K. Weiss

Rydberg transitions in X-ray absorption spectroscopy of alkanes: The importance of matrix effects

Precise ab initio electronic structure calculations on isolated alkanes (propane, butane) and on propane embedded in a matrix of several propane molecules were used in order to characterize the near-edge x-ray absorption fine structure (NEXAFS) resonances of condensed saturated hydrocarbons. The results demonstrate that upon condensation the dominating NEXAFS spectral features, i.e., the Rydberg resonances located between 287.4 and 288.1 eV, have a significant blue shift from the corresponding transition energies of an isolated alkane molecule. Furthermore, the theoretical results confirm the excitonic character of the final state. Additional calculations carried out for different conformations (trans/gauche) of isolated butane demonstrate that the transition energies of the corresponding NEXAFS resonances do not differ significantly. Previously observed changes in the electronic structure of disordered thin organic films of calciumarachidate are thus attributed to changes in the intermolecular spacing of...

Near edge x-ray absorption fine structure study of benzene adsorbed on metal surfaces: Comparison to benzene cluster complexes

Near edge x-ray absorption fine structure (NEXAFS) spectroscopy has been used to study benzene adsorbed on several close-packed metal surfaces [Au(111), Rh(111), Pt(111), and Ru(0001)] under the same experimental conditions. The NEXAFS spectra reveal significant differences with regard to the shapes and positions of resonances and the dichroism observed for the resonance intensities. These differences correlate directly with the binding energy and can be attributed to differences in the electronic coupling of the benzene molecule to the substrate and to adsorption in- duced changes in the molecular structure of the benzene molecule. A comparison to the NEXAFS spectra recorded for the benzene metal cluster complexes [Ru(η6–C6H6)(CH3CN)3]2+ and Ru3(CO)9(μ3:η2:η2:η2–C6H6) indicates that the μ3-arene complex, where the benzene ring is bonded to three metal atoms, can be regarded as a reasonable cluster analog for the adsorption of benzene on the Ru(0001) transition metal surface.

Template-mediated synthesis of polycyclic aromatic hydrocarbons: Cyclodehydrogenation and planarization of a hexaphenylbenzene derivative at a copper surface

The deposition of polycyclic aromatic hydrocarbons on substrate surfaces is a key step for using such disklike molecules in nanoelectronic devices. An alternative to the (frequently problematic) preparation by evaporation or deposition from solution is to use the substrate surface as a template for the planarization that accompanies the cyclodehydrogenation (see picture).

Pyridine adsorption on the polar ZnO(0001) surface: Zn termination versus O termination

The interaction of pyridine (C5H5N) with the two differently terminated ZnO(0001) surfaces has been investigated using thermal desorption spectroscopy (TDS), x-ray photoelectron spectroscopy (XPS), and x-ray absorption spectroscopy (NEXAFS). The binding energy of pyridine on the O-terminated ZnO(000-1) surface amounts to 57 kJ/mol and the spectroscopic data reveal only small modifications of the pyridine electronic structure, indicating the presence of a physisorbed species. On the Zn-terminated ZnO(0001) surface, the binding energy is substantially higher, 112 kJ/mol, and the N1s NEXAFS data for the pyridine π* resonance shows a shift of 0.6 eV toward higher binding energies. This observation indicates a substantial interaction between the nitrogen lone pair and Zn, as is confirmed by precise ab initio calculations of the core-level excitation.

Saturated hydrocarbons on a Cu surface: a new type of chemical interaction?

Abstract Ab initio Hartree–Fock (HF) calculations for cyclopropane adsorbed on a Cu-cluster reveal that the so-called M*-resonance seen in X-ray absorption (NEXAFS)-spectra for saturated hydrocarbons adsorbed on metal surfaces is due to a strong electronic mixing between the metal and the molecule. Since this type of chemical interaction is unexpected and has not been observed previously the different types of interactions were carefully analyzed. In addition to the importance of correlation effects the results reveal that a small but significant interaction is due to a charge-donation from the metal states to the molecule, most likely to states of Rydberg character.

Orbital rehybridization in n-octane adsorbed on Cu(110)

We have investigated the local electronic structure of n-octane adsorbed on the Cu(110) surface using symmetry-resolved x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES) in combination with density functional theory (DFT) spectrum calculations. We found new adsorption-induced states in the XE spectra, which we assign to interaction between the bonding CH orbitals and the metal surface. By performing a systematic investigation of the influence of different structural parameters on the XA and XE spectra, we conclude that the molecular geometry is significantly distorted relative to the gas-phase structure. The bonding to the surface leads to a strengthening of the carbon–carbon bonds and a weakening of the carbon–hydrogen bonds, consistent with a rehybridization of the carbons from sp3 to sp2.8.

The adsorption of small hydrocarbons on Cu(111): A combined He-atom scattering and X-ray absorption study for ethane, ethylene, and acetylene

Ethane (C2H6), ethylene (C2H4), and acetylene (C2H2) adsorbed on Cu (111) are investigated using high-resolution helium atom scattering and x-ray absorption spectroscopy (NEXAFS). For C2H6/Cu(111) and C2H4/Cu(111) the excitation energies of the frustrated molecular translation normal to the surface (FTz) amount to 6.7 meV, suggesting the presence of a physisorbed species which is consistent with the NEXAFS data for ethylene. In contrast, for C2H2/Cu(111) the NEXAFS data indicate strong intramolecular distortions of the acetylene adsorbate compatible with a tilt of both CH ends away from the molecular axis. While the latter finding is in agreement with recent theoretical studies the theoretically predicted chemisorbed ethylene species could not be observed by the experiment. However, more detailed theoretical studies of the ethylene-Cu(111) interaction potential reveal two minima separated by an activation barrier. The minimum closer to the surface refers to strongly distorted chemisorbed C2H4 whereas the ...

XPS and XAS investigation of condensed and adsorbed n-octane on a Cu(110) surface

Using x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES) and x-ray photoelectron spectroscopy (XPS) in combination with density functional theory (DFT) the changes in electronic and geometric structure of hydrocarbons upon adsorption are determined. The chemical bonding is analyzed and the results provide new insights in the mechanisms responsible for dehydrogenation in heterogeneous catalysis.In the case of alkanes, n-octane and methane are studied. XAS and XES show significant changes in the electronic structure upon adsorption. XES shows new adsorption induced occupied states and XAS shows quenching of CH*/Rydberg states in n-octane. In methane the symmetry forbidden gas phase lowest unoccupied molecular orbital becomes allowed due to broken symmetry. New adsorption induced unoccupied features with mainly metal character appear just above the Fermi level in XA spectra of both adsorbed methane and n-octane. These changes are not observed in DFT total energy geometry optimizations. Comparison between experimental and computed spectra for different adsorbate geometries reveals that the molecular structures are significantly changed in both molecules. The C-C bonds in n-octane are shortened upon adsorption and the C-H bonds are elongated in both n-octane and methane.In addition ethylene and acetylene are studied as model systems for unsaturated hydrocarbons. The validity of both the Dewar-Chatt-Duncanson chemisorption model and the alternative spin-uncoupling picture is confirmed, as well as C-C bond elongation and upward bending of the C-H bonds.The bonding of ethylene to Cu(110) and Ni(110) are compared and the results show that the main difference is the amount of back-donation into the molecular π* orbital, which allows the molecule to desorb molecularly from the Cu(110) surface, whereas it is dehydrogenated upon heating on the Ni(110) surface. Acetylene is found to adsorb in two different adsorption sites on the Cu(110) surface at liquid nitrogen temperature. Upon heating the molecules move into one of these sites due to attractive adsorbate-adsorbate interaction and only one adsorbed species is present at room temperature, at which point the molecules start reacting to form benzene. The bonding of the two species is very similar in both sites and the carbon atoms are rehybridized essentially to sp2.

Synthesis of Polycyclic Aromatic Hydrocarbons and Graphite Islands Via Surface-Induced Reaction of Small Molecules.

Attractive materials for molecular electronics are the polycyclic aromatic hydrocarbons (PAHs), the hydrogen-terminated segments of a graphitic plane. Since the processing of larger PAHs, for example hexa-peri-hexabenzocoronene, is complicated by their extremely low solubilities and very small vapor pressures, the possibility of a surface-assisted synthesis of planar PAHs is investigated starting from diphenylacetylene (left) and hexaphenylbenzene (right), as shown in the picture.

PHOTOEMISSION FROM LONG CHAIN ALKANES ADSORBED ON A METAL SURFACE AND THE ELECTRONIC STRUCTURE OF TRANS-POLYETHYLENE (–CH2–)n

The adsorption geometry of n-alkanes on the surface of noble metals (Cu, Au) has been investigated by ultraviolet photoemission spectroscopy (UPS) in order to resolve apparent inconsistencies with regard to the orientation of the C–C–C plane relative to the surface. Measurements were carried out for octane (C8H18), hexatriacontane (C36H74) and triacontanoic acid (C29H59COOH) on Au(111) and Cu(111). Analysis of the data requires a detailed symmetry analysis of the electronic structure of trans-polyethylene (–CH2–), which is also presented and yields an orientation of the C–C–C plane parallel to the surface for all molecules.