A femtosecond laser-induced filament in air was investigated by detecting the C-3 Pi(u)-B-3 Pi(g) (0,0) fluorescence of N-2. 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 N-2 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. (C) 2013 Elsevier B.V. All rights reserved.