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

Abstract

We investigate experimentally spatiotemporal characteristics of fluorescence emission from fs-laser-induced filaments in air. Emissions accompanying the transitions of N2 (C3Πu-B3Πg) and N2+ (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 N2 fluorescence is about three orders shorter than its spontaneous emission lifetime, indicating that most of the N2 molecules in the excited state (C3Πu) are de-excited through collision. Different de-excitation mechanisms of N2 (C3Πu) molecules contributing to fluorescence decay constants, e.g., the e--N2, N2-N2, and O2-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 N2 molecules and N2+ ions with a synthetic spectral model. Our results suggest that the fast and slow decay processes originate from the e--N2 and O2-N2 collisions, respectively.