Revisiting the mechanism of nitrogen fluorescence emission induced by femtosecond filament in air

The backward propagating and side emitted fluorescence during the femtosecond filamentation in air is experimentally investigated in this paper. By comparing the fluorescence emission in the circular and linear polarization states, we find that in the shorter focal length case, the direct ionization of _{N 2} greatly affects the fluorescence emission behaviors: the fluorescence from _N^{2 +} and _{N 2} is always stronger in the linear and circular polarization cases, respectively. Based on the observation, the emission mechanism of nitrogen fluorescence emission induced by a femtosecond filament is discussed.