependences and those predicted by ideal blast wave theory. The DLRAP diagnostic clearly shows that the ablated material (CN molecule from polyimide and ground state neutral aluminum atoms from laser-ablated aluminum) resides in the ablation plume. CN molecules are detected in both argon and air environments proving that CN is generated as an ablation product and not by reaction with the background gas. As the background gas pressure and the time after ablation is increased, the film darkening due to the laser-ablated material begins to fade leaving only the nonresonant shadowgraphy component of the plume. The plume dynamics observed by DLRAP are discussed in terms of gas dynamics, plume chemical kinetics, material diffusion in the plume, and cluster/particulate formation.