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

We align nonadiabatically an ensemble of N-2 in air, and irradiate the ensemble with linearly polarized intense few-cycle laser pulses to generate population-inverted N-2(+) ions. By probing the self-seeded lasing signals of N-2(+) at 391 nm, we show that the ultrafast population inversion in N-2(+) between the electronic ground X-2 Sigma(+)(g) state and the electronically excited B-2 Sigma(+\)(u) state is sensitively influenced by the extent of the alignment of N2 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-2 Sigma(+)(u) state from the X-2 Sigma(+)(g) state of N-2(+) in the ultrashort laser field is mediated by the A(2)Pi(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 N-2 and the laser pulse ionizing N-2 into N-2(+) as well as by the difference in the laser polarization directions of these laser pulses.