Rotational Coherence Encoded in an “Air-Laser” Spectrum of Nitrogen Molecular Ions in an Intense Laser Field

Abstract

We investigate lasing action in aligned nitrogen molecular ions (N 2 þ) produced in an intense laser field. We find that, besides the population inversion between the B 2 AE u þ À X 2 AE g þ states, which is responsible for the observed simulated amplification of a seed pulse, a rotational wave packet in the ground vibrational state (v ¼ 0) of the excited electronic B 2 AE u þ state has been created in N 2 þ. The rotational coherence can faithfully encode its characteristics into the amplified seed pulses, enabling reconstruction of rotational wave packets of molecules in a single-shot detection manner from the frequency-resolved laser spectrum. Our results suggest that the air laser can potentially provide a promising tool for remote characterization of coherent molecular rotational wave packets.

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

“…[20,21]. The femtosecond laser beam (1 kHz, 800 nm, ~40 fs) produced by a commercial Ti:sapphire laser system (Legend Elite-Duo, Coherent, Inc) was divided into two arms.…”

“…Pump-probe measurements based on these pulses can provide a sufficient temporal resolution to reveal the rotational dynamics of many kinds of molecules such as the nitrogen molecule investigated in this work. Second, the bandwidth of the strong-field-ionization induced molecular lasing spectrum is typically on the scale of ~0.1 nm, providing a sufficient spectral resolution to distinguish the different rotational states [15,21]. In our point of view, the combination of these advantages opens new possibilities for investigation of 6 strong field molecular physics.…”

“…The molecular rotational wave packet is a coherent superposition of rotational eigenstates excited from the initial state, [21]. However, in this work, only the integrated signal of all the lasing lines in the P-branch or R-branch was investigated with the pump-probe measurement, which cannot provide the dynamic information on evolution of the rotational state distribution in the wave packet of molecular ions of nitrogen.…”

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

“…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.…”

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

“…Recently, Zhang et al observed that the rotational coherence of the molecular ions can encode its characteristics into the air laser [16]. 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.…”