Using high energy resolution angle resolved photoemission spectroscopy, we
have resolved the bilayer splitting effect in a wide range of dopings of the
bilayer cuprate Bi2Sr2CaCu2O8+δ. This bilayer splitting is
due to a nonvanishing intracell coupling t⊥, and contrary to
expectations, it is not reduced in the underdoped materials. This has
implications for understanding the increased c-axis confinement in underdoped
materials