The breakdown of the single active electron approximation for strong field laser-atom interactions*

The single active electron (SAE) approximation is a cornerstone of how we treat the interaction of light with matter. Notable examples are high harmonic generation and attosecond science.  However, the SAE approximation fails in systems where the multi-electron character of the atomic wavefunction plays a significant role. In this work, we solve the Schrodinger equation for atoms and ions in strong laser fields within the adiabatic response regime (e.g. long wavelength). The single electron response to the laser field is modeled in the dipole approximation for laser field strengths near the saturation intensity of tunneling ionization. This intensity corresponds to approximately 5×10¹⁴ W/cm² for the single ionization of neon atoms for example. Independent electron spin-orbital based Hartree-Fock wavefunctions are calculated self-consistently for the atomic and ionic wavefunction. We present the SAE and many active electron (MAE) cases for closed and open shell species such as neon, krypton, and sodium in various charge states. By quantifying the MAE influence on tunneling ionization, we provide new insights into the limitations of the SAE model and highlight the importance of incorporating MAE effects on tunneling ionization.