We investigate the microscopic excited-state dynamics initiated by macroscopic propagation of femtosecond laser pulses in low-pressure ${\mathrm{N}}_2$ and ${\mathrm{N}}_2–\mathrm{Ar}$ gas mixtures. Through combined measurements of forward laser transmission and spectrally resolved side-fluorescence emissions of ${\mathrm{N}}_2{}^{+}$ and ${\mathrm{N}}_2$ under various experimental conditions, we reveal a counterintuitive spatial mismatch between the localized on-axis peak intensity and the clamped macroscopic plasma volume. Our results indicate that the sharp decrease of the 391-nm fluorescence signal with increasing pressure is primarily driven by electron-ion collisional quenching, while the reduction in ionization efficiency caused by a slight decrease in peak intensity plays only a minor role. In contrast, the anomalous depletion of the 380-nm fluorescence at relatively higher pressures is caused by the synergistic effect of microscopic collisional quenching, clamping of the effective plasma volume, and plasma density. The experimental observations are well reproduced by solving a comprehensive three-dimensional spatiotemporal nonlinear Schrödinger propagation equation and the proposed fluorescence emission model. This work bridges the gap between macroscopic nonlinear propagation and microkinetic collisional processes, shedding light on the physical picture of the generation and depletion of active species in low-density plasma channels.

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• Atomic & molecular processes in external fields
• Ultrafast optics
• Ultrafast phenomena