Intensity clamping measurement of laser filaments in air at 400 and 800 nm

Molecular N2 fluorescence excited by laser filaments formed from laser pulses at 400 and 800 nm propagating in air is investigated. A comparison showed that, when excited with 400 nm photons, the fluorescence from the first negative band of N2+ was enhanced by a factor of 6.4 while that of the second positive band of neutral N2 remained relatively constant. The enhanced N2+ signal is attributed to a more efficient inner-shell multiphoton process (to the B2Σu+ state) at 400 nm leaving a larger population of N2+ ions in the excited state. On the other hand, the stable fluorescence from neutral N2 is due to the fact that the plasma density is more or less the same at both wavelengths. Using these results, a theoretical model is developed to determine the clamped intensities of the laser filaments at 400 and 800 nm.