We investigate postionization population transfer processes among the three low-lying electronic states—$\mathrm{X}{}^2{\mathrm{\Sigma }}_{\mathrm{g}}^{+}$, $\mathrm{A}{}^2{\mathrm{\Pi }}_{\mathrm{u}}$, and $\mathrm{B}{}^2{\mathrm{\Sigma }}_{\mathrm{u}}^{+}$—of ${\text{N}}_2^{+}$ created in an ultrashort-pulsed intense laser field using a sudden turn-on model by explicitly including the ionization timing effect throughout the entire laser pulse. By adopting the rovibronic model in which the electronic, vibrational, and rotational degrees of freedom of ${\text{N}}_2^{+}$ are included, we have revealed that the population inversion between the $\mathrm{X}{}^2{\mathrm{\Sigma }}_{\mathrm{g}}^{+}(v=0)$ state and $\mathrm{B}{}^2{\mathrm{\Sigma }}_{\mathrm{u}}^{+}(v=0)$ state can be realized in the specific rotational quantum number ranges in the R- and P branches when ionization occurs around the central peak of the temporal shape of the laser field intensity. Based on the quasistationary Floquet theory, we have examined the laser field intensity dependence of the final population in the upper Floquet state, corresponding to the $\mathrm{B}{}^2{\mathrm{\Sigma }}_{\mathrm{u}}^{+}(v=0)$ state of ${\text{N}}_2^{+}$, and revealed that the population transfer can be interpreted by the ionization at the ionization timing of $t=0$ as long as the laser field intensity is less than $4\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$, while the ionization processes occurring in the entire range of the laser pulse need to be taken into account when the field intensity becomes larger.

• Received 24 August 2022
• Revised 7 November 2022
• Accepted 5 December 2022

DOI:https://doi.org/10.1103/PhysRevA.106.063109