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 B²{{Σ}}ᵤ⁺$ state) at 400 nm leaving a larger population of ${N}₂{}⁺$ ions in the excited state. On the other hand, the stable fluorescence from neutral ${N}₂$ 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.