Electronic State Population Dynamics upon Ultrafast Strong Field Ionization and Fragmentation of Molecular Nitrogen

TL;DR

This study uses ultrafast nitrogen K-edge spectroscopy to track electronic state populations and fragmentation dynamics of N₂ molecules under strong laser fields, revealing detailed femtosecond to picosecond processes.

Contribution

It provides the first quantitative measurement of time-dependent electronic state populations of N₂⁺ during ultrafast ionization and fragmentation using high-order harmonic probe pulses.

Findings

Low population of the A²Π_u state in N₂⁺

Nearly equal populations of X²Σ_g⁺ and B²Σ_u⁺ states

Fragmentation into N and N⁺ occurs over tens of picoseconds

Abstract

Air-lasing from single ionized N$_2^+$ molecules induced by laser filamentation in air has been intensively investigated and the mechanisms responsible for lasing are currently highly debated. We use ultrafast nitrogen K-edge spectroscopy to follow the strong field ionization and fragmentation dynamics of N$_2$ upon interaction with an ultrashort 800 nm laser pulse. Using probe pulses generated by extreme high-order harmonic generation, we observe transitions indicative of the formation of the electronic ground X$^2\Sigma_{g}^{+}$, first excited A$^2\Pi_u$ and second excited B$^2\Sigma^+_u$ states of N$_2^+$ on femtosecond time scales, from which we can quantitatively determine the time-dependent electronic state population distribution dynamics of N$_2^+$. Our results show a remarkably low population of the A$^2\Pi_u$ state, and nearly equal populations of the X$^2\Sigma_{g}^{+}$ and B$^2\Sigma^+_u$ states. In addition, we observe fragmentation of N$_2^+$ into N and N$^+$ on a time scale of several tens of picoseconds that we assign to significant collisional dynamics in the plasma, resulting in dissociative excitation of N$_2^+$.