On the gas dynamics of laser-pulse sputtering of polymethylmethacrylate

The laser-pulse sputtering of polymers should have two limiting cases. In the one there is ongoing release of particles from the target surface, the particles then form a Knudsen layer (KL), and there is finally an unsteady adiabatic expansion (UAE) ('effusion' model). In the other limit, bond-breakage occurs rapidly over a characteristic depth and the resulting gaslike particles then flow out directly in a UAE without a formal KL ('outflow' model). To test these idealized gas-dynamic descriptions, we discuss experiments in which ∼ 20 ns excimer laser pulses are incident on polymethylmethacrylate in air at 193 or 248 nm and the release process is photographed with a ∼ 1 ns probe pulse. The results not only give explicit support to the gas-dynamic description of the problem, but also indicate that the KL-UAE model is more appropriate. For example, only this model accommodates the observation that the release process continues for ∼ 6 μs, which is ∼ 500 times the laser pulse length. © 1991.