  Molecular rotational spectroscopy based on strong-field-ionization-induced
nitrogen laser is employed to investigate the time evolution of the rotational
wave packet composed by a coherent superposition of quantum rotational states
created in a field-free molecular alignment. We show that this technique
uniquely allows real-time observation of the ultrafast dynamics of the
individual rotational states in the rotational wavepacket. Our analysis also
shows that there exist two channels of generation of the nitrogen laser,
shedding new light on the population inversion mechanism behind the air laser
generated by intense femtosecond laser pulses.
