**5. Conclusions**

The photoionization cross-section of the lowest excited states of neon are presented. The R-matrix approach is applied with a variety of different parameters, and the comparison of the usability of real and virtual orbitals is carried out. Results are presented for the metastable states (total angular momentum *J* = 0 , 2) ionized by unpolarized light and for the dipole-allowed states (*J* = 1) ionized by the light with the same polarization as those causing their excitation: unpolarized, linearly and circularly polarized. The spectroscopic features such as Cooper minimum, particle-hole and doubly excited autoionizing states are found to appear differently depending on polarization.

It was shown that the model based on virtual orbitals provides much better convergence in terms of length and velocity gauge, as well as in terms of target states involved. It predicts a deeper Cooper minimum placed at higher energy. On the other hand, V-model has a tendency to become overcorrelated (unrealistically compact wave functions), which imposes the problem of a correct positioning of the narrow AISs. Based on real orbitals, the C-model turns out to be more stable and better reproduces the AIS structures. The hole-particle AISs are found to be much broader than the doubly excited ones.

This work may serve as building blocks for the investigation of multi-photon, probably two-color ionization by VUV radiation.

**Author Contributions:** M.D.K. is responsible for the V-model and M.M.P. for the R-, C-models. All of the authors took part in discussions of the results and preparing the manuscript. Conceptualization E.V.G. and A.N.G.-G.; investigation, formal analysis M.M.P. and M.D.K.; visualization M.M.P.; resources S.M.B.; writing—original draft preparation E.V.G., M.M.P. and M.D.K.; writing—review and editing M.M.P., M.D.K., S.M.B., E.V.G. and A.N.G.-G.; funding acquisition A.N.G.-G. and S.M.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** The development of the R- and C-models and the analysis of the corresponding crosssections' energy dependencies are supported by the Russian Foundation for Basic Research (RFBR) under project No. 20-52-12023 and by the Ministry of Science and Higher Education of the Russian Federation (grant No. 075-15-2021-1353). The preparation of the V-model and analysis of AIS are supported by the Russian Science Foundation (project No. 21-42-04412). The calculations of the photoionization processes are performed using resources of the Shared Services "Data Center of the Far-Eastern Branch of the Russian Academy of Sciences" and supported by the Ministry of Science and Higher Education of the Russian Federation (project No. 0818-2020-0005).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors benefited greatly from discussions with Oleg Zatsarinny. The authors are grateful to Jon Grumer for many useful pieces of advice.

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