Ch. Ostertag

The influence of a linear photon polarisation light on measurements of the circular dichroism in photoemission

Abstract We measured the CDAD from NO on Pt(111) at the SX 700- III monochromator at the synchrotron BESSY in Berlin. We found high asymmetries up to 60% using a photon energy of 40 eV. Surprisingly, this is higher than the degree of circular polarisation that should be between 30% and 50% for the experimental parameters used. This obvious contradiction can be explained if an additional dichroism arising from a strongly linear polarised component of the radiation at the SX 700- III is taken into account. The polarisation of this component has the orientation of ±45° with respect to the synchrotron plane. This linear polarisation changes its sign if the circular polarisation is changed, too. First experiments on this linear dichroism show large asymmetries for CO Pt (111) , comparable to the CDAD asymmetries.

Circular dichroism in angular resolved photoemission from pure and Rb-doped C60 and C22H14 layers on platinum and tungsten

Polycrystalline C60 and Pentacene films grown on W(110) and Pt(111) have been studied in valence band photoemission using circularly polarised synchrotron radiation from BESSY with special emphasis on circular dichroism in photoemission. For thin films of C60, dichroic asymmetries of about 10% occur independent of the temperature and the substrate hinting that the rotation of the topmost layer is hindered even at room temperature. For Pentacene we found asymmetries up to 50% in the region of the σ-electrons. Moreover, we found for this molecule a dichroic asymmetry in normal emission, that is a forbidden geometry. This hints on adsorption with the molecules perpendicularly oriented at the surface. Doping the films with rubidium causes a filling of unoccupied electronic states in both molecules. In Pentacene all valence level binding energies are shifted depending on the amount of exposed Rb. For the Rb-doped C60 and C22H14 films a dichroism in emission from the Rb 4p signal occurs that is much stronger than in the case of pure, disordered Rb films at Pt(111) where the asymmetry almost vanishes. This hints on a high correlation of the Rb p-electrons in both materials that may not be neglected in investigations of the electronic structure.

Circular dichroism in photoemission from Xe and Kr on Pt(111)

We investigated circular dichroism in photoemission from Kr and Xe adsorbed on Pt(111). A large CDAD has been observed in emission from the p states of both rare gases depending on the observation angle and the photon energy. Special interest was put on the CDAD in emission from the near-threshold region. Strong CDAD was found in constant final state mode spectra for very low kinetic energies of the photoelectrons. Our results show, that in a completed monolayer the Xe 5p states cannot be treated as atom-like.

Magnetic circular and linear dichroism in VUV-photoemission from thin iron films on W(110)

Abstract We report on two different photoemission techniques for ferromagnetic surfaces: the phenomena of magnetic circular and linear dichroism in the angular distribution of photoelectrons MCDAD and MLDAD. MCDAD had already been studied in detail for different systems. Now we are able to compare our MCDAD results from thin iron films on W(110) to new data from magnetic linear dichroism MLDAD. Both effects (MCDAD and MLDAD) have been studied in the identical photon energy region (up to hv = 36 eV). The results will be discussed with respect to a bandstructure calculation.

Investigation of alkali metals and noble gases adsorbed on transition metals using Circular Dichroism in Angular resolved photoemission

Abstract We investigated the adsorption of alkali metals an noble gases on transition metal surfaces. These serve as atom-like adsorbates to be comparable to calculations of the Circular Dichroism in the Angular Distribution of photoelectrons (CDAD) including backscattering at the substrate surface and photoelectron diffraction within the adsorbate layer. Our calculations show the influence of scattering and diffraction, that even can generate a dichroism where none should be. For the case of the 4p 1 2 - subshell of Rb our calculations agree well with our measurements. For the 4p 3 2 - subshell we found a different behaviour due to a polarisation of the state in the adsorbed layer. We also present first results for CDAD from noble gases. The 5p - states from Xe show less atomic-like behaviour, because they are much more weakly bound and form bands.

Circular dichroism in X-ray photoemission from Pd(111) and CO/Pd(111)

It is shown experimentally that in the soft X-ray region a large circular dichroism in the photoelectron angular distribution (CDAD) exists for both valence orbitals and core levels of CO molecules adsorbed on Pd(111). From theoretical consideration it follows that in the case of a spherically symmetric ground state wave function, like the 1σ and 2σ orbitals of CO, CDAD appears due to the lack of spherical symmetry in the final state. For carbonK-shell experimental results are compared to model calculations. Investigations at the Pd core levels proved that CDAD does also arise in X-ray photo-emission from non-magnetic crystals.

DICHROISM IN ANGLE-RESOLVED PHOTOEMISSION FROM Pt(111)

The angular dependence of the circular dichroism in photoemission from Pt(111) was investigated for excitation with VUV and soft X-ray radiation. VUV excitation was used to probe band structure and the circular dichroism for valence band emission. The measurements are compared to full relativistic single step photoemission calculations. XPS was used to investigate the circular dichroism in emission from the 4f core level. In this case, the dichroism is induced by photoelectron diffraction. First results from single step core level calculations are compared to the experimental observations.

Oxygen adsorbed on rare earth surfaces

Abstract The adsorption of oxygen on the surfaces of the rare earth metals Gd, Nd, and Tb was investigated using VUV photoelectron spectroscopy. As substrates served thin films showing the (0001) surfaces of crystals. This study utilized the technique of circular dichroism in the angular distribution of photoelectrons CDAD. The CDAD asymmetry, determined by the normalized difference of the photoelectron intensities for right and left circularly polarized radiation, is reduced with increasing oxygen exposure. The behavior of the asymmetry of the O 2p induced structure is rather similar for oxygen on neodymium and terbium. On gadolinium, in contrast, the observed feature is different including a change of sign. This observation points to different electronic properties of the adsorbate overlayers.

Oxidation of rubidium at platinum (111)

Abstract We have studied the adsorption of Rb and the co-adsorption of O 2 and Rb at Pt(111) using LEED, AES, EELS, ARUPS excited by circularly polarised light (CDAD) and workfunction measurements. At T = 300 K different ordered structures occur as indicated by LEED. The pure Rb monolayer exhibits a (7 × 7) structure. Even at very low oxygen exposure ( L ) this structure is changed to a (√3× √3) R 30° structure with the same lattice spacing within 1%. At high oxygen doses the adlayer becomes disordered. A (4 × 1) structure is observed after an additional heat treatment. Simultaneously, different stages of local order can be distinguished clearly by use of CD AD from the Rb4p states. At T = 90 K disordered Rb adlayers were produced that show an almost vanishing CDAD. For high Rb coverage EELS and ΔΦ measurements indicate that the suboxide Rb 9 O 2 occurs during the early stage of oxidation.

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.