Abstract Laser-induced fluorescence (LIF) diagnostics for molecular ions remain far less developed than their noble gas counterparts, despite growing interest in molecular propellants for electric propulsion (EP). In this work, a LIF diagnostic targeting the N₂^+\, (B²ᵤ^+ - X²g^+) First Negative System is developed and applied to an EP relevant plasma discharge. A practical excitation–fluorescence scheme is defined, supported by a spectral model to analyze absorption and fluorescence processes and to guide excitation-line selection. The LIF diagnostic is applied to the plume of a hidden anode plasma, operating on nitrogen feed gas, enabling spatially resolved measurements of ion velocity distribution functions (IVDFs). The IVDFs obtained reveal distributed ionization and acceleration, with fast ions appearing several millimeters downstream of the source, consistent with probe measurements. These results represent the first application of LIF for IVDFs of molecular ions in EP relevant systems. Finally, a comparative analysis of LIF signal-levels between molecular and atomic (noble gas) ions, including consideration of linestrengths and population fractions, is presented.
Valle et al. (Wed,) studied this question.