On determining the fraction of metastable ions produced by direct ionization

Laser-induced fluorescence (LIF) on metastable ions is a powerful tool in studying wave-particle interactions, velocity-space diffusion and other phenomena in plasmas under the proper conditions. However, measurements which probe particle interactions place restrictions on the metastable lifetime. Metastable states are produced both from direct ionization of neutral gas particles and ions in other electronic states, but the former metastable population will only faithfully represent processes which act on the ion dynamics in a time shorter than the metastable lifetime. A numerical simulation is performed to study the metastable lifetime effects using a Lagrangian approach for LIF. A theoretical model in determining the fraction of metastable ions produced from direct ionization is reported to provide corrections to the LIF measurements.