What is the order of increasing ionization between lead cesium and barium?

The order of increasing ionization energy among lead (Pb), barium (Ba), and cesium (Cs) is cesium < barium < lead. Cesium, being in Group 1, has the lowest ionization energy due to its larger atomic radius and lower effective nuclear charge. Barium, an alkaline earth metal, has a higher ionization energy than cesium but lower than lead, which is a post-transition metal with a higher effective nuclear charge and smaller atomic radius. Thus, the increasing order is Cs < Ba < Pb.