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

Molecular rotational spectroscopy based on a 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 molecular rotational wave packet. Our analysis also shows that there exist two channels of generation of the nitrogen laser, shedding light on the population inversion mechanism behind the air laser generated by intense femtosecond laser pulses.

Figure 1

A typical spectrum of the 391 nm nitrogen lasing emission.

Figure 2

(a) The intensity of the total emission of the $P$-branch band as a function of the time delay between the pump and the probe pulses. (b) The Fourier transform of the curve in (a).

Figure 3

The signal intensities of the $R$-branch lines presented individually as a function of the time delay between the pump and probe pulses. Insets show the details in a window of time delay between 5 and 10 ps.

Figure 4

The Fourier transforms of the corresponding curves in Fig. 3.

Figure 5

Experimental measured data (stars) and theoretical curves (dashed line) of the slow oscillation frequency as a function of rotational quantum number.

Figure 6

Energy level diagrams of the lasing emissions from (a) the direct channel and (b) the Raman-like cascade channel.