**5. Conclusions**

We computed a theoretical photoelectron spectrum for photoelectrons of low kinetic energies for the second step of sequential two-photon double ionization (2PDIII PES) of Ar at a photon excitation energy of 65.3 eV, exceeding the 3*s* ionization threshold. The calculation takes into account the most significant electron correlations stemming from single- and double-electron excitations.

The calculation predicts that in the low-energy part of the 2PDIII PES, correlation satellites should be observed whose intensities exceed the intensity of the main (direct) transition to the Ar2<sup>+</sup> <sup>3</sup>*s*23*p*<sup>4</sup>(<sup>1</sup>*S*, <sup>1</sup>*D*, <sup>3</sup>*P*) states. Correlation satellites consist of lines associated with conventional photoelectron emissions and the Auger decay of the <sup>3</sup>*s*13*p*<sup>4</sup>*n*<sup>1</sup> states; the cross sections of both processes are comparable in magnitude. The lines of the first type should change their energy with changing photon excitation energy, while the energy of the Auger lines should remain unchanged.

The dependence of the intensities of correlation satellites may have resonant characters associated with the excitation of discrete levels at certain energies. In this case, the excitation of resonances at the second step of the two-photon process (e.g., those stemming from the second-step configuration <sup>3</sup>*s*13*p*43*d*2) will contribute to the 2PDIII PES even at small exciting radiation fluxes. At large fluxes, there should be a contribution from resonances excited at the first step (e.g., those stemming from the first-step configuration <sup>3</sup>*s*13*p*53*d*1). The estimates of the lifetime of the <sup>3</sup>*s*13*p*53*d*<sup>1</sup>(*LSJ*) states showed that the contribution of these processes can be significant at fluxes exceeding 10<sup>16</sup> W/cm2.

**Author Contributions:** Conceptualization and methodology V.L.S. and A.E.; data curation, B.M.L., I.D.P., V.A.K. and N.M.N.; formal analysis, validation, visualization, and writing—original draft, B.M.L., I.D.P. and V.L.S.; funding acquisition, project administration and supervision, P.V.D. and A.E.; investigation, B.M.L., I.D.P., V.L.S., V.A.K. and N.M.N.; software, B.M.L., I.D.P., V.L.S. and V.A.K.; writing—review and editing, B.M.L., I.D.P., V.L.S., P.V.D. and A.E. All authors have read and agreed to the published version of the manuscript.

**Funding:** V.L.S., N.M.N., P.V.D., and A.E. were funded by the Deutsche Forschungsgemeinschaft (DFG)— project no. 328961117-SFB 1319 ELCH (extreme light for sensing and driving molecular chirality).

**Data Availability Statement:** All the data reported in this work are available from the correspondence author on reasonable request.

**Acknowledgments:** The authors are grateful to Markus Ilchen for drawing their attention to the problem existing in interpreting the 2PDI of Ar, for his permanent interest in the work, and for discussing the results obtained.

**Conflicts of Interest:** The authors declare no conflict of interest.
