Femtosecond Laser Excitation in Argon-Nitrogen Mixtures
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abstract
Copyright 2017 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Argon gas is demonstrated to enhance femtosecond laser electronic excitation tagging at atmospheric pressure and temperature for unseeded velocimetry applications, primarily through two to four orders of magnitude increase of excited species that may radiate through nitrogen's second positive system at early timescales of interest. The first positive system continues to play an important role in maintaining this emission at longer delays. A detailed kinetic model is implemented to explain this observed behavior in nitrogen and argon mixtures. Dominant processes governing the creation of N2(B3IIg) and N2(C3IIa) include a slower decay of electron temperature through increased ionization processes, reduced nitrogen quenching of electrons, nitrogen atom creation and recombination, the formation and dissociation of N4+, and a number of argon-nitrogen direct and indirect excitation pathways. The production of ArN+ ions through charge-transfer reactions Ar+ + N2 ArN+ + N and Ar + N2+ ArN+ + N affect excited C- and B-state nitrogen population delays at later times (t > 100 ns). The pooling reactions N2(A3+) + N2(A3a+) (N2(B3IIg);N2(C3IIg)) + N2 play minor roles in the formation of N2(B3IIg) and N2(C3IIu) at timescales useful for measurements in the femtosecond laser electronic excitation tagging argon plasma chemistry. In mixtures where nitrogen is dominant, metastable argon species are less instrumental in direct nitrogen excitation transfer, Ar+(43P2) + N2 Ar + (N2(B3IIg);N2(C3IIu)), than in facilitating further reactions through maintaining a higher electron temperature, whereas this excitation transfer begins to play a larger role as the percentage of argon is further increased. The model yields results that agree with sub-100 torr argon-nitrogen discharge experiments and theoretical results derived from other studies. It is concluded that not one single process can be credited for the enhancement, but a combination of ionization and heating produces the increased emission observed in argon mixtures.
