The spin structure of quantum well states (QWSs) in threemonolayer-thick gold overlayers on W(110) and Mo(110) is studied experimentally by spin-and angle-resolved photoelectron spectroscopy and theoretically by ab initio calculations. The spin-orbit coupling effects manifest themselves differently in the spin structure of atomic-like d and delocalized sp QWSs depending on how strongly the states are influenced by the substrate. The QWSs of a d character show a strong spin polarization with an

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... lmost identical structure for W(110) and Mo(110), suggesting a weak interaction with the substrate, whereas for sp QWSs, the interaction is much stronger, which to a large extent determines their splitting and spin polarization. The theoretical model yields a qualitative agreement with the experiment and explains the observed behavior.