Langmuir probe study of laser ablation plume dynamics

Abstract

For many applications of pulsed laser ablation it is necessary to have an understanding of the expansion dynamics of the ablation plume both in vacuum and in low pressure gases. Knowledge of the ablation plume hydrodynamics can also contribute to the understanding of the laser ablation process. In this paper we will consider some of the existing theoretical models of laser ablation plume expansion and draw some conclusions as to which model is most appropriate for the low temperature plasmas which arise in pulsed laser deposition. For ablation plumes which are significantly ionised, Langmuir probes have proved to be a relatively simple and inexpensive tool for measuring the plume shape, ion energy distribution and electron temperature. We describe some recent work on the development of Langmuir probes for laser ablation plume diagnosis. Typically in laser ablation plasma the flow velocity is supersonic, which complicates the interpretation of the I-V probe characteristic. We describe some new work on the behaviour of a flat probe lying parallel to the plasma flow. For nanosecond ablation of silver, we also show how a planar Langmuir probe can be used to obtain a fairly comprehensive description of the expansion dynamics of the ionised part of the ablation plume, including plume shape, ion energy distribution and electron temperature.