Gas dynamics of laser ablation: Influence of ambient atmosphere

A two-stage two-dimensional (2D) gas-dynamic model of laser ablation in an ambient gas atmosphere is proposed. The initial one-dimensional stage of th e process is related to the ablation plume formation under the action of a laser pulse (duration of the order of 10 ns; fluence about several J/cm(2); laser spot diameter about 1 mm) and describes heating, melting, and evapor ation of the target, the target-vapor interaction in the Knudsen layer, and the vapor dynamics. The final 2D stage is responsible for the formation of the energy and angular distributions of the ablated material. Considerable compression of the ambient gas around the expanding plume of the laser-eva porated material and a shock front propagating through the undisturbed ambi ent gas are found. The pressure of the compressed ambient gas behind the sh ock may be much higher than the ambient one. However, at the investigated a mbient pressures below 100 Pa, it remains still much lower than the vapor p ressure during laser evaporation. Therefore, the initial stage of laser abl ation is essentially independent of the ambient atmosphere. Once the laser pulse is over, the vapor pressure eventually drops down to a value comparab le to the compressed ambient gas pressure. From this time on, the gas consi derably suppresses vapor expansion. There is a noticeable difference betwee n the vapor distribution in vacuum and the one in the ambient atmosphere: t he vapor fills the entire plume volume in vacuum while in the presence of a mbient atmosphere it is accumulated near the plume boundary and tends to fo rm a thin shell. The angular and energy distributions of the ablated materi al are especially sensitive to the nature and pressure of the ambient gas. Both the kinetic energy of the ablated atoms and the width of their angular distribution decrease with the ambient pressure. (C) 2000 American Institu te of Physics. [S0021-8979(00)02914-5].