  We report the fluorescence emission which is driven by femtosecond laser
pulses with a repetition rate of 100 MHz and a center wavelength of 1040 nm in
a nitrogen gas jet. The experiment is performed in a femtosecond enhancement
cavity coupled with high repetition rate laser for the first time to the best
of our knowledge. In contrast to previous observation at low repetition rate
with a nitrogen gas jet, where the 391 nm radiation was observed but the 337 nm
emission was missing, the 337 nm emission is 3 times stronger than the 391 nm
emission in our experiment. By examining the dependence of the radiation
intensity on the flow rate of the nitrogen gas and the polarization of the pump
pulse, the formation mechanism of the N2(C3{\Pi}u) triplet excited state, i.e.,
the upper state of the 337 nm emission, is investigated. We attribute the main
excitation process to the inelastic collision excitation process, and exclude
the possibility of the dissociative recombination as the dominate pathway. The
role of the steady state plasma that is generated under our experimental
conditions is also discussed.
