Returning electron wave packets for He in linearly polarized laser pulses at 780 nm. (a) The screening factor is chosen as α = 1.0, and the peak intensities are 5 × 10¹⁴ W/cm², 10 × 10¹⁴ W/cm², and 15 × 10¹⁴ W/cm², respectively. (b) The peak intensity is 15 × 10¹⁴ W/cm² and the screening factors are α = 1.0, 2.0, 3.0, and 4.0, respectively. The calculations include integration over the laser focus volume. In panel (a), the vertical solid line marks the excitation threshold of He⁺ in the field-free case, while the vertical dotted line marks the corresponding threshold for excitation in the field at a peak intensity of 15 × 10¹⁴ W/cm². Adapted from Ref. [35].