Spatial Distribution Of Nitrogen Fluorescence Emission Induced By Femtosecond Laser Filamentation In Air

The nonlinear effects caused by intense femtosecond laser pulse propagation in air,such as filamentation,have attracted much attention.When femtosecond laser propagates in the air,the self-focusing phenomenon caused by Kerr effect and the defocusing effect caused by plasma reach the dynamic balance,and a long and bright low-density plasma channel is formed,which is called "filament".The plasma undergoes complex transition processes and radiates characteristic fluorescence spectrum.By measuring the fluorescence spectrum induced by femtosecond laser filamentation,we can obtain the information of laser intensity,electron temperature and plasma density,and also help to understand the excitation,ionization and other dynamic processes of atoms and molecules.The generation mechanism of femtosecond laser-induced plasma can be well described by studying the emission spectrum of femtosecond laser-induced plasma in air.As the major component in the air,nitrogen emits fluorescence when it interacts with intensive laser field.The fluorescence comes from the first negative band system(B2?u+?X2?g+ transition)of N2+ and the second positive band system(C3?u+?B3?g+ transition)of N2.Under the action of high-intensity femtosecond laser,N2 can be directly photo-ionized to generate N2+(B2?u+),which results in fluorescence emission of N2+.In the process of femtosecond laser filament formation,the dynamic processes such as ionization and excitation of nitrogen molecules are affected by the laser intensity distribution and laser polarization direction.The products show different distributions in the propagation direction and radial space,which conversely affects its light emission.Therefore,it is necessary to further explore its generation mechanism through the spatial distribution of nitrogen fluorescence.In this experiment,the spatial distribution of the nitrogen fluorescence emission generated by linearly polarized femtosecond laser pulses filaments in air is measured.By changing the polarization direction of the laser to study the distribution of nitrogen fluorescence on the radial plane,it is found that the fluorescence emission of N2+ is more intense in the direction perpendicular to the laser polarization,while it is weaker in the direction parallel to the laser polarization.Nitrogen fluorescence emission has the same intensity in all directions.The ionization probability of a linear molecule depends on the angle between the laser polarization direction and the molecular axis,which is maximum(minimum)when the angle is 0°(90°).N2 are more likely to be ionized in the laser polarization direction,the nitrogen molecular ions N2 and electrons are separated in the direction parallel to the laser polarization.Therefore,more ions(N2+)are generated in the direction parallel to the laser polarization,and the fluorescence emission of N2 is more intense.Along the propagation direction of the laser,it is found that the fluorescence of N2 appears before the fluorescence of N2+ and disappears after the fluorescence of N2+ disappears.This is due to the fact that N2 can be ionized to generate N2+(B2?u+)at the position of high enough laser intensity,thus emitting fluorescence of N2.However,the laser energy is not enough to ionize nitrogen at the beginning and end of laser transmission,but it can generate N2*,which emits nitrogen fluorescence through the process of intersystem crossing N4++e?N2(C3?u+)+N2.The spatial distribution of nitrogen fluorescence emission during femtosecond laser filament formation shows that the intersystem crossing scheme can explain the formation of N2(C3?u+).This research is helpful to understand the mechanism of nitrogen fluorescence emission.