Reinhard Denecke

Surface structure of MBE-grown α-Fe2O3(0001) by intermediate-energy X-ray photoelectron diffraction

Abstract We have used intermediate-energy X-ray photoelectron diffraction to determine the surface structure of epitaxial α-Fe 2 O 3 (0001) grown on α-Al 2 O 3 (0001). Comparison of experiment with quantum mechanical scattering theory reveals that the surface is Fe-terminated, and that the first four layer spacings are −41, +18, −8, and 47% of the associated bulk values, respectively. These results agree reasonably well with the predictions of molecular mechanics and spin-density functional theory previously reported in the literature for the Fe-terminated surface. However, we find no evidence for an O-terminated surface predicted to be stable by spin-density functional theory.

Dichroism in angular resolved XPS from gadolinium core-levels

This work reports on measurements of the magnetic dichroism in angular resolved photoemission from the Gd 4l (l5s, p,d, f ) core-levels of in-plane magnetised Gd films excited with linearly and circularly polarised, as well as unpolarised, light. High resolution spectra were taken by means of the advanced photoelectron spectrometer-diffractometer end-station at the Advanced Light Source in Berkeley. The dichroism measurements were performed by physically rotating the sample azimuth to change the direction of the magnetisation axis relative to the incident light. Strong asymmetries were observed, even when exciting the spectra with unpolarised light. This paper demonstrates how magnetic dichroism utilising unpolarised light easily gives information about the relative orientation of the magnetisation with respect to the plane of photon incidence.

Magnetic dichroism in core-level photoemission from Gd(0001)

This work reports on magnetic dichroism in photoemission from the Gd 4l(l=s,p,d,f ) core level of in-plane magnetized Gd films excited with linearly, circularly, and unpolarized light. Thin Gd films of about 100 ML thickness were evaporated onto a W(110) substrate at room temperature, and subsequently annealed to 700 K to form an ordered layer exhibiting a (0001) surface. The dichroism measurements were performed by physically rotating the sample azimuth so as to change the direction of the magnetization axis relative to the incident light. The magnetic dichroism asymmetries were obtained by calculating the normalized difference between spectra obtained with the two mutually orthogonal orientations of the magnetization. Strong asymmetries were observed, even if exciting the spectra by means of unpolarized light. The experimental results are compared to three-step photoemission cluster calculations based on a relativistic full potential algorithm respecting magnetic exchange.

X-ray photoelectron diffraction study of hexagonal GaN(0001) thin films

We report on the first scanned-angle X-ray photoelectron diffraction measurements on GaN(0001) in the wurtzite structure, as grown on sapphire substrates using LPCVD. These as-grown samples reveal forward scattering peaks in agreement with a theoretical calculation using a multiple scattering cluster calculation. From the combination of experiment and theoretical calculation and from a simple intensity ratio argument the surface polarity for these samples could be determined to be N. The surface contamination by O and C does not exhibit any clear structure. The data also indicate that C is on average closer to the GaN surface than O.

Study of the oxidation of W(110) by full-solid-angle photoelectron diffraction with chemical state and time resolution

The brightness of third-generation synchrotron radiation from beamline 9.3.2 at the Advanced Light Source has been combined with the high-intensities and energy resolutions possible with its advanced photoelectron spectrometer/diffractometer experimental station in order to study the time dependence of the oxidation of the W(110) surface. This has been done via chemical-state-resolved core-level photoelectron spectroscopy and diffraction. This system has been studied previously by other methods such as LEED and STM, but several questions remain as to the basic kinetics of oxidation and the precise adsorption structures involved. By studying the decay and growth with time of various peaks in the W 4f{sub 7/2} photoelectron spectra, it should be possible to draw quantitative conclusions concerning the reaction kinetics involved. The authors have also measured full-solid-angle photoelectron diffraction patterns for the two oxygen-induced W states, and these should permit fully defining the different structures involved in this oxidation process.