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

Enhanced Population on Ionic Excited States by Synchronized Ionization and Multiphoton Resonance