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

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${}^2\hat{I}{\pounds }_g^{+}$, first excited A${}^2{\hat{I}}_u$ and second excited B${}^2\hat{I}{\pounds }_u^{+}$ 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${}^2{\hat{I}}_u$ state, and nearly equal populations of the X${}^2\hat{I}{\pounds }_g^{+}$ and B${}^2\hat{I}{\pounds }_u^{+}$ 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${}_2^{+}$.