  We study population distributions and lasing actions of N_2^+ driven by
femtosecond lasers with various wavelengths, and uncover an efficient ionic
excitation mechanism induced by synchronized ionization and multiphoton
resonance. Our results show that the strongest N_2^+ lasing appears around 1000
nm pump wavelength. At the optimal wavelength, the pump-energy threshold for
air lasing generation is reduced by five folds compared with that required by
the previous 800 nm pump laser. Simulations based on the ionization-coupling
model indicate that although the Stark-assisted three-photon resonance can be
satisfied within a broad pump wavelength range, the optimal pump wavelength
arises when the dynamic three-photon resonance temporally synchronizes with the
ionization injection. In this case, the ionic dipoles created at each half
optical cycle have the same phase. The dipole phase locking promotes the
continuous population transfer from ionic ground state to the excited state,
giving rise to a dramatic increase of excited-state population. This work
provides new insight on the photoexcitation mechanism of ions in strong laser
fields, and opens up a route for optimizing ionic radiations.
