  We carry out a combined theoretical and experimental investigation on the
population distributions in the ground and excited states of tunnel ionized N2
molecules at various driver wavelengths in the near- and mid-infrared range.
Our results reveal that efficient couplings (i.e., population exchanges)
between the ground state and the excited states occur in strong laser fields.
The couplings result in the population inversion between the ground and the
excited states at the wavelengths near 800 nm, which is verified by our
experiment by observing 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.
