Giant Enhancement of Air Lasing by Complete Population Inversion in N2+

A fine manipulation of population transfer among molecular quantum levels is a key technology for control of molecular processes. When a light field intensity is increased to the TW-PW cm(-2) level, it becomes possible to transfer a population to specific excited levels through nonlinear light-molecule interaction, but it has been a challenge to control the extent of the population transfer. We deplete the population in the X-2 Sigma(+)(g) (v = 0) state of N-2(+) almost completely by focusing a dual-color (800 nm and 1.6 mu m) intense femtosecond laser pulse in a nitrogen gas, and make the intensity of N-2(+) lasing at 391 nm enhanced by 5-6 orders of magnitude. By solving a time-dependent Schrodinger equation describing the population transfer among the three lowest electronic states of N-2(+), we reveal that the X-2 Sigma(+)(g) (v = 0) population is depleted by the vibrational Raman excitation followed by the electronic excitation A(2)Pi(u)(v = 2,3,4) <- X-2 Sigma(+)(g)(v = 1) <- X-2 Sigma(+)(g) (v = 0), resulting in the excessive population inversion between the B-2 Sigma(+)(u) (v = 0) and X-2 Sigma(+)(g) (v = 0) states. Our results offer a promising route to efficient population transfer among vibrational and electronic levels of molecules by a precisely designed intense laser field.