Effect of Knudsen-layer formation on the initial expansion and angular distribution of a laser-produced copper plasma at reduced pressure of air

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Abstract
The initial expansion of a copper plasma generated at reduced pressure (20 mTorr air) by XeCl excimer laser irradiation of 308 nm has been studied by fast intensified charge‐coupled device photography of the overall visible plume emission with a time resolution of 3 ns. The spatial distribution and its temporal development along the target normal as well as in a plane parallel to the sample surface were measured in a single laser shot (28 ns). We report the first in situ measurement and observation of a one‐dimensional plasma expansion with very small divergence during the initial stage (δt−2). The related angular distribution of ablated particles has a cosn Θ form where n≳36 for δt≤150 ns and Θ3 m s−1 has been observed. A particle velocity at the surface of uk=2×103 m s−1, in good agreement with the theory of unsteady adiabatic expansion, has been confirmed through a measured excitation temperature of T≊104 K revealing an image of the kinetic energy.