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This work is dedicated to Professor Ahmadou Wagué for guiding me as I embarked on my first steps in research.

For supervising both my PhD Thesis (2007) and my National PhD Thesis in Physical Sciences (2013).

May he find expressed in these few lines, my entire recognition as well as my full congratulations in his being the first African to be elected to the council of the American Physical Society (APS) in August 2017.

Screening Constant by Unit Nuclear Charge Method

Description and Application to the Photoionization of Atomic Systems

Ibrahima Sakho