PART 1 -- Different Photoionization Processes, Rydberg Series -- Experimental and Theoretical Methods of Photoionization -- General Formalism of the Screening Constant by Unit Nuclear Charge Method Applied to Photoionization -- Applications in the Calculations of Energies and Natural Widths of the Resonance States of Multi-Electron Atomic Systems. Applications in the Calculations of Energies and Natural Widths of the Resonance States of Multi-Electron Atomic Systems -- Application to the Calculation of Energies of Two-electron Atomic Systems (Helium-like Systems) -- Calculating the Energies of Three-electron Atomic Systems (Lithium-like Systems) -- Application in the Resonant Photoionization of Atomic Systems of Atomic Numbers Z = 4-12 -- Resonant Photoionization of Sulfur (S) and Ar+, Se+, Se2+ and Kr+ Ions -- Conclusion: Advantages and Limits of the SCUNC Formalism -- Detailed Calculation of the Screening Constant by Unit Nuclear Charge Relative to the Ground State of Two-electron Atomic Systems -- Formalism of Slater's Atomic Orbital Theory -- Modified Formalism of the Atomic Orbital Theory -- Bibliography -- Index -- Other titles from iSTE in Waves.