Why cesium has a lower first ionization energy then rubidium?

Caesium has more electron levels than rubidium, because the electrons are further away the attraction between the outer electrons and protons in the nucleus is less. But that's not just the answer. There is also a thing called electron shielding, the negative charges, of the electrons in the energy levels closer to the nucleus, sort of repel the outer electrons so they can't get closer to the nucleus. Since the electrons are further away and have the attraction from the positively charged nucleus reduced, it takes less energy to break the attraction for caesium than rubidium.