A first principles study of the adsorption and dissociation of CO<Subscript>2</Subscript> on the δ-Pu (111) surface

A complete understanding of the nature of the 5f electrons has been and continues to be a major scientific problem in condensed matter physics. Bulk and surface electronic structure studies of the actinides as also atomic and molecular adsorptions on the actinide surfaces provide a path towards this understanding. In this work, ab initio calculations within the framework of density functional theory have been used to study the adsorption of molecular CO₂ and the corresponding partially dissociated (CO + O) and completely dissociated (C + O + O) products on the δ-Pu (111) surface. The completely dissociated C + O + O configurations exhibit the strongest binding with the surface (7.92 eV), followed by partially dissociated products CO + O (5.08 eV), with molecular CO₂ adsorption having the lowest binding energies (2.35 eV). For all initial vertically upright orientations, the CO₂ molecule physisorbs or do not bind to the surface and the geometry and orientation do not change. For all initial flat lying orientations chemisorption occurs, with the final state corresponding to a bent CO₂ molecule with bond angles of 117°–130° and the elongation of the CO bond. For CO + O co-adsorption, the stable configurations corresponded to CO dipole moment orientations of 100°–172° with respect to the surface normal and the elongation of the CO bond. The most stable chemisorption cases correspond to anomalously large rumpling of the top Pu layer. The interactions of the CO₂ and CO with the Pu surface have been analyzed using the energy density of states and difference charge density distributions. The nature and the behavior of the 5f electrons have also been discussed in detail in the context of this study.     

Scanning tunneling microscopy and spectroscopy of individual C60 molecules on Si(100)-(2 × 1) surfaces

Individual C₆₀ molecules chemisorbed on Si(100)-(2 × 1) surfaces have been studied by scanning tunneling microscopy and spectroscopy. Chemisorption was realized by annealing the samples with room-temperature deposited adsorbates to 600°C. Spectroscopic results on individual adsorbates reveal a transition of their electronic structure from that of a near-free adsorbate to that of SiC, as the adsorbate-substrate interaction increases.     

Mechanism of the oxidation and combustion of normal paraffin hydrocarbons: Transition from C<Subscript>1</Subscript>–C<Subscript>6</Subscript> to C<Subscript>7</Subscript>H<Subscript>16</Subscript>

A previously proposed algorithm for constructing an optimal mechanism of the high- and low-temperature oxidation and combustion of normal paraffin hydrocarbons was used, which includes the major processes that determine the rate of reaction and the formation of the main intermediate and final products. The mechanism has the status of a nonempirical detailed mechanism, since all the constituent elementary reactions have a kinetic substantiation. The mechanism has two specific features: it included no reactions of so-called double addition of oxygen and no isomeric compounds and derivatives thereof as intermediate species. Realization of this algorithm leads to fairly compact models, a circumstance important for studies of chemical processes involving paraffin hydrocarbons C _n with large n. Previously, based on this algorithm, compact mechanisms of oxidation and combustion of propane, n-butane, n-pentane, and n-hexane were constructed. In this paper, we develop a nonempirical detailed mechanism of oxidation and combustion of n-heptane. The most important feature of the new mechanism is its ability to predict the staging of the process in the form of cool and blue flames at low autoignition temperatures. A comparison of the simulation results with the available experimental data is conducted.     

Theoretical investigations of 3d-metal adsorption on the α-AL<Subscript>2</Subscript>O<Subscript>3</Subscript> (0001) surface

Theoretical investigations of adsorption of 3d-metals from Ti to Cu on the α-Al₂O₃ (0001) surface are presented. The influence of adsorbates on the atomic and electronic structure of the aluminum oxide surface is considered. Values of the adsorption energy are calculated, and the equilibrium adatom positions on the surface are determined. A comparative analysis of the properties and mechanisms of 3d-metal interaction with atoms of the substrate is performed.     

Mechanisms of the oxidation and combustion of normal alkanes: Transition from C<Subscript>1</Subscript>–C<Subscript>5</Subscript> to C<Subscript>6</Subscript>H<Subscript>14</Subscript>

A previously proposed algorithm of constructing optimal mechanisms of the low- and high-temperature oxidation and combustion of normal alkanes was applied to n-hexane. The proposed mechanism can be considered a nonempirical detailed mechanism, since all the constituent reactions have a solid kinetic substantiation. The mechanism features two main peculiarities: it contains no reactions of double oxygen addition (first to the peroxide radical and then to its isomerized form) and (2) involves no isomeric compounds and derivatives thereof. Application of the algorithm to n-hexane made it possible to create a new compact kinetic mechanism. The mechanism was demonstrated to correctly describe the multistage character of low-temperature self-ignition: the appearance of a cool and then a blue flame.     

Scanning tunneling microscopy study of molecular growth structures of Gd@C82 on Cu（111）

The coverage and temperature-dependent nucleation behaviors of the Gd@C82 metallofullerenes on Cu(111) have been studied by low-temperature scanning tunneling microscopy (LT-STM) in detail. Upon molecular deposition at low temperature, Gd@C82 molecules preferentially decorate the steps and nucleate into single layer islands with increasing coverage. Further annealing treatment leads some of the Gd@C82 molecules to assemble into bright and dim patches, which are correlated to the adsorption induced substrate reconstruction. Upon sufficient thermal activation, Gd@C82 molecules sink into the Cu(111) surface one-copper-layer-deep, forming hexagonal close-packed molecular islands with intra-molecular details observed as striped patterns. By considering the commensurability between the Gd@C82 nearest-neighbor distance and the lattice of the underlying Cu(111), we clearly identified two kinds of in-plane molecular arrangements as (19(1/2)×19(1/2))R23.4°and (19(1/2)×19(1/2))R36.6°with respect to Cu(111). Within the assembled Gd@C82 molecular, island molecules with dim-bright contrast are spatially distributed, which may be modulated by the preexisted species on Cu(111).     

Si<Subscript>x</Subscript>Ge<Subscript>1-x</Subscript> ultrahigh-vacuum chemical vapor deposition on Si(111)(7×7) from GeH<Subscript>4</Subscript>/Si<Subscript>2</Subscript>H<Subscript>6</Subscript> mixtures

We report the results of a study on ultrahigh-vacuum chemical vapor deposition of Si_xGe_1-x layers on Si(111)(7×7) with GeH₄ and Si₂H₆ mixtures. Using combined scanning tunneling microscopy and X-ray photoelectron spectroscopy, structural properties, the growth kinetics and the composition of the deposited alloys are analyzed as a function of the growth temperature for two different GeH₄:Si₂H₆ mixture ratios. The mutual influence of the precursors is shown by comparing the structures formed during deposition and the sticking coefficients of Si₂H₆ and GeH₄ with results obtained from exposure of Si(111) to the pure gases. Received: 28 July 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-731/502-5452, E-mail: hubert.rauscher@chemie.uni-ulm.de     

<Emphasis Type="Italic">X</Emphasis><Subscript>α</Subscript> discrete variational analysis and <Superscript>1</Superscript>H nuclear magnetic resonance study of the dissociation of water molecules on the surface of TiO and TiO<Subscript>2</Subscript> oxides

The disordered state of hydrogen in titanium oxyhydrate has been investigated by ¹H nuclear magnetic resonance. The protonation of TiO and TiO₂ oxides has been simulated by the X _α discrete variational method. It is established that an increase in the number of protons decreases the stability of coordination polyhedra of titanium atoms, while an increase in the O-H bond length enhances the ability of water molecules to dissociate.     

Low-temperature scanning tunneling microscopy and near-edge X-ray absorption fine structure investigation of epitaxial growth of F<Subscript>16</Subscript>CuPc thin films on graphite

Low-temperature scanning tunneling microscopy (LT-STM) and near-edge X-ray absorption fine structure (NEXAFS) measurements are used to study the epitaxial growth and molecular orientation of organic thin films of copper hexadecafluorophthalocyanine (F₁₆CuPc) on highly oriented pyrolytic graphite (HOPG). Our results show that F₁₆CuPc molecules lie flat on HOPG up to 5 nm thickness, stabilized by interfacial and interlayer π–π interactions. LT-STM experiments reveal the coexistence of two different in-plane orientations of the F₁₆CuPc monolayer on HOPG. On the second layer of F₁₆CuPc on HOPG, however, all F₁₆CuPc molecules possess the same in-plane orientation.     

Dipoles and band alignment for benzene/Au(111) and C<Subscript>60</Subscript>/Au(111) interfaces

A local orbital DFT-approach combined with a “scissor”-operator is used to obtain the Charge Neutrality Level and the screening parameter in the benzene/Au(111) and C₆₀/Au(111) interfaces. The “pillow” dipole and interface Fermi level are also calculated. The total dipole induced across the interface is compared with the experimental evidences: while the agreement for C₆₀/Au(111) is excellent, for benzene/Au(111), some discrepancies appear that are discussed in the light of other models.     

Scanning tunneling microscopy at multiple voltage biases of stable “ring-like” Ag clusters on Si(111)–(7 × 7)

Since more than twenty years it is known that deposition of Ag onto Si(111)–(7 × 7) leads under certain conditions to the formation of so-called “ring-like” clusters, that are particularly stable among small clusters. In order to resolve their still unknown atomic structure, we performed voltage dependent scanning tunneling microscopy (STM) measurements providing interesting information about the electronic properties of clusters which are linked with their atomic structure. Based on a structural model of Au cluster on Si(111)–(7 × 7) and our STM images, we propose an atomic arrangement for the two most stable Ag “ring-like” clusters.     

Effect of pressure on the interlayer transport and the electronic structure in the organic quasi-two-dimensional bilayer metal θ-(BETS)<Subscript>4</Subscript>HgBr<Subscript>4</Subscript>(C<Subscript>6</Subscript>H<Subscript>5</Subscript>Cl)

The behavior of the interlayer resistance and the magnetoresistance in the organic quasi-twodimensional bilayer metal θ-(BETS)₄HgBr₄(C₆H₅Cl) is studied at normal pressure and a hydrostatic pressure of 10 kbar. The interlayer transport under atmospheric pressure is found to occur in an incoherent mode. The applied pressure does not change the electronic structure of the conducting layers and causes a transition to a weakly coherent mode at low temperatures.     

Vacancy creation on the Si(111)−7 × 7 surface due to sulfur desorption studied by scanning tunneling microscopy

Ultra high vacuum scanning tunneling microscopy (STM) has been used to study the adsorption and subsequent thermal desorption of a sulfur overlayer deposited in situ, at room temperature, on the Si(111)-7 × 7 surface. Little ordering is visible in the STM images of this overlayer 1–2 monolayers thick, with the underlying silicon surface retaining the (7 × 7) reconstruction with weakened low energy electron diffraction (LEED) spot intensity. STM images of this surface following a thermal anneal at 375°C revealed the presence of a number of monolayer deep “holes” or voids in the (7 × 7) surface. The appearance of these voids is consistent with a coalescence of vacancy defects induced by the sulfur desorption process as also observed for oxygen induced etching of the silicon surface. In addition, we have observed small regions of the (√3 × √3)R30° and c(4 × 2) reconstructions within areas exhibiting a high degree of surface disorder, with the metastable (5 × 5) reconstruction also being found. Both reconstructions and disorder are attributed to vacancy diffusion to step edges causing a redistribution of surface silicon atoms.     

Effect of γ irradiation on the thermochromic phase transition in [(C<Subscript>2</Subscript>H<Subscript>5</Subscript>)<Subscript>2</Subscript>NH<Subscript>2</Subscript>]<Subscript>2</Subscript>CuCl<Subscript>4</Subscript> crystals (as derived from heat capacity measurements)

The heat capacity of [(C₂H₅)₂NH₂]₂CuCl₄ crystals, both nonirradiated and γ-irradiated to a dose of 10⁷R, was studied in the temperature interval 90–330 K by adiabatic calorimetry. The temperature dependence of C_p(T) was found to have a peak-shaped anomaly in the region of the thermochromic phase transition (PT) at T = 322.7 K. Smoothened experimental heat capacity data were used to calculate the changes in the thermodynamic functions. The changes in the entropy and enthalpy of the thermochromic PT were determined to be ΔS = 42 J K^?1 mol^?1 and ΔH = 13653 J mol^?1 for the nonirradiated crystals and ΔS = 39 J K^?1 mol^?1 and ΔH = 12120 J mol^?1 for the irradiated crystals, respectively. Irradiation of a [(C₂H₅)₂NH₂]₂CuCl₄ crystal by γ rays to a dose of 10⁷ R was shown to shift the PT point toward lower temperatures by ΔT ≈ 1.7 K.     

Crossing of the πd<Subscript>5/2</Subscript> and πg<Subscript>7/2</Subscript> orbitals in the <Subscript>51</Subscript>Sb isotopes

Self-consistent calculations using the D1S Gogny force have been performed in order to study the mechanism involved in the crossing of the πd _5/2 and πg _7/2 orbitals in the Sb isotopes. This inversion is well predicted by the HFB + blocking calculations with spherical symmetry performed for the odd-A Sb isotopes. In addition, several HFB and HF calculations have been performed for even-even nuclei of the five neighbouring isotopic chains (Z = 46 to 54, from the proton dripline to N = 82). The results obtained for the binding energies of the two proton orbitals indicate that the radii of the systems play an important role in the crossing, even though some particular πν interactions also give a contribution. The spin-orbit interaction, which is known to be concentrated mainly at the nuclear surface, is proposed to be the main responsible of the crossing.