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 (N-2(+)) produced in an intense laser field. We find that, besides the population inversion between the B-2 Sigma(+)(u) - X-2 Sigma(+)(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 B-2 Sigma(+)(u) state has been created in N-2(+). 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.