Near-threshold laser sputtering of gold

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Abstract
This work characterizes the laser sputtering of gold by 248 nm laser pulses at near‐threshold fluences (material removal rates ≤10 Å/pulse) using time‐of‐flight plume diagnostics, scanning electron microscope analysis of the surface topography, and thermal analysis of the transient near surface conditions. Pulsed laser irradiation leads to development of surface topography characterized by droplet and ridge formations, and to the liberation of micrometer‐sized droplets into the plume. The development of surface topography has been identified with a hydrodynamic response to phase change occurring at the surface of the target. Drawing upon a Rayleigh–Taylor instability description of the melt surface, the readily observable ∼5 μm periodicity in topography formation can be theoretically predicted. Additionally, the preferential formation and liberation of ∼1 μm diameter droplets at the target surface is observed. Nevertheless, the majority of sputtered mass flux is not comprised of droplets, but of neutral gold atoms with almost perfect Boltzmann translational energy distribution. The mean translational energy of the gold atoms, however, is much too high to reconcile with a simple thermal vaporization model. The yield, translational energy, and angular characteristics of the plume are strongly influenced by the surface topography. Local variations in the light absorption and heat transfer explain the qualitative trends in the experimental results.

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