Numerical Investigation of Femtosecond Laser Electronic Excitation Tagging in Pure Nitrogen

Femtosecond Laser Electronic Excitation Tagging (FLEET) has emerged as a critical tool in high-speed flow velocimetry, hypersonics, and combustion diagnostics. FLEET relies on femtosecond laser pulses to excite electronic states in molecular nitrogen, enabling high-resolution flow visualization through fluorescence tagging. Despite its established utility, the precise chemical processes involved in the generation of fluorescence and the effects of pressure on the excitation and relaxation dynamics of nitrogen species remain inadequately explored. This limits its broader application in extreme flow environments such as hypersonic flows and combustion processes. This contribution represents a numerical investigation of the processes that drive the long-lived emission in FLEET at various pressures. Based on plasma kinetics modeling and high-speed diagnostics, the FLEET signal originates primarily from the first positive system of nitrogen (N_2 (B^3 Π_g→A^3 Σ_u^+)) and, to a lesser extent, the second positive system (N_2 (C^3 Π_u→B^3 Π_g)), as the observed persistence results from re-population of these short-lived states via pooling and recombination kinetics. By combining the solution of the nonlinear Schrödinger equation with advanced plasma kinetics models and computational fluid dynamics (CFD), this research reproduces experimental signal behavior. Simulations were conducted at three distinct pressure conditions (1 bar, 10 bar, and 30 bar) in pure nitrogen to isolate the effects of pressure on the signal's generation and its decay pathways. We report that elevated pressures exhibit faster decay rates of N_2 (B^3 Π_g) and N_2 (C^3 Π_u), suggesting that the FLEET signal is shorter lived. Moreover, at higher pressures, an additional channel of fluorescence becomes important N_2^+ (B→X), and contributes significantly to the observed signal.