Reconstruction of a Rotational Wavepacket of Inverted Molecular Ions in an Intense Femtosecond Laser Field

TL;DR

This paper demonstrates the generation and decoding of a rotational wavepacket in N2+ ions within a femtosecond laser plasma, using an air laser as a probe to remotely characterize molecular rotational states.

Contribution

It introduces a method to generate and decode rotational wavepackets in molecular ions using an air laser spectrum in a femtosecond laser plasma.

Findings

Rotational wavepacket observed in N2+ ions.

Modulation of seed signals correlates with wavepacket dynamics.

Theoretical calculations match experimental results.

Abstract

We report on generation of a rotational wavepacket in the ground vibronic state (v = 0) of excited electronic B2{\Sigma}u+ state of N2+ in a femtosecond laser induced plasma spark. Decoding of the rotational wavepacket is achieved with the frequency-resolved seed-amplified air laser spectrum resulting from the population inversion between the B2{\Sigma}u+-X2{\Sigma}g+ states of N2+ in the plasma. We also observe that the rotational wave-packet leads to modulation of the amplified seed signals in the time domain using a pump-probe scheme, which can be well reproduced by theoretical calculation. Our results demonstrate that the air laser provides an ideal probe for remote characterization of molecular rotational states distribution in a femtosecond laser induced filament.