Lawrence Berkeley National Laboratory

We present systematic $k_ \parallel $-dependent measurements of the Fermi surface and underlying bandstructure of quantum well states in Cu/Co/Cu(001). Compared to bands from normal emission, we find a complicated evolution of "split" QW states as a function of the thicknesses of both the copper overlayer and the cobalt barrier layer. Self-consistent calculations show that the penetration of the quantum well states into the cobalt barrier layer is significant and leads to the observed very non-free-electron behavior of these states.