Abstract:
This paper presents the results of the study of the generation parameters in the forward direction on the first negative system (transition $B^2\Sigma^+_u - X^2\Sigma^+_g$) of molecular nitrogen ions in the laser plasma, which was created in air or pure nitrogen by laser radiation with a central wavelength of $950$ nm, pulse duration of $50$–$60$ fs, and energy of up to $10$ mJ. The effect of nitrogen pressure and energy pumping on the emission intensity at $391.4$ nm ($0$–$0$ vibrational transition) and $427.8$ nm ($0$–$1$) was studied. It is shown that by changing the pressure of nitrogen, it is possible to obtain generation of different vibrational transitions of the molecular nitrogen ion and to change their intensities. The maximum intensities at $391.4$ nm and $427.8$ nm were observed at nitrogen pressures of $\sim 30$ and $\sim 500$ mbar, respectively. Optimal generation pressures are determined by an increase in intensity due to an increase in the number of ionized nitrogen molecules in the excited state, on the one hand, and a decrease in gain due to quenching of excited states by neutral nitrogen molecules and plasma electrons, on the other hand. It is also shown that starting the generation process requires powerful seed radiation, which are photons of a broadband supercontinuum appearing in the same laser plasma.
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