Population Redistribution among Multiple Electronic States of Molecular Nitrogen Ions in Strong Laser Fields

TL;DR

This study combines theory and experiments to explore how strong laser fields cause population redistribution among electronic states of nitrogen ions, revealing mechanisms behind nitrogen laser generation in air.

Contribution

It provides new insights into population exchange mechanisms in N2+ ions under strong laser fields, supported by both experimental and theoretical analysis.

Findings

Population exchange occurs between ground and excited states in strong laser fields.

Population inversion near 800 nm leads to nitrogen laser amplification at 391 nm.

Experimental verification of population inversion via seed amplification.

Abstract

We carry out a combined theoretical and experimental investigation on the population distributions in the ground and excited states of tunnel ionized N2 molecules at various driver wavelengths in the near- and mid-infrared range. Our results reveal that efficient couplings (i.e., population exchanges) between the ground state and the excited states occur in strong laser fields. The couplings result in the population inversion between the ground and the excited states at the wavelengths near 800 nm, which is verified by our experiment by observing the amplification of a seed at ~391 nm. The result provides insight into the mechanism of free-space nitrogen ion lasers generated in remote air with strong femtosecond laser pulses.