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 exci mer KrF laser beam is studied in typical conditions of pulsed laser film de position. For the examination of transport phenomena of ejected species, th e space and time resolved evolution of the luminous plume is investigated b y fast imaging as a function of laser fluence (from 7 to 200 J/cm(2)) and n itrogen background pressure (from 5 x 10(-3) to 500 Pa) for five different target materials (boron nitride, graphite, alumina, molybdenum, a supercond ucting oxide YBCO). Under "vacuum" (5 x 10(-3) Pa) and for nitrogen backgro und pressures up to 10(-1)-1 Pa, the plume expands freely. For higher backg round 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 d rag force model correctly describes the plume shape evolution. Velocities o f the luminous plasma front and of the "mass center" of the plume are deter mined versus laser fluence and background nitrogen gas pressure.