ON THE DIRECT OBSERVATION OF THE GAS-DYNAMICS OF LASER-PULSE SPUTTERING OF POLYMERS .1. ANALYTICAL CONSIDERATIONS

We bring together a wide range of ideas relating to the gas-dynamic effects that are now recognized to play a leading role in laser-pulse sputtering. The ideas are grouped according to three basic models. (a) In the "effusion" model one deals with particles which are released from a target surface, form a Knudsen layer (KL), and then enter an unsteady adiabatic expansion (UAE). When the release terminates at time t = tau-r there is an abrupt change at the surface from positive to zero flow velocity, which means that particles moving towards the surface are reflected. The flow breaks up into three regions and analytical solutions exist for all aspects of this flow. (b) In the "recondensation" model the comportment of the target is initially like that of the effusion model but when the release terminates at t = tau-r the change at the surface is from positive to negative flow velocity, meaning that particles which move towards the surface recondense. Only numerical solutions presently exist (due to Sibold and Urbassek) but they are sufficient to show that the flow breaks up into two (not three) regions. (c) The "outflow" model could be described as the escape of gas from a finite reservoir, a well-known problem since it describes some aspects of guns. In its application to laser sputtering it is assumed that bond-breakage occurs rapidly over a characteristic depth and the resulting gas-like particles then flow out in a UAE; there is no formal KL.