Figure 2: Comparison of experimental and theoretical results.

(a) Illustration of the RESI mechanism dominating NSDI at the experimental parameters: the laser electric field as a function of time is drawn along with schematics of the electronic configuration of the outer shell of Ar at three key time steps of the process. First, an electron e₁ is liberated from Ar, leaving a hole (small circle) in the outer shell of Ar⁺ (light blue). Second, an excited Ar⁺ ion (pink colour) is produced upon recollision of electron e₁ near the zero-crossing of the electric field. Finally, a second electron e₂ is liberated from the excited Ar⁺ ion forming Ar²⁺ (purple). (b) Calculated two-electron momentum spectrum averaged over CEP for the experimental parameters using the semiclassical RESI model, described in the text. (c) Experimental recoil-ion momentum spectra (for momenta p_|| parallel to the polarization axis) for single (Ar⁺ ions, blue open dots) and double ionization (Ar²⁺ ions, black crosses), respectively, averaged over CEP. The experimental data are compared with the model results for single ionization (blue line), and double ionization ((e, 2e)-mechanism (dashed red line) and RESI-mechanism (black line)). Scattering angles β=0° and β=20° are used to simulate the (e, 2e) and RESI processes, respectively. (d) Comparison of the experimental and calculated asymmetry A as a function of the CEP φ for Ar⁺ and Ar²⁺ (using the same symbols and colour coding as in (c)). All the theoretical results shown in (b,c,d) were obtained from the same simulation with the parameters given in the text.