If one electron has been removed from an atom, it becomes increasingly difficult to remove the second and subsequent electrons from the resulting positively charged ions on account of electrostatic attraction.
Suppose a gas mixture of F, Cl, Br, and I is irradiated with photons of frequency appropriate to ionize Cl. What ion(s) will be present in the mixture?
Cl+, Br+, I+ only
If the frequency has enough energy to ionize Cl, the radiation can ionize Br and I also. This is because the ionization energy of Br and I would be lesser than Cl;