A theoretical prediction of a new spin effect by Tamura, Piepke, and Feder has been experimentally verified: Photoelectrons can be polarized even if the photoemission is performed with linearly polarized radiation and even if it is studied in the highly symmetrical setup of normal incidence and normal emission. Radiation with energies between 21 and 22.4 eV ejects photoelectrons from Pt(l 11) with a degree of polarization between 10% and 40%. The spin direction coincides with a plane parallel to the surface and changes its sign when the crystal is rotated by 60° about the surface normal.PACS numbers: 79.60.Cn, 71.25.PiThe existence of spin-polarized photoelectrons obtained with circularly polarized radiation from unpolarized targets (free atoms, molecules, adsorbates, and nonferromagnetic solids) has been proved to be a common phenomenon rather than exceptional. 1 The spinpolarization information is an important tool to characterize the symmetry of the states and bands involved, i.e., to perform a symmetry-resolved band mapping of solids or a characterization of quantum numbers, dipole matrix elements, or phase-shift differences of wave functions in the photoionization of free or adsorbed atoms. XylThat linearly or even unpolarized radiation is able to eject polarized electrons in photoemission of ferromagnetic solids, 3 in which the photoelectron polarization is primarily an effect of the initial states, is well known. In photoionization of free unpolarized atoms and molecules 4 or in photoemission of nonferromagnetic solids 5 it has been found in angle-resolved off-normal photoelectron emission as a final-state effect; in these cases it is a consequence of a quantum-mechanical interference between different photoelectron partial waves in atomic photoionization, 6 or due to spin-dependent photoelectron diffraction or phase-matching conditions at the solidvacuum interface in photoemission. 7 In spin-resolved photoemission from noncentrosymmetric crystals spin polarization can arise from difference in spin-up and spin-down conduction-band hydridization with valence p states and from surface-transmission effects. 8 Normal incidence of linearly polarized light along centrosymmetric cubic crystals and normal photoelectron emission was, however, commonly assumed to yield no spin polarization at all. 7 ' 9 Very recently, Tamura, Piepke, and Feder 10 refuted this belief and predicted normal-emission photoelectron spin polarization by linearly polarized light for (111) surfaces of centrosymmetric cubic crystals. Their prediction is based upon a one-step photoemission theory using a relativistic multiple-scattering formalism and they identify the spin-orbit interaction in the half-space initial states as its main cause. In general, symmetry arguments show that for this special geometry electron spin polarization P can be nonzero. Because of the invariance of the total system (semi-infinite crystal with surface, incident light, electron detection direction) under a symmetry operation, photoelectrons can only be polarized p...

Spin resolved photoelectron spectroscopy with Ir(lll) was performed in normal incidence of circularly polarized VUV radiation and normal electron emission. Using the spin information the spectra were separated with regard to the symmetry of the initial states. Besides the dominant transitions into the free electron like parts of the unoccupied bands, transitions into a secondary unoccupied band were unambiguously identified. Transitions into this secondary band cannot be evidenced without spin analysis. The data are in excellent agreement with a fully relativistic first-principles band structure calculation of Noffke and Fritsche, except for an overall shift of AE = 0.8 eV ±0.3 eV for the energy of the unoccupied final bands.

A theoretical prediction of Tamura and Feder has been experimentally verified: Photoelectrons from the fourfold-symmetric surface of a centrosymmetric crystal, Pt(100), can be polarized even if the incident radiation is unpolarized and the electrons are emitted normal to the surface. For 21.2and 16.9-eV photon energies, a spin-polarization component P~p erpendicular to the reaction plane is found. The degree of polarization is up to 15% and does not change when the crystal is rotated about its surface normal. This supports strongly the prediction that the effect is due to phase-shift differences.This work reports on the experimental verification of an effect in photoemission that produces spin-polarized electrons without making use of magnetically ordered electron spins (as in photoemission from magnetic materials') or of optical spin orientation by excitation with circularly polarized light. It was predicted in a recent theoretical work by Tamura and Feder and yields spinpolarized electrons with unpolarized light even in the normal photoemission from the fourfold-symmetrical Pt(100) surface, for which other earlier reported spinpolarization effects with linearly and unpolarized radiation from nonmagnetic crystals are forbidden by symmetry (see below}. The effect has its origin in a broken symmetry due to off-normal light incidence in combination with hybridization [b,656 hybrids for (100) surfaces] and yields spin-polarized electrons due to phase-shift differences. The spin-polarization vector P is perpendicular to the reaction plane of the incident radiation and the emitted electrons. The effect is only predicted for the "one-step model" of photoemission, while it is absent in the "three-step model. " It is expected to occur for practically each crystal surface, which makes it widely applicable. In photoemission from ferromagnetic samples, for example, it competes with exchange-induced effects and might influence the interpretation of spinresolved photoemission spectra and conclusions concerning ferromagnetic properties. There have been previous reports in the angle-resolved photoemission from nonmagnetic materials with linearly and unpolarized radiation but they all differ fundamentally from the effect reported in this work: In Ref. 5 spinpolarized electrons arise from spin-dependent photoelectron diffraction or phase matching conditions at the solid vacuum interface and are only obtained when the electron emission occurs a+normal Spin-polarized e. lectron emission with linearly and unpolarized light was also observed in the photoionization of free unpolarized atoms and molecules where it is a consequence of a quantummechanical interference between different photoelectron partial waves. It was, however, until now not clear whether a counterpart of this effect for solids can exist at all. Even for normal photoemission with linearly and unpolarized light from nonmagnetic crystals spin-polarized electrons were found when the sample is noncentrosymmetric.Phase shifts do not, however, play any role in this case and...

Au/Pt (111) has been studied by spin-, angle-and energyresolved photoemission with normal incident circularly polarized synchrotron radiation of BESSY and normal photoelectron emission for different Au coverages. The prepared layers were characterized by LEED and Augerelectron spectroscopy and turned out to grow up two dimensional and epitaxially. In the photoemission experiments the development of the 3-dimensional bandstructure in the A-direction could be observed. For a coverage of 2.6 layers the highest occupied spin-orbit split bands are located at about 0.6 eV lower binding energy than the corresponding bands for a 3D-Au crystal and show dispersion which is, however, weaker than in a 3D-Au crystal. A 5 layer Au adsorbate was found to have already the same dispersion and energetic location as a Au(lll)-crystal. For thick gold layers, which behave in photoemission like Au(lll)-crystals, we find structures that cannot be due to direct transitions into a free electron like final band. The coverage dependence and spin polarization of these structures show that some of them are due to surface resonances, while the origin of one strong peak could not yet be explained conclusively. In addition we find strong hybridization and two avoided crossings in the occupied part of the bandstructure.