Simulation of transformations of thin metal films heated by nanosecond laser pulses

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Abstract
The ablation of free-standing thin aluminum films by a nanosecond laser pulse was investigated by time-resolved transmission electron microscopy and numerical simulation. It was established that thin film geometry is particularly suited to furnish information on the mechanism of evaporation and the surface tension of the melt. In the case of aluminum the surface tension σ as function of temperature can be approximated by two linear sections with a coefficient −0.3×10−3 N/K m from the melting point 933 K up to 3000 K and −0.02×10−3 N/K m above 3000 K, respectively, with σ(933 K)=0.9 N/m and σ(8500 K)=0. At lower pulse energies the films disintegrated predominantly by thermocapillary flow. Higher pulse energies produced volume evaporation, and a nonmonotonous flow, explained by recoil from evaporating atoms and thermocapillarity. The familiar equations of energy and motion, which presuppose separate and coherent vapor and liquid phases, were not adequate to describe the ablation of the hottest zone. Surface evaporation seemed to be marginal at all laser pulse energies used.