Shear instability of gamma-Fe in bulk and in ultrathin films

Distinct Diffusion and Recombination Dynamics of Atomic Carbon and Oxygen on Face-Centered Cubic and Body-Centered Cubic Iron Multilayers

This study investigates the distinctly different dynamics of atomic carbon and oxygen diffusion, both on the surface and into the bulk of iron multilayer films with face-centered cubic (FCC) (100) and body-centered cubic (BCC) (110) structures, and how these processes impact the recombination behavior of carbon and oxygen, particularly at elevated temperatures. On FCC-iron (γ-iron), CO dissociation occurs around 300 K, leading to the formation of segregated carbide and oxide islands on the surface upon annealing. Above the onset temperature of 600 K, mobile oxygen atoms diffuse to the edge of the carbide islands, where they combine with carbon to form CO. In contrast, on BCC (α-iron) surfaces, a disordered, atomically mixed carbide-oxide phase forms upon CO dissociation. Carbon does not remain on the surface but migrates to the subsurface during heating, leaving oxygen on the surface. Carbon remains predominantly subsurface following CO dissociation, enabling a direct recombination pathway between subsurface carbon and surface oxygen. This subsurface activity requires lower activation, resulting in CO recombination and then desorption at lower temperatures compared to the FCC system. These distinct pathways observed on γ-FCC and α-BCC iron surfaces have significant implications for materials science, metallurgy, and catalysis, highlighting the critical role of thermodynamic and kinetic factors in governing atomic diffusion and recombination processes.

Ferromagnetic Fe on Cu(001) throughout the fcc-like phase: arguing from the viewpoint of the electronic structure

The scientific enthusiasm for ultrathin Fe films on Cu(001) has now lasted for more than 20 years. Is there ferromagnetic iron with a face-centred cubic (fcc) structure? Does ferromagnetism in Fe hinge on the body-centred cubic (bcc) structure? In this contribution, we try to establish that the electron system gives evidence of ferromagnetic behaviour with fcc-like electronic bands. We examine a crystal-induced surface state, which is characteristic of fcc surface order. Furthermore, we compare electronic signatures of fcc and bcc: the d-band exchange splitting, image-potential-state energies and the work function. We conclude that, from the viewpoint of the electronic structure, Fe on Cu(001) is found to be ferromagnetic throughout the fcc-like phase. This result raises a new question: how much deviation from the relaxed fcc order is acceptable without losing the electronic signature of fcc?

Multiple photon excitation and ionization of NO in and on helium droplets

The photoexcitation of NO embedded in superfluid Hen nanodroplets having n approximately 10(4) has been examined. Two-photon excitation prepares electronically excited states (NO(*)), most notably, the embedded analog of the A 2Sigma state of gas phase NO. Vertical excitation to this low Rydberg state is blueshifted and broadened relative to its gas phase counterpart because of the repulsive electron-helium interaction. Transport to the droplet surface is believed to be facile in the superfluid. For example, NO* prefers (energetically) to reside at the droplet surface rather than at the droplet center, in contrast to NO. Photoionization of surface-bound NO* occurs over a significant photon energy range. This yields small cluster ions NO+Hek) with approximately 90% of these clusters having k< or =10. The variation of ion yield with photon energy displays a precipitous change in the region of 24 300-24 400 cm(-1) for all values of k. Possible photoionization mechanisms are discussed and it is suggested that intermediate levels with high-n Rydberg character play a role. This work underscores the important role played by transport in the photophysics of species embedded in the superfluid host.

Surface structure of ultrathin Fe films on Cu(001) revisited

The structure and magnetism of ultrathin Fe films epitaxially grown on a Cu(001) surface are investigated by grazing scattering of fast H and He atoms or ions. By making use of a new variant of ion beam triangulation based on the detection of the number of emitted electrons, we obtain direct information on the structure of the film surface. We observe for room temperature growth a dominant and defined fcc-like structure. Complex surface reconstructions as reported in recent STM and LEED studies are observed only for cooling and H2 dosing.

Dewetting near the glass transition: transition from a capillary force dominated to a dissipation dominated regime

Dynamics and corresponding morphology of dewetting of thin polystyrene films at temperatures close to the glass transition were investigated by measuring simultaneously dewetted distance and width of the rim. Comparing the opening of cylindrical holes with the retraction of a straight contact line revealed (i). a drastic influence of the geometry (planar or radial symmetry) on the dynamics at early stages, (ii). a new logarithmic dewetting regime, and (iii). transitions between four dewetting regimes clearly indicated by changes in the shape of the rim. The complete dewetting scenario can be understood as an initial dominance of capillary driving forces, which is progressively overtaken by dissipation related to the increasing size of the rim.