Quenching Effect of O2 on Cavity-free Lasing of N2 Pumped by Femtosecond Laser Pulses
Résumé
Nitrogen gas pumped by circularly polarized femtosecond laser pulses gives rise to bidirectional lasing emission，which holds unique potential for remote optical sensing application. However，the presence of oxygen molecules strongly suppresses this lasing effect. The influence of O2，Kr，Ar and He on the lasing effect of nitrogen molecules was compared，and the fluorescence of nitrogen molecules in pure nitrogen and ambient air was examined. It is observed that the lasing presents a similar quenching effect with the partial pressure of O2 and Kr，since Kr has a close ionization potential compared to O2. In contrast， for He which has a much higher ionization potential，there is no significant quenching effect. Therefore it is suggested that the quenching effect of O2 on nitrogen molecules mainly stems from the fact that O2 leads to a reduction of the clamped laser intensity inside the plasma filaments，which results in a decrease of the kinetic energy of the free electrons and an inefficient collision excitation.
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