Free-space air molecular lasing from highly excited vibrational states pumped by circularly-polarized femtosecond laser pulses

We experimentally investigate N₂⁺ lasing from highly excited vibrational states with ultrashort, intense 800-nm laser pulses. We observe laser emissions at multiple wavelengths (353.3, 353.8, and 354.9 nm) corresponding to B²Σ_u⁺(v′ = 5, 4, 3) → X²Σ_g⁺(v = 4, 3, 2) transitions. Specifically, it is found that two strong signals corresponding to the (5-4) transition at 353.3 nm and the (4-3) transition at 353.8 nm, which can be achieved only at relatively low pressures, show a strong dependence on the ellipticity of the pump pulses. This observation suggests that the ions could be populated to high vibrational energy levels of the B²Σ_u⁺ state by electron impact excitation.