Expansion of the ablation plume created by ultraviolet laser irradiation of various target materials

The plasma plume created during photoablation of various targets by an excimer KrF laser beam is studied in typical conditions of pulsed laser film deposition. For the examination of transport phenomena of ejected species, the space and time resolved evolution of the luminous plume is investigated by fast imaging as a function of laser fluence (from 7 to 200 J/cm(2)) and nitrogen background pressure (from 5 x 10(-3) to 500 Pa) for five different target materials (boron nitride, graphite, alumina, molybdenum, a superconducting oxide YBCO). Under "vacuum" (5 x 10(-3) Pa) and for nitrogen background pressures up to 10(-1)-1 Pa, the plume expands freely. For higher background pressures (greater than or equal to 10 Pa), three successive regions above the target can be distinguished: at first the expansion is free, then the plume expands according to a shock wave-like behaviour, and lastly a drag force model correctly describes the plume shape evolution. Velocities of the luminous plasma front and of the "mass center" of the plume are determined versus laser fluence and background nitrogen gas pressure.