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...

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Abstract. Unpolarized light ejects spin polarized electrons from Pt(111) and Au(ll 1) even if the electron emission occurs normal to the surface. For off normal incidence of 11.8 eV, 16.9eV, and 21.2 eV radiation, and for the main peaks in the photoemission spectra, a degree of spin polarization of up to 30% or more is found for the spin polarization component Py perpendicular to the reaction plane. A crystal rotation about its surface normal does not change Py. Py is largest for transitions from bands with symmetry A~. All these experimental findings agree with a recent theoretical prediction [1] of a new spin effect by Tamura and Feder. 79.60.Cn, 71.25.Pi Spin analysis in the angle-and energy-resolved photoemission is a well established tool to gain a wealth of additional information about the electronic structure of both ferromagnetic [2] and nonmagnetic [4] crystals, surfaces and adsorbates. Most studies on nonmagnetic targets use circularly polarized radiation, since for the photoemission with unpolarized and linearly polarized radiation from non-ferromagnetic centrosymmetric [6] crystals it was for a long time generally believed (see references cited in [7,8]) that the electrons emitted normal [9] to the surface can only be unpolarized. This belief results from the description of photoemission as a "three-step process": As a consequence of the space inversion symmetry, which the infinite crystal exhibits in contrast to the semi-infinite one, no spin polarization is produced by linearly or unpolarized light in the bulk excitation step. Since transport and perpendicular surface transmission are spin independent, the photoelectrons remain unpolarized [1,10]. A theoretical treatment in the "one-step model" [7] and experiment [8] yielded, on the other hand, spin polarized electrons for the special case of normal incidence of linearly polarized radiation and three-fold surfaces. The effect shows the three-fold symmetry in the rotation diagram of the spin polarization, i.e. a sign change when the crystal is rotated about 60 degrees and it was only observed when the time reversed states with A34 and A~ symmetry are involved in the transition. The occurrence of spin polarized electrons due to this effect was, however, an exception rather than the rule; up to now the effect was only observed for transitions into the bandgap, i.e. for transitions into evanescent states [8], which emphasizes the surface sensitive nature of this effect. PACS:A spin polarization effect for far more general conditions can be expected if the radiation incidence occurs off normal. This has been predicted in recent theoretical work [1] (again in the one-step model) by Tamura and Feder. Their work deals with normal photoemi,ssion with linearly and unpolarized radiation from Pt(100) (it is in addition stated [1], that the effect should occur for practically every crystal surface): Symmetry arguments show that a spin polarization component perpendicular to the reaction plane defined by photon and photoelectron momenta is not forbidden; the ...

The normally emitted photoelectrons of the reconstructed (1 x 2) surface of Pt(ll0) caused by off-normally incoming, unpolarized light are spin polarized. Measurements have been performed with the light of a gas discharge tube and spin polarization of up to 20% has been found for the spin polarization perpendicular to the reaction plane, defined by the momenta of incoming photon and outgoing electron beam. The polarization depends significantly on a rotation of the crystal about the surface normal. The experimental data are in good agreement with theoretical calculations of Feder's group [1] based upon the theory describing the photoemission in the "one-step model".

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