Population Redistribution among Multiple Electronic States of Molecular Nitrogen Ions in Strong Laser Fields

We carry out a combined theoretical and experimental investigation on the population distributions in the ground and excited states of tunnel-ionized nitrogen molecules at various driver wavelengths in the near- and midinfrared range. Our results reveal that efficient couplings (i.e., population exchanges) between the ground N2+(X2Σg+) state and the excited N2+(A2Πu) and N2+(B2Σu+) states occur in strong laser fields. The couplings result in a population inversion between the N2+(X2Σg+) and N2+(B2Σu+) states at wavelengths near 800 nm, which is verified by our experimental observation of the amplification of a seed at ∼391 nm. The result provides insight into the mechanism of free-space nitrogen ion lasers generated in remote air with strong femtosecond laser pulses.