Alignment-dependent population inversion in N2+ in intense few-cycle laser fields

Abstract

Listen

Translate article

We align nonadiabatically an ensemble of ${\mathrm{N}}_{2}$ in air, and irradiate the ensemble with linearly polarized intense few-cycle laser pulses to generate population-inverted ${\mathrm{N}}_{2}{}^{+}$ ions. By probing the self-seeded lasing signals of ${\mathrm{N}}_{2}{}^{+}$ at 391 nm, we show that the ultrafast population inversion in ${\mathrm{N}}_{2}{}^{+}$ between the electronic ground $X\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Sigma}}_{\mathrm{g}}$ state and the electronically excited $B\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Sigma}}_{\mathrm{u}}$ state is sensitively influenced by the extent of the alignment of ${\mathrm{N}}_{2}$ with respect to the polarization direction of the few-cycle laser field. The observed alignment dependence of the air lasing is reproduced well by numerical calculations performed based on a theoretical model in which the population transfer to the $B\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Sigma}}_{\mathrm{u}}$ state from the $X\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Sigma}}_{\mathrm{g}}$ state of ${\mathrm{N}}_{2}{}^{+}$ in the ultrashort laser field is mediated by the $A\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Pi}}_{\mathrm{u}}$ state. Our findings show that the intensity of the air lasing can be manipulated precisely by the timing between the laser pulses inducing the alignment of ${\mathrm{N}}_{2}$ and the laser pulse ionizing ${\mathrm{N}}_{2}$ into ${\mathrm{N}}_{2}{}^{+}$ as well as by the difference in the laser polarization directions of these laser pulses.