Ultrafast gain dynamics in N₂⁺ lasing from highly excited vibrational states pumped by circularly polarized femtosecond laser pulses

We experimentally demonstrate N₂⁺ lasing actions at the wavelengths of 353.3, 353.8, and 354.9 nm using a circularly polarized femtosecond laser. The three laser lines correspond to the B²Ʃ_u⁺ (v'=5, 4, 3) →X²Ʃ_g⁺ (v'=4,3,2) transitions, respectively. Particularly, we reveal the pressure-dependent gain dynamics of these lasing actions from highly excited vibrational states with a pump-Cprobe scheme. Our experimental results confirm that electron collisional excitation plays an important role in the establishment of a population inversion of N₂⁺ lasing at these wavelengths.