  We report on generation of a rotational wavepacket in the ground vibronic
state (v = 0) of excited electronic B2{\Sigma}u+ state of N2+ in a femtosecond
laser induced plasma spark. Decoding of the rotational wavepacket is achieved
with the frequency-resolved seed-amplified air laser spectrum resulting from
the population inversion between the B2{\Sigma}u+-X2{\Sigma}g+ states of N2+ in
the plasma. We also observe that the rotational wave-packet leads to modulation
of the amplified seed signals in the time domain using a pump-probe scheme,
which can be well reproduced by theoretical calculation. Our results
demonstrate that the air laser provides an ideal probe for remote
characterization of molecular rotational states distribution in a femtosecond
laser induced filament.
