Comparison of the simulated results, with the shift of the continuum threshold energy taken into account, for the yields of doubly charged ions with the experimental data for He in linearly polarized laser pulses at 780 nm. The solid circles represent the measurements of Walker et al,^[5] while the lines are simulated results for α = 1.0, 2.0, 3.0, and 4.0, respectively. The theoretical predictions are relative and were normalized to the experimental data at the peak intensity of 4.5 × 10¹⁴ W/cm² with a fixed factor for each screening factor. The inset shows the ratio of the contributions to double ionization from ionization and excitation. Adapted from Ref. [35].