New surfaces stabilized by adsorbate-induced faceting

Faceting is a form of self-assembly of single-crystal surfaces at the nanometer-scale in which an initially planar surface converts to a 'hill-and-valley' structure, exposing new faces of low-index surfaces. Our recent studies revealed that, upon annealing in O(2), three-sided nanoscale pyramids form on Ir(210) exposing smooth {311} and partially restructured (110) faces. Through a combination of scanning tunneling microscopy and density functional theory, we identify this structure to be a stepped double-missing-row reconstruction, which is only stable on nanopyramidal facets, not on a planar Ir(110) surface. This faceting-enabled stabilization of a hitherto unstable surface points to a new approach to prepare nanoscale model catalysts for structure-sensitivity studies in heterogeneous (electro-)catalysis with high selectivity and reactivity.     

Simulation of adsorbate-induced faceting on curved surfaces

A simple solid-on-solid model, proposed earlier to describe overlayer-induced faceting of bcc(1 1 1) surface, is applied to faceting of curved surfaces covered by an adsorbate monolayer. Surfaces studied in this paper are formed by a part of sphere around the [1 1 1] pole. Results of Monte Carlo simulations show that the morphology of a faceted surface depends on the annealing temperature. At an initial stage the surface around the [1 1 1] pole consists of 3-sided pyramids and step-like facets, then step-like facets dominate and their number decreases with temperature, finally a single big pyramid is formed. It is shown that there is a reversible phase transition at which a faceted surface transforms to an almost spherical one. It is found that the temperature of this phase transition is an increasing function of the surface curvature. Simulation results show that measurements of high temperature properties performed directly and after fast cooling down to a low temperature lead to different results.     

Bremsstrahlung-induced Auger peaks

For highly-polar nitro-substituted aromatic compounds, characterization of the nitro-N 1s core-hole states from the standpoint of intramolecular charge-transfer (CT) interaction is obtained through analysis of the energy difference between the satellite and the main peak, and the relative intensity of the satellite compared to the main peak.Two-dimensional configuration interaction (CI) is considered between the no-bond (covalent) and the CT (ionic) structures in both the initial and the final states. The off-diagonal element between these two structures is given in terms of the core resonance integral between the ring carbon and the nitro nitrogen, and can therefore be reduced by steric hindrance, and is also closely related to the π-electron density at the adjacent carbon in the donor orbital. The absolute magnitude of the off-diagonal element and the ionization potential of the donor are important factors determining the relative intensity of the CT satellite.It was found that in the case of some nitro-N 1s core-hole ions the CT structure is located below the no-bond structure, and that two-dimensional CI calculations are valid as far as the ground and the CT states are concerned.     

Auger spectrum of ammonia

The Auger spectrum of ammonia has been measured in the vapor phase, completing the isoelectronic series Ne, HF, H₂O, NH₃, and CH₄. Eleven transitions are possible, but three leading to triplet final states are expected to be very weak. Two others are expected on theoretical grounds to be unresolvable. The spectrum showed seven resolvable peaks, in keeping with these expectations. The peaks range in width from 2.2 to 7.4 eV, the narrowest being associated with vacancies in non-bonding lone-pairs and the broadest with vacancies in bonding orbitals. There is reasonably good agreement between the experimental results and recent theoretical calculations for both energy and intensity.     

Large photonic band gaps and strong attenuations of two-segment-connected Peano derivative networks

In this Letter, based on ancient Peano curves we construct four kinds of interesting Peano derivative networks composed of one-dimensional (1D) waveguides and investigate the optical transmission spectra and photonic attenuation behavior of electromagnetic (EM) waves in one- and two-segment-connected networks. It is found that for some two-segment-connected networks large photonic band gaps (PBGs) can be created and the widths of large PBGs can be controlled by adjusting the matching ratio of waveguide length and are insensitive to generation number. Diamond- and hexagon-Peano networks are good selectable structures for the designing of optical devices with large PBG(s) and strong attenuation(s).     

Auger photoelectron coincidence spectroscopy

Auger photoelectron coincidence spectroscopy (APECS) is a technique that provides us with unique information and a chance to gain insight into the significance of processes in the Auger spectra of atoms in solids. Hence it is a great aid in our understanding of the Auger process in atoms where electron correlations are strong. Despite the first demonstration of the technique more than 20 years ago, there are still very few working experiments. The reasons why, and the ways forward are discussed.     

Auger recombination of electron-hole drops

Phonon-assisted Auger recombination is calculated for indirect band gap semiconductors in the strongly degenerate case. It follows a reciprocal lifetime τ^?1=Cn² with C=7.19×10³¹ cm⁶ sec^?1 for Si and C=2.94× 10^?31 cm⁶ sec^?1 for Ge. These results are in good agreement with experimental values of the decay of electron-hole drops. Therefore one can conclude that phonon-assisted Auger recombination is the essential nonradiative recombination process in this case.     

Auger spectra of lithium hydride

Auger electron emission spectra have been observed for lithium hydride in three conditions : (1) cleaved in vacuum, (2) prepared by the reaction of hydrogen gas with clean lithium metal, and (3) by annealing slightly oxidized lithium hydride single crystals in vacuum. The dominant Auger line (40 ± 1 eV) was found to be a KVV transition involving valence electrons from the anion and was indistinguishable from a similar transition for lithium oxide at room temperature. Lithium hydride surfaces lose hydrogen in vacuum causing the formation of a lithium metal phase at room temperature and a significant reduction in surface hydride stoichiometry at 600°C.     

Rule of corresponding Auger spectra

The normalized core-valence-valence Auger spectra A(?) of the simple metals Be, Mg, and A$\text{l}$, when plotted as a function of ?/2λ, where λ is the free-electron Fermi energy, are all nearly the same. Li, with λ=4.1±0.1 eV, also has the same spectrum. This suggests that the Auger spectra of simple metals depend primarily on the electron gas density and are almost independent of details of the band structure.     

The Auger spectra of uranium

TheL, M, andK-Auger andL-Coster-Kronig spectra of uranium were measured and are compared to other experimental and theoretical work. The meanL-shell fluorescence yield of uranium was measured as \(\bar \omega _L \) =0.53±0.06.      

The effect of adsorbate-induced surface reconstruction on the apparent arrhenius parameters for desorption

The surface reconstruction described in terms of the second-order phase transition theory, is shown to result in a decrease in the apparent pre-exponential factor for desorption in comparison with the value predicted on the basis of the standard transition state theory provided that adsorbed particles suppress the phase transition. If adsorbed particles facilitate the phase transition, the reconstruction leads to an increase in the pre-exponential factor. The effect may be strong (of the order of 10^±4. The pre-exponential factors for CO desorption from the Ni(111), Ru(001) and Ir(110) surfaces are compared and discussed. The low value of the desorption pre-exponential factor for the latter surface is explained by the surface reconstruction.     

Anomalous PL brightening caused by partial Auger recombination of a (D, X) complex during an impact ionization avalanche in n-GaAs

Partial Auger recombination of the (D⁰, X) bound-exciton in n-GaAs at 4.2 K during an impact ionization avalanche under an applied pulse voltage has been investigated. The bright photoluminescence (PL)-pattern observed by applying the pulse voltage, characterizes well the formation of a current density filament. The observation of the bright PL spectra inside the current density filament gives a quite new result concerning the partial Auger recombination process of the (D⁰, X) complex, leaving the neutral donor in the excited states.