Laser-pulse sputtering of aluminum: Vaporization, boiling, superheating, and gas-dynamic effects

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Abstract
We have developed a numerical method to describe laser-pulse sputtering of Al in a thermal regime. The irradiation consists of a single pulse of triangular form having a duration of 30 ns. The laser light is assumed to be absorbed according to a simple exponential mechanism. Heat transport in the Al is described by the heat flow equation with boundary conditions for vaporization, with or without boiling. Vaporization rates are evaluated by the Clausius-Clapeyron law and the boiling mechanism (when boiling is assumed to be possible) is implemented as soon as the vapor pressure reaches 1 atm. A critical analysis of the time scales necessary for true boiling, as well as for superheating above the boiling temperature, is made in order to understand the relevance of these phenomena with respect to particle emission from the Al surface. Moreover, on the basis of the calculated vaporization rates, it is possible to distinguish between different gas-dynamic regimes. When the rate is less than ∼1 ML in 20 ns, the particles emerging from the surface do not achieve local thermal equilibrium, and therefore undergo free flight describable by a modified Maxwellian. When the rate is ∼1 ML in 20 ns, a Knudsen layer forms, at the boundary of which, particles achieve local thermal equilibrium and only subsequently undergo free flight. Finally, when the rate is sufficiently greater than ∼1 ML in 20 ns, the gas dynamics of the particles leaving the Knudsen layer may be described with the gas-dynamic equations, if the density is high enough, or, otherwise, by the Boltzmann equation. Numerical results concerning the effectiveness of laser sputtering in producing craters in irradiated Al, as well as the main features of the gas dynamics (including recondensation or reflection of the gas at the Al surface), are illustrated.