Research On The Rotational Spectral Characteristics Induced By The Femtosecond Laser Pulses Filamentation In Pure Nitrogen Gas

In recent years,with the rapid development of laser technology,ultrafast laser pulses play a critical role in medical,life science,physics,atmospheric sounding,aerospace,optical communication,and chemical reaction control due to their low thermal effect,high single pulse energy,strong peak power,and wider frequency range.The femtosecond laser filamentation in air is accompanied by a series of nonlinear effects,such as diffraction,light intensity clamping,self-phase modulation,supercontinuum spectrum,and filamentation-induced fluorescence emission laser-induced fluorescence spectroscopy.Among these techniques,fluorescence emission spectroscopy can be used to diagnose information such as temperature,flow rate,concentration and pressure of gas molecules,making diagnosis more convenient and efficient based on its high sensitivity,contactless detection,easy operation and high accuracy.Thus,rotational temperature diagnostic information can be obtained by exploring rotationally resolved spectroscopy.When the strong laser pulse interacts with nitrogen gas,the part of the fluorescence transition is mainly the first negative band system(B2Σu+→X2Σg+ transition)of nitrogen molecule ions and the second positive band system(C3Πu+→B3Πg+ transition)of nitrogen molecules.In this paper,we use femtosecond laser pulse-induced fluorescence emission spectroscopy to investigate the rotational spectral properties of nitrogen molecular ions.The chosen band is the R branch in the spectral system of nitrogen molecular ion B2Σu+→X2Σg+ transitional.The spectra in the range of 389-392 nm at position z=6.5 mm were obtained for a pressure of 10,000 Pa.In addition,the fluorescence signal intensity of nitrogen molecular ions with propagation distance at different rotational quantum numbers(J=5,9,13 and 21)of the R-branch at pressures of 200 Pa and 10,000 Pa,respectively,and the(B2Σu+(v’=0)→X2Σg+(v"=0)transition)fluorescence spectra measured at different positions were also obtained,focusing on the analysis of the R-branch rotation-resolved spectra of nitrogen molecular ions during fluorescence transitions.The experimental results show that the higher the air pressure,the higher the rotational temperature;and the change of air pressure affects the spatial distribution of the rotational temperature of nitrogen molecular ions at different air pressures.The results of this paper provide a strong indication of the spatial distribution of the rotational temperature and contribute to the application of fluorescence emission spectroscopy.