An experimental and theoretical investigation of femtosecond laser excitation in N₂ + O₂ mixtures

Oxygen is found to have both a quenching and amplifying effect on Femtosecond Laser Electronic Excitation Tagging at different mixture fractions with nitrogen. The strongest signal is still achieved in a pure nitrogen gas flow. When oxygen is initially added, the total signal quickly decreases before increasing again to a local maximum level around 50% nitrogen + 50% oxygen, and then decreases as the relative amount of oxygen in the mixture continues to increase. No obvious changes in the first or second positive emission are observed to explain this effect. Experiments and modeling both suggest that long-lived secondary species, NO and O₃, could be responsible for these effects. A zero-dimensional kinetics model is developed to study and explain the effects of oxygen on FLEET emission in mixtures of nitrogen and oxygen at atmospheric pressure and temperature.