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

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<150 ns) at reduced pressure for low laser fluence (5.4 J cm−2). The related angular distribution of ablated particles has a cosn Θ form where n≳36 for δt≤150 ns and Θ<20°. The observed effect corresponds to a highly collisional plasma, where strongest forward peaking beyond the Knudsen-layer edge takes place. For times greater than 150 ns, the plasma switches into a three-dimensional expansion corresponding to an effusion model with recondensation. A nearly constant center-of-mass velocity in the range of 6×103 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.