Quantum well behavior without confining barrier observed via dynamically screened photon field.

Angle-resolved photoemission spectra from Na adlayers on Al(111) reveal features which behave like quantum well resonances although the substrate provides no confining barrier. These features are observed in a narrow photon energy range where overlayer collective excitations cause resonant enhancement of the photoemission intensity. The quantum well behavior is shown to be due to surface resonances of the Na/Al system. The resonances are observable using photoemission because of spatial confinement and dynamical enhancement of the local electric field within the Na films.