Panels (a)–(c) show the variation of electron collision events with t_R–t₁ and t_R, respectively. Panel (a) is for He atoms driven by 10¹⁶-W/cm² field, and the insert shown the variation with t_R. Panels (b) and (c) are for Ar atoms driven by 2.0 × 10¹⁴-W/cm² field. Each curve is normalized by its maximum, and t₁ and t_R denote the time of the first ionization and the electron collision, respectively. Panel (d) shows the time variation of the two-electron distance distribution. Vertical red dot line indicates the correspondence between the electron collision peaks and the two-electron distances. The driving field is the 800 nm–400 nm CRTC field, and the optical cycle denotes the period of 800-nm field. The x component of the electric field is also marked by the light gray lines in panels (a) and (b).