Rotational coherence encoded in an "Air-Laser" spectrum of nitrogen molecular ions in an intense laser field

We investigate lasing action in aligned nitrogen molecular ions (N₂ ⁺) produced in an intense laser field. We find that, besides the population inversion between the B² ∑_u ⁺ - X² ∑_g+ states, which is responsible for the observed simulated amplification of a seed pulse, a rotational wave packet in the ground vibrational state (v=0) of the excited electronic B² ∑_u ⁺ state has been created in N₂ ⁺. The rotational coherence can faithfully encode its characteristics into the amplified seed pulses, enabling reconstruction of rotational wave packets of molecules in a single-shot detection manner from the frequency-resolved laser spectrum. Our results suggest that the air laser can potentially provide a promising tool for remote characterization of coherent molecular rotational wave packets.