Why is the first ionisation energy of barium larger than the first ionsiation energy of caesium?

Barium has one more proton than caesium. The outer electrons of both elements are in the same shell so they feel the same shielding effect. However, as barium has one more proton, barium's outer electrons will experience a greater nuclear attraction. Therefore, more energy will be required to remove an electron from barium than caesium, resulting in a larger first ionisation energy.