Comparison of the simulated results 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 dotted and solid curves are the QRS simulations with and without taking into account the shift of the continuum threshold energy due to the presence of an electric field at the collision time, respectively. The screening factor was set to α = 1.0. The simulated results are relative and normalized to the experimental data individually at the intensity of 4.5 × 10¹⁴ W/cm². The curves on the right are the calculated sequential He²⁺ yields using ADK and PPT. Adapted from Ref. [35].