N.A. Cherepkov

Influence of the symmetry on the circular dichroism in angular resolved core-level photoemission

The model for angular resolved photoemission from adsorbed atoms is extended to account for the non-axial symmetry of atoms in a crystal field. In particular, circular dichroism in the angular distribution (CDAD) is theoretically investigated. A special emphasis is put on the case when incident photons are propagating along the principal axis of an atom in Cnv symmetry. The model, although mainly developed for adsorbates, may also be used as a base for emission from solid surfaces. An extension to simple bulk symmetries, like D for hexagonal or O for cubic crystals, is included. The CDAD 6 hh for normal incidence does not vanish in the extended model and reflects the symmetry of the adsorption site. Scattering induced final state effects are discussed for alkali metal adsorption. A numerical calculation of the emission from the shallow 4p core level of Rb atoms adsorbed in a (ˇ33ˇ3)2R308 structure on a Pt(111) surface is presented. In this case even the extended photoemission model predicts the absence of CDAD. The appearance of CDAD is only possible due to the scattering of photoelectrons from the neighbouring atoms of the solid.

Fe 2p photoemission magnetic dichroism with linearly polarized synchrotron radiation and with unpolarized A1 Kα radiation

We report on the measurements and analysis of Fe 2p magnetic dichroism in the angular distribution of the photoelectrons from remanently magnetized Fe(100)surfaces with unpolarized, monochromatized A1 Kα X-rays of 1486 eV energy (MDAD)and with linearly polarized synchrotron radiation of energy 800 eV (LMDAD). The two experiments verify the atomic photoionization model of (L)MDAD including the dichroism effect between field-averaged experiments of different chirality.

Spin polarisation and dichroism in ARUPS from thin rare earth films

Abstract In the present study, spin polarisation and dichroism were investigated in angular resolved VUV photoemission ARUPS from magnetised as well as paramagnetic rare earth surfaces. Thin Gd(0001) films were prepared epitaxially on W(110). The photoelectrons were excited by polarised synchrotron radiation from the BESSY I synchrotron radiation source. A SPLEED detector attached to an electron spectrometer mounted on a single axis goniometer was used for angular resolved spin analysis. The main part of this work focused on the characteristic surface state of Gd(0001) which shows a splitting, as previously observed by linear magnetic dichroism and spin polarised photoemission. The spin analysis shows that the two observed states have opposite spin characters. Additionally, the non-magnetic circular dichroism suggests j =3/2 character for the surface state. The experimental findings are compared to atomic models and one-step photoemission calculations.

Dichroism in VUV photoemission from the (0001)-surfaces of ultrathin Gd and Nd films on W(110)

Abstract Ultrathin layers of Gd and Nd epitaxially grown on W(110) have been investigated using linearly and circularly polarised light at the 6.5 m NIM beamline at the BESSY storage ring in Berlin. The high quality of the films was monitored by the sharp and dispersionless surface state just below the Fermi energy. The CDAD (circular dichroism in the angular distribution of photoelectrons) measurements showed strong effects in the surface states as well as in the valence bands. The LMDAD (linear magnetic dichroism in…) from Gd(0001) showed a small, but significant splitting of the surface state. This splitting can be reproduced by numerical one-step calculations based on spin-dependent surface barriers. The photon energy-dependent intensities as well as the magnetic dichroism could be explained qualitatively by calculations within an atomic model giving evidence of a Fano-type resonance.

Linear dichroism in the angular distribution of photoelectrons from oriented molecules

This paper presents a theoretical treatment of Linear Dichroism in the Angular Distribution of photoelectrons (LDAD), that is a difference between photoelectron currents ejected at a definite angle by photon beams of two different linear polarizations rotated by 90° relative to each other. Apart from a trivial term existing already in the cases of unpolarized atoms and unoriented molecules, for oriented molecules additional terms appear in the electric dipole approximation. Their measurement can give additional information on the molecular structure complementing measurements of the circular dichroism in the angular distribution investigated earlier. As an example, LDAD in a model case of an oriented diatomic molecule is investigated.

Magnetic interface formation at Fe/Cr/Fe(100)

Photoemission magnetic dichroism of Fe and Cr 3p core levels was employed to investigate the magnetic order at the Fe/Cr/Fe(100) interface. For submonolayer and monolayer Fe coverages on Cr(100) the interface system appears magnetically frustrated, with no net magnetization along the parallel (antiparallel) direction of the substrate magnetization. The analysis of the Fe 3p photoemission dichroism allows to conclude that, at these low coverages, Fe is magnetically ordered but along an axis lying at 90° from the substrate Fe(100) magnetization, favoring the interpretation of biquadratic coupling between the ultrathin iron overlayer and the bulk iron substrate, across the Cr interlayer.