No abstract

The second development step of the SMART project, i.e. an energy-filtered but not yet corrected photoelectron emission microscope, operates at the undulator U49/1-PGM beamline at BESSY II. It already demonstrates the variety of methods of the final version: microscopy, spectroscopy and electron diffraction. Some recent experimental results are reported for these three operation modes. In addition, the theoretical improvement of lateral resolution and transmission of PEEMs in general by using an energy filter is discussed for systems without and with aberration correction.

The growth of the first ten layers of organic thin films on a smooth metallic substrate has been investigated in real-time using the model system PTCDA on Ag(111). The complex behaviour is comprehensively studied by electron microscopy, spectroscopy and diffraction in a combined PEEM/LEEM instrument revealing several new phenomena and yielding a consistent picture of this layer growth. PTCDA grows above room temperature in a Stranski-Krastanov mode, forming three-dimensional islands on a stable bi-layer, in competition with metastable 3rd and 4th layers. Around room temperature this growth mode changes into a quasi layer-by-layer growth, while at temperatures below about 250 K a Vollmer-Weber-like behaviour is observed. By means of laterally resolved soft X-ray absorption spectroscopy the orientation of all adsorbed molecules is found to be homogeneously flat lying on the surface, even during the growth process. The films grow epitaxially, showing long-range order with rotational domains. For the monolayer these domains could be directly analysed, showing an average size of several micrometers extending over substrate steps.

Over the last few years it has been shown that information about the bulk band structure can be obtained by resonant inelastic X -ray scattering (RIXS) in the soft X -ray regime. However, conceptual differences exist about the underlying pictures involving keywords such as "resonant Xray emission" processes, "electronic resonant Raman scattering", and "core exciton formation". To understand and clarify the applicability of the different concepts, we have studied the properties of a new class of materials -beryllium chalcogenides (i.e., BeS, BeSe, and BeTe) -with soft X-ray K α emission following a resonant Be 1s core hole formation. Be chalcogenides constitute interesting candidates for several electronic applications of II-VIsemiconductors, e.g., in ZnSe-based blue light-emitting devices or Mn-containing spin aligner systems. We will show that even for such large band gap materials with an indirect gap, the observed spectral variations can be nicely correlated to the band structure derived from accompanying LDA-, EXX-, and GW-calculations taking the involved matrix elements into account. Furthermore, we observe Raman-like inelastic scattering features below the absorption threshold. Finally, "core excitonic" features above the valence band maximum can be observed for sufficiently high excitation energies. In the case of BeS, we will demonstrate that such information can even be obtained from insulating powder samples. The obtained results will be discussed in correlation with the above mentioned theoretical band structures, in view of the different conceptual approaches, and with a discussion of the underlying physical processes contributing to a loss of k-conservation and the appearance of a major non-coherent fraction in the observed spectra.The experiments were performed at beamline 8.0

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