Mechanism of population inversion in laser-driven N2+

On the basis of our theoretical model of the population transfer between two levels interacting suddenly with an intense laser pulse (Zhang et al 2017 J. Phys. B: At. Mol. Opt. Phys. 50 185603), we investigate the mechanism of the population inversion between the electronically excited B-2 Sigma(+)(u), state and the electronic ground X-2 Sigma(+)(g) state of N(2)(+ )achieved by the irradiation of a near-IR (800 nm) intense laser pulse, leading to the generation of the coherent emission at 391 nm. We show that one of the Floquet states generated at the moment when N(2)(+ )starts interacting with an intense laser pulse is adiabatically connected to the ground vibrational state of the electronic B-2 Sigma(+)(u), state and keeps a constant population throughout the period of the interaction with the light field, indicating that the efficient off-resonance B-2 Sigma(+)(u)-X-g(+) transition proceeds.