Time dependent metastable dynamics in low temperature argon plasmas.*

In the development of optical emission spectroscopy (OES) based plasma diagnostics, it is critical to have a detailed understanding of the atomic processes that give rise to the emission of light.  It is known that relative metastable population abundances in a plasma (the so-called “metastable fractions”) play an important role in the population of excited states. Typical rate coefficients for excitation from metastable states are large compared to coefficients for ground state excitation. Thus, metastable excitation can play a large role in the spectral emission of a plasma, even when the relative population of these states is small.

We report on recent discoveries with regards to the evolution of metastable states in low temperature argon plasmas, using atomic data from a Breit-Pauli R-Matrix with PseudoStates (BPRMPS) calculation and a Generalized Collisional Radiative (GCR) model. We show that the time dependent evolution of metastable populations can impact relative line heights from spectral emission, and discuss the impact this effect can have on line ratio diagnostics using OES. Finally, we discuss when metastable fractions can be well approximated by their equilibrium values.