Gas dynamics of laser ablation: Influence of ambient atmosphere

Abstract

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 the 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/cm2; laser spot diameter about 1 mm) and describes heating, melting, and evaporation 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-evaporated material and a shock front propagating through the undisturbed ambient gas are found. The pressure of the compressed ambient gas behind the shock may be much higher than the ambient one. However, at the investigated ambient pressures below 100 Pa, it remains still much lower than the vapor pressure during laser evaporation. Therefore, the initial stage of laser ablation is essentially independent of the ambient atmosphere. Once the laser pulse is over, the vapor pressure eventually drops down to a value comparable to the compressed ambient gas pressure. From this time on, the gas considerably suppresses vapor expansion. There is a noticeable difference between the vapor distribution in vacuum and the one in the ambient atmosphere: the vapor fills the entire plume volume in vacuum while in the presence of ambient atmosphere it is accumulated near the plume boundary and tends to form a thin shell. The angular and energy distributions of the ablated material 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.