Real-time observation of dynamics in rotational molecular wave packets by use of "air laser" spectroscopy

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.