Laser intensity has a great influence on the excitation, ionization and other dynamical processes of atoms and molecules. During the femtosecond laser filamentation, the different values of intensity along the propagation direction will have a great influence on the spatial distribution of the products generated by these processes, and that of the fluorescence emitted by these products. In this paper, by measuring the spatial distribution of fluorescence induced by femtosecond laser filament in air, we find that along the propagation direction, the fluorescence signal from N₂ appears in front of that from ${\text{N}}_2^{+}$. Based on the experimental observations, we discuss the mechanism for the formation of N₂ ($C^3{\mathrm{\Pi }}_{\text{u}}$), and arrive at the conclusion that in the case of shorter focal length, the intersystem crossing process plays a main part in the formation of the $C^3{\mathrm{\Pi }}_{\text{u}}$ state of N₂.

激光强度对原子和分子的激发，电离和其他动力学过程有很大影响。在飞秒激光灯丝化过程中，沿着传播方向的不同强度值将对这些过程生成的产品的空间分布以及这些产品发出的荧光的空间分布产生很大影响。在本文中，通过测量飞秒激光灯丝在空气中感应的荧光的空间分布，我们发现沿着传播方向，来自N ₂的荧光信号出现在来自于N ₂的荧光信号的前面。${\text{ñ}}_2^{+}$。根据实验观察结果，我们讨论了N ₂（$C^3{\mathrm{\Pi }}_{\text{ü}}$），并得出结论，在较短的焦距情况下，系统间的交叉过程在形成光圈中起主要作用 $C^3{\mathrm{\Pi }}_{\text{ü}}$N _2的状态。