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

Abstract

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 molecular rotational wave packet. 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. PACS number(s):

Coupling ofN2+rotational states in an air laser from tunnel-ionized nitrogen molecules

“…The results are shown in Fig. 2, which is consistent with our previous observation [19]. Figure 2(a) is a typical forward lasing spectrum and the inset is a close-up view of R-branch lasing lines (indicated in the box) from the individual rotational quantum number J labeled on each peak.…”

“…Therefore we take a different approach to make a qualitative estimation of the linewidth of each P -branch line. In our previous work [19], we have measured the P -branch intensity as a function of the time delay between the 800-nm pump pulse and the 400-nm probe pulse. The linewidth of the P -branch lines can be estimated by performing a Fourier transform of the curve measured in the time domain.…”

“…The coherent rotational wave packet of the molecular ions was achieved either by ionization-induced alignment [17] or by Raman-type interaction of the pump laser with the molecules [18], or a combination of the two. Motivated by this finding, Zeng et al further investigated the ultrafast dynamics of the rotational coupling in the lasing action by carrying out frequency-resolved pump-probe measurements of the laser lines corresponding to R-branch transitions ( J = −1) and discovered that the laser lines show a fast oscillation [19] which can be ascribed to the quantum beat between two adjacent rotational states |J and |J + 2 in the rotational wave packet. In the current work, we carry out a more systematic investigation of the unusual behavior in the pump-probe measurement of the individual laser lines corresponding to R-branch transitions ( J = −1) and analyze the result by theoretical simulations.…”

Ultrafast Rotational Spectroscopy Based on a Free-space Nitrogen Ion Laser

“…We can see that the frequencies of the fast oscillations increase with the increasing rotational quantum number J, as indicated by the red dashed line which can be expressed by the equation ∆ −2, = (4 − 2) , where is the rotational constant of the excited state 2 Σ + of 2 + . Other frequencies such as ∆ −4, −2 = (4 − 10) , ∆ , +2 = (4 + 6) can also be observed [5].…”

“…Here, we show that the nitrogen ion laser can be applied to investigate the rotational coherences of molecular wave packets. The ultrashort pump laser pulses not only generate the population inversion between the excited state 2 Σ + and the ground states 2 Σ + of 2 + , which is responsible for the simulated amplification seeded by an external pulse, but also creates a rotational wave packet in the ground vibrational state (υ = 0) of the excited electronic state 2 Σ + [4][5]. The rotational coherences of 2 + will faithfully encode its characteristics into the amplified seed pulses.…”