Determining metastable ion lifetime and history using wave-particle interaction and laser-induced fluorescence*

For practical purposes, laser-induced fluorescence (LIF) is frequently performed on metastable states that are produced directly from neutral gas particles and also from ions in other electronic states. Here rises an important question: when can Doppler-resolved LIF on metastable ions be used to infer the velocity distribution of ground-state ions, the majority ion population in many laboratory plasmas? Previous experimental results suggest that there are limitations of this laser diagnostic technique due to the finite lifetime of metastable ions. Simulations based on our newly developed Lagrangian model for LIF show that under circumstances where the metastable ion population is produced from direct ionization of neutrals, the velocity distribution measured using LIF will only faithfully represent processes which act on the ion dynamics in a time shorter than the metastable lifetime.¹ However, the LIF performed on the metastable population produced from pre-existing ions is not affected by metastable lifetime. Unlike the well-known optical pumping broadening in LIF measurements, this systematic error caused by the finite metastable lifetime has never been studied before in experiments. Understanding the behavior of each metastable population, whether produced from neutral particles or pre-existing ions, is important in LIF measurements as it provides a path for avoiding or correcting this new type of systematic error. It is a long-standing problem, however, to trace the production history of metastable ions. This paper presents the experimental measurement of ion metastable lifetime in a plasma as well as the relative fraction of metastables produced directly from neutral atoms as opposed to pre-existing ions. The technique relies on measuring the ionic wave response.

¹F. Chu and F. Skiff, Phys. Plasmas (1994–Present) 25, 013506 (2018).