Laser-driven fluorescence emission in a nitrogen gas jet at 100 MHz repetition rate

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. 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 three 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 N-2(C (3)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.