Coherent emission and attosecond transient absorption of N +2 in strong fields

The ionic dynamics induced by strong-field ionization are essential to understand the fundamental physics and chemical reactions. By solving the ionization-coupling equation theoretically, we can simultaneously address strong-field ionization and coupling dynamics in ions. By employing the driving pulse at the wavelength of 1580 nm, we show that the B²Σ ⁺_u state of the strong field ionization created N ⁺₂ could be populated by polarization effect and five-photon resonance but there is no population inversion between X²Σ ⁺_g and B²Σ ⁺_u states for the nitrogen molecular ions aligning along the laser polarization. In addition, both the ultraviolet supercontinuum and the attosecond transient absorption spectroscopy (ATAS) are calculated to illustrate the characteristics of population and coherence. The Stark shift observed from the transient absorption confirms the origin of ultraviolet supercontinuum. Our results show the evolution of the absorption spectral lineshape, varying from Lorentzian to Fano to inverted Lorentzian and back forth and the optical gain is achieved at 394 eV due to the vibrational coherent dynamics. This study offers valuable insights into the strong-field quantum optics of molecular ions.