Image-potential states on bcc Ñ110Ö surfaces of iron and tungsten

These so-called image-potential states have been studied in recent years mainly by inverse photoemission and with higher accuracy by two-photon photoemission ~2PPE!. Investigations on a variety of metal surfaces revealed that the binding energy En depends on the position relative to the band gap. This influence is accounted for by introducing the quantum defect a in Eq. ~1!. For magnetic materials the energetic position of the bulk band gap differs for the two spin orientations. As a result, also the energies En @and from Eq. ~1! also the quantum defect a# depend on the spin orientation of the electron in the image-potential state. Because the quantum defect a is in good approximation independent of n, the spin splitting decreases with increasing quantum number n. For the 3d transition metals spin splittings have been reported for the n 51 image-potential state of the order of several tens of meV depending on material and surface orientation. The W~110! surface has not been studied with 2PPE before. The first image-potential state has been detected by inverse photoemission. We report experimental studies on the image-potential states at Fe~110! surfaces. Due to the experimental difficulties with the preparation of bulk Fe single crystals, we prepared epitaxial Fe~110! films of thicknesses above 10 monolayers ~ML! on W~110! ~Ref. 14! and Cu~100! ~Refs. 15 and 16! substrates. The present study resolves the spin splitting of the image-potential states by polarization-dependent 2PPE measurements. The results agree with previous studies by spin-selective inverse photoemission with lower energy resolution and two-photon photoemission without spin sensitivity.