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 N-2(+)(X-2 Sigma(+)(g)) state and the excited N-2(+)(A(2)Pi(u)) and N-2(+)(B-2 Sigma(+)(u)) states occur in strong laser fields. The couplings result in a population inversion between the N-2(+)(X-2 Sigma(+)(g)) and N-2(+)(B-2 Sigma(+)(u)) states at wavelengths near 800 nm, which is verified by our experimental observation of the amplification of a seed at similar to 391 nm. The result provides insight into the mechanism of free-space nitrogen ion lasers generated in remote air with strong femtosecond laser pulses.