【作者】 张云；

【导师】 金明星；

【作者基本信息】 吉林大学 ， 原子与分子物理， 2021， 硕士

【摘要】 强飞秒激光脉冲在空气中传输引起的非线性效应比如成丝现象一直备受关注。自聚焦现象是由空气的光克尔效应产生的,而散焦效应是由电离后产生的等离子体所带来的。自聚焦现象和散焦效应共同作用,从而实现了一种动态的平衡。会形成长而明亮的低密度等离子体通道,这就是“丝”。等离子体经历复杂的跃迁过程会辐射特征荧光光谱。通过测量飞秒激光成丝过程中的荧光光谱,可以了解激光强度、电子温度、等离子体密度及其它信息,也有助于理解原子分子经历的激发、电离和其他动力学过程。对空气中飞秒激光诱导等离子体发射光谱的研究可以更好地对其产生机制进行描述。氮气作为空气中的主要成分,在与强激光场相互作用时发出荧光,荧光来源于N2+的第一负带系(B2∑u+→X2∑g+跃迁)和N2的第二正带系(C3 ∏u+→B3 ∏g+跃迁)。研究表明,强飞秒激光与N2相互作用后,N2会直接发生光电离产生N2+(B2∑u+),经由B2∑u→X2∑g+跃迁,产生N2+荧光发射。我们主要研究氮分子和氮分子离子的荧光发射,因为在飞秒激光成丝过程中,氮分子的电离和激发等动力学过程受激光强度分布和激光偏振方向的影响,其产物在传播方向和径向呈现出不同的分布情况,进而影响其光发射。因此,有必要进一步通过氮气荧光的空间分布来研究其产生机制。本论文研究了线偏振飞秒激光脉冲在空气中成丝产生的氮荧光发射的空间分布。通过改变激光的偏振方向研究成丝过程中氮荧光发射的径向角分布,发现N2+荧光发射在垂直于激光偏振方向上更强,而在平行于激光偏振方向上较弱;N2荧光发射在所有方向上具有近乎相同的强度。通过实验结果从理论上讨论了N2+荧光发射强度在垂直于激光偏振方向时更强的原因。这是由于线性分子的电离几率取决于激光偏振方向和分子轴之间的夹角,夹角为0°时电离概率最大,夹角为90°时最小。若激光偏振方向固定,当N2+转动到分子轴的方向与激光偏振方向一致时,其荧光发射较强。沿着激光传播方向,发现N2荧光先于N2+荧光出现且在N2+荧光消失之后消失。这是由于在激光强度足够高的位置,N2可以被电离产生N2+(B2∑u+),发射N2+荧光。但是,在激光传输的始末位置,激光能量不足以使氮分子电离,但足以激发产生N2*,通过系间窜越过程产生N2(C3∏u+),发出N2荧光。本论文基于氮荧光信号的空间分布情况进行了理论分析,对N2(C3∏u+)的形成机理进行了探讨,此研究有助于理解氮荧光发射的机理。更多还原

【Abstract】 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.更多还原