State-to-State Modeling of N₂(C) and N₂⁺(B) Generation in Femtosecond Filaments in Air

This contribution presents a state-to-state model of femtosecond plasma filament generated in atmospheric pressure air. Previous experimental studies using optical emission spectroscopy have shown that the short-lived early emission is dominated by the N2(C-B) and N2+(B-X) transitions. Surprisingly, N2(C-B) emission is observed only within the first 6 ns after the pulse, while N2+(B-X) is limited to the first 2 ns. The main goal of this study is to develop a detailed state-to-state plasma kinetic model that can provide a better understanding of the main pathways leading to the formation and destruction of these excited nitrogen species. It was found numerically that N2(C) is primarily formed via the dissociative recombination of the intermediate N4+ ion. In contrast, the N2+(B) comes primarily from the electron impact excitation reaction with N2+(X), which is generated during the fs ionization process. The model’s predictions of species lifetimes show good agreement with experimental measurements. These results are important for improving the understanding of early-time emission from femtosecond filaments, with implications for applications such as femtosecond laser-induced breakdown spectroscopy (fs-LIBS), pre-ionization schemes, directed energy systems, or plasma-assisted combustion.