Amplified spontaneous C3Πu–B3Πg emission and rotational and vibrational state distributions in C3Πu state of N2 in femtosecond laser induced filament in air

A femtosecond laser-induced filament in air was investigated by detecting the C3Πu–B3Πg (0, 0) fluorescence of N2. The intensity of the backward fluorescence increased exponentially as a function of the filament length, showing the amplification of the spontaneous emission. The vibrational and rotational temperatures of N2 in the C state determined by spectroscopic analyses were found to take respectively almost the same values of 2800(200) and 450(100) K in the wide laser intensity range between 0.5 and 6 mJ/pulse, which can be regarded as evidence of the clamping of the laser field intensity in the filament.