Determination of electron densities by diode-laser absorption spectroscopy in a pulsed ICP

A novel method to determine electron densities in a low pressure (1 Pa) pulsed ICP discharge via absorption spectroscopy on argon metastables is presented. By use of an external cavity diode laser tuned at a vacuum wavelength of 696.73 nm the time behaviour of the absorption from metastable argon atoms in the Ar1s(5) state in the afterglow is recorded. An analytical model motivates the assumption of a homogeneous metastable density distribution under our experimental conditions. Further, a detailed model for the metastable density decay is developed, including the spatial electron density distribution and its temporal decay. This allows determination of the electron density from the measured data. Comparison between densities obtained by this technique and by Langmuir probe measurements shows excellent agreement. Furthermore, the electron density decay time, the ambipolar diffusion constant and the effective electron temperature in the afterglow are determined comparing data and model. The scaling of the electron decay time with density indicates recombination. The obtained low electron temperature reduces diffusion very much and can be expected to cause an increased recombination rate.