Silver's unique ability to selectively oxidize ethylene to ethylene oxide under an oxygen atmosphere has long been known. Today it is the foundation of ethylene oxide manufacturing. Yet, the mechanism of selective epoxide production is unknown. Here we use a combination of UHV and in situ experimental methods along with theory to show that the only species that has been shown to produce ethylene oxide, the so-called electrophilic oxygen appearing at 530.2 eV in the O 1s spectrum, is the oxygen in adsorbed SO4 (SO4,ad). This adsorbate is part of a 2D Ag/SO4 phase, where the nonstoichiometric surface variant, with a formally S(V+) species, facilitates selective transfer of an oxygen atom to ethylene. Our results demonstrate the significant and surprising impact of a trace impurity on a well-studied heterogeneously catalyzed reaction.

SummaryThe implementation of imaging techniques with low-energy electrons at synchrotron laboratories allowed for significant advancement in the field of spectromicroscopy. The spectroscopic photoemission and low energy electron microscope, SPELEEM, is a notable example. We summarize the multitechnique capabilities of the SPELEEM instrument, reporting on the instrumental aspects and the latest developments on the technical side. We briefly review applications, which are grouped into two main scientific fields. The first one covers different aspects of graphene physics. In particular, we highlight the recent work on graphene/Ir(100). Here, SPELEEM was employed to monitor the changes in the electronic structure that occur for different film morphologies and during the intercalation of Au. The Au monolayer, which creeps under graphene from the film edges, efficiently decouples the graphene from the substrate lowering the Dirac energy from 0.42 eV to 0.1 eV. The second field combines magnetism studies at the mesoscopic length scale with self-organized systems featuring ordered nanostructures. This example highlights the possibility to monitor growth processes in real time and combine chemical characterization with X-ray magnetic circular dichroism–photoemission electron microscopy (XMCD–PEEM) magnetic imaging by using the variable photon polarization and energy available at the synchrotron source.

We study the interplay between competitive substrate–C interaction processes occurring during chemical vapour deposition (CVD) of ethylene on Re (0 0 0 1). At T < 500 K dissociative ethylene adsorption leads to the formation of a dimer species, producing an ordered (4×2) structure. In the range 500–700 K, the formation of a high-quality single-layer of graphene is strongly opposed by the formation of a surface carbide characterised by C trimer units, and, at higher temperatures, by carbon dissolution into the bulk. Our experimental and theoretical results demonstrate that, under UHV conditions, the formation of a long-range ordered graphene layer on Re (0 0 0 1) without carbon bulk saturation is confined to a narrow window of growth parameters: substrate temperature, hydrocarbon gas pressure and exposure time. Our combined experimental and theoretical approach allowed us to validate a concept which had already been anticipated in some earlier works on Rh, Fe and Ni, namely that the epitaxial growth of graphene is not necessarily restricted to surfaces where carburisation is precluded, but could take place, under given appropriate conditions, also on other metallic substrates exhibiting a strong C–substrate interaction

We demonstrate for the first time that seeded harmonic generation on electron storage rings can produce coherent optical pulses in the vacuum ultraviolet spectral range. The experiment is performed at Elettra, where coherent pulses are generated at 132 nm, with a duration of about 100 fs. The light source has a repetition rate of 1 kHz and adjustable polarization; it is very bright, with a peak power several orders of magnitude above that of spontaneous synchrotron radiation. Owing to high stability, the source is used in a test photoemission electron microscopy experiment. We anticipate that seeded harmonic generation on storage rings can lead to unprecedented developments in time-resolved femtosecond spectroscopy and microscopy.

The switchable bipolar ground state is at the heart of research into ferroelectrics for future, low-energy electronics. Polarization switching by an applied field is a complex phenomenon which depends on the initial domain ordering, defect concentration, electrical boundary conditions and charge screening. Injected free charge may also to be used to reversibly switch in-plane polarized domains. We show that the interaction between the initial domain order and the bulk screening provided by very low energy electrons switches the polarization without the collateral radiation damage which occurs when employing a beam of high energy electrons. Polarization switching during charge injection adds a new dimension to the multifunctionality of ferroelectric oxides.