Collision-mediated ultrafast decay of N₂ fluorescence during fs-laser-induced filamentation

We investigate experimentally spatiotemporal characteristics of fluorescence emission from fs-laser-induced filaments in air. Emissions accompanying the transitions of N₂ (C³Π_u-B³Π_g) and N 2+ (B²Σu+-X²Σg+) are dominant. The decay dynamics of fluorescence from different radial positions and longitudinal sections of a filament column are obtained along with high resolution spectra. A decay curve contains two exponential components: a fast one (with a decay time constant ∼10s ps), and a slow one (∼sub-ns). The lifetime of the N ₂ fluorescence is about three orders shorter than its spontaneous emission lifetime, indicating that most of the N ₂ molecules in the excited state (C³Π_u) are de-excited through collision. Different de-excitation mechanisms of N ₂ (C³Π_u) molecules contributing to fluorescence decay constants, e.g., the e ^--N₂, N ₂-N₂, and O ₂-N₂ collisions, are elucidated. We analyze the variations of decay constants together with corresponding fluorescence intensities, and obtain temperature distributions by fitting band spectra of N ₂ molecules and N 2+ ions with a synthetic spectral model. Our results suggest that the fast and slow decay processes originate from the e ^--N₂ and O ₂-N₂ collisions, respectively.