Laser Driven Fluorescence Emission in a Nitrogen Gas Jet at 100 MHz Repetition Rate

TL;DR

This study demonstrates laser-driven fluorescence in a nitrogen gas jet at 100 MHz repetition rate using a femtosecond enhancement cavity, revealing new insights into excitation mechanisms and emission intensities.

Contribution

First implementation of high repetition rate femtosecond laser in a cavity for nitrogen fluorescence, showing enhanced 337 nm emission and elucidating excitation processes.

Findings

337 nm emission is 3 times stronger than 391 nm emission

Inelastic collision excitation is the main process for N2(C3Πu) state formation

Steady state plasma influences fluorescence emission

Abstract

We report the fluorescence emission which is driven by femtosecond laser pulses with a repetition rate of 100 MHz and a center wavelength of 1040 nm in a nitrogen gas jet. The experiment is performed in a femtosecond enhancement cavity coupled with high repetition rate laser for the first time to the best of our knowledge. In contrast to previous observation at low repetition rate with a nitrogen gas jet, where the 391 nm radiation was observed but the 337 nm emission was missing, the 337 nm emission is 3 times stronger than the 391 nm emission in our experiment. By examining the dependence of the radiation intensity on the flow rate of the nitrogen gas and the polarization of the pump pulse, the formation mechanism of the N2(C3{\Pi}u) triplet excited state, i.e., the upper state of the 337 nm emission, is investigated. We attribute the main excitation process to the inelastic collision excitation process, and exclude the possibility of the dissociative recombination as the dominate pathway. The role of the steady state plasma that is generated under our experimental conditions is also discussed.