Formation Mechanism of Excited Neutral Nitrogen Molecules Pumped by Intense Femtosecond Laser Pulses.

Backward amplified spontaneous emission of neutral nitrogen molecules has been reported from laser-induced plasma filaments. The cavity-free UV emission has great potential applications in remote atmospheric sensing. However, the formation mechanism for the excited nitrogen molecules inside filaments remains controversial. Here we study the formation mechanism of excited nitrogen molecules pumped by intense femtosecond laser pulses. After modification of the electron energy distribution by inclusion of the recollision between the electron and its parent ion as well as modification of the electron collision cross section by inclusion of the secondary electron contribution, the theoretical calculations reproduce the experimental observations very well. The results clearly demonstrate that excited nitrogen molecules are generated through collisions between energetic electrons and neutral nitrogen molecules.