Femtosecond TALIF and Optical Emission Calibration for Quantifying N-Atom Production in RF Plasma Sources

This work aims to quantify the ground-state atomic nitrogen produced by a radiofrequency (RF) remote plasma source (RPS) using a femtosecond two-photon absorption laser-induced fluorescence (fs-TALIF) technique. These measurements targets N(2p3 4S), at the outlet of the plasma device, where two-photon excitation (λ=2×206.6 nm) populates the 3p ⁴S₃/₂ level, and the resulting fluorescence is collected from its radiative decay to the 3s ⁴P triplet state of atomic nitrogen (λ ~745 nm). The experiments are performed in the non-saturated regime, and krypton is used as a calibration species due to its nearby two-photon resonance and naturally atomic ground state. Atomic nitrogen densities were measured and validated using two complementary diagnostics: fs-TALIF and optical emission spectroscopy (OES). In particular, the fs-TALIF signal was compared against emission from N_2 (B^3 Π_g, v=11), populated primarily through the three-body recombination pathway N (^4 S)+N (^4 S) +M→N_2 (B^3 Π_g, v=11)+M in the post-discharge region. This approach provides practical means to monitor N-atom production when direct TALIF measurements are not feasible, enabling corresponding OES-based measurements of atomic nitrogen under comparable operating conditions. This manuscript presents N-atom number densities at various mass-flow rates and pressure conditions for the plasma source, along with results from both diagnostics. Together, these measurements demonstrate that fs-TALIF offers a reliable method for direct atomic nitrogen quantification in low-pressure RF plasmas and supports the development of diagnostics for broader plasma-based applications.