Electronic structure of a Ag(111) quantum film covered with a (√3×√3) R30o Bi/Ag ordered suface alloy, which shows Rashba spin-split surface states, is investigated with high-resolution spin- and angle-resolved photoemission spectroscopy. Quantum well states (QWS) of the Ag film are significantly modified by the interaction with the spin-split surface states (SS). The band dispersion of the QWS forms energy gaps at the crossing points with the SS bands when spins of the QWS and the SS are parallel, while it remains free-electron-like when those spins are anti-parallel. The present results give a direct evidence of breakdown of the spin-degeneracy in the quantum film of a non-magnetic metal by the surface Rashba effect.
