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

Air lasing from single ionized ${\mathrm{N}}_{2}^{+}$ 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 ${\mathrm{N}}_{2}$ 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 $X^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$, first excited $A{^{2}\mathrm{\ensuremath{\Pi}}}_{u}$, and second excited $B^{2}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$ states of ${\mathrm{N}}_{2}^{+}$ on femtosecond timescales, from which we can quantitatively determine the time-dependent electronic state population distribution dynamics of ${\mathrm{N}}_{2}^{+}$. Our results show a remarkably low population of the $A^{2}{\mathrm{\ensuremath{\Pi}}}_{u}$ state, and nearly equal populations of the $X^{2}{\mathrm{\ensuremath{\Sigma}}}_{g}^{+}$ and $B^{2}{\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$ states. In addition, we observe fragmentation of ${\mathrm{N}}_{2}^{+}$ into N and ${\mathrm{N}}^{+}$ on a timescale of several tens of picoseconds that we assign to significant collisional dynamics in the plasma, resulting in dissociative excitation of ${\mathrm{N}}_{2}^{+}$.

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