We investigate lasing of a nitrogen gas induced by intense femtosecond laser pulses at around 400 nm. By examining both the self-induced and externally seeded forward emission spectra, we unambiguously identify the ${{\mathrm{N}}_2}^{+}$ lasing actions at 427.8 and 423.6 nm assigned respectively to the $B{}^2\mathrm{\Sigma }{{}_u}^{+}-X{}^2\mathrm{\Sigma }_g^{+}$ (0, 1) and (1, 2) emissions and show that the lasing mechanism is totally different from the lasing induced by near-infrared (800 nm) laser pulses, in which the population transfer from $X{}^2\mathrm{\Sigma }_g^{+}$ to $A{}^2\mathrm{\Pi }_u$ proceeds through the resonant $X{}^2\mathrm{\Sigma }_g^{+}-A{}^2\mathrm{\Pi }_u$ transition, facilitating the population inversion between the $B{}^2\mathrm{\Sigma }{{}_u}^{+}-X{}^2\mathrm{\Sigma }_g^{+}$ states [Phys. Rev. Lett. 123, 203201 (2019)]. We simulate the population distributions among the vibrational levels in the three lowest electronic states of ${{\mathrm{N}}_2}^{+}$ by the 400-nm laser pumping and find that the population is efficiently transferred between the $X{}^2\mathrm{\Sigma }_g^{+}$ state and the $B{}^2\mathrm{\Sigma }{{}_u}^{+}$ state by a Rabi oscillation combined with a Raman-type transition, leading to the population inversion so that the lasing occurs at the $B{}^2\mathrm{\Sigma }{{}_u}^{+}-X{}^2\mathrm{\Sigma }_g^{+}$ (0, 1) and (1, 2) emissions.

• Received 4 July 2021
• Accepted 24 August 2021

DOI:https://doi.org/10.1103/PhysRevA.104.032823