D. Steinmüller

Ordered overlayers of aniline and phenol on Pd(110): Surface structure and bonding

Abstract The surface structure and bonding of aniline and phenol on Pd(110) has been studied by angle resolved UV photoemission (ARUPS) using synchrotron radiation, LEED, and thermal desorption spectroscopy (TDS). Both molecules form ordered c(4 × 2) surface structures on Pd(110), and ARUPS and TDS results suggest that aniline and phenol split off an H atom from the functional group to form C 6 H 5 NH and C 6 H 5 O species in the monolayer phases. The molecules coordinate to the surface via the it electrons of the aromatic rings and via the heteroatoms as indicated by the stabilisation of the π states and of the N or O lone pair electrons in the ARUPS spectra; thus they adopt an adsorption geometry with the ring plane in close proximity to the metal surface. The results do not, however, rule out some tilting of the molecules which would be expected from steric considerations. There is preferential azimuthal orientation in the densely packed adlayers of the monolayer phases, and the molecules align with their functional groups along the [001] azimuth.

Cyanogen on Ni(110): an experimental and theoretical study

Abstract Adsorption of cyanogen, C 2 N 2 , on a Ni(110) surface has been investigated by combining low energy electron diffraction, thermal desorption and angular resolved ultra-violet photoemission experiments as well as model cluster calculations using the linear combination of Gaussian-type orbitals local density functional method as well as a force field approach. Results of mirror plane photoemission experiments on the ordered c(2 × 2) monolayer could be rationalized by invoking adsorbates bonded to the surface via the π electrons, with their axis oriented along the [001] direction, across the grooves of the (110) surface, leading to an assignment of all six adsorbate-derived valence orbitals. However, unlike in a previous study on the analogous chemisorption system Pd(110)/C 2 N 2 , only one mirror plane was detected. A possible tilt of the adsorbates in the (110) plane was related to crowding on the closer spaced nickel surface by estimating the lateral interaction within the adsorption layer using force field models. Electronic structure calculations on various chemisorption model clusters confirmed the experimental orbital assignment and the orientation of the adsorbate axis essentially along the [001] direction. Best agreement with UPS data was found for the orbital splitting pattern of an adsorption geometry where the C-N groups bind sideways on-top of nickel atoms in the first crystal plane. On the other hand, bonding along in the troughs of the (110) surface leads to a distinctly different ordering of the valence orbitals, at variance with the experimental assignment.

Compressed benzene on Ni(110)

Abstract The adsorption of benzene on Ni(110) provides an example of benzene in a significantly crowded surface configuration. This is shown from studies with angle resolved photoemission, near-edge X ray absorption fine structure, LEED and thermal desorption spectroscopy. The benzene molecules are incorporated in a well-ordered, densely packed c(4 × 2) overlayer with preferential azimuthal orientation. It is suggested that compressive intermolecular interaction induces C 2v symmetry of the flat lying molecules. The high packing density in the overlayer leads to a considerable orbital overlap, which is reflected in a large C-H band dispersion of 0.75 eV.

Phenol on Ni(110): characterisation of surface phenoxide

Abstract The interaction of phenol with a Ni(110) surface has been investigated by angle resolved UV photoelectron spectroscopy (ARUPS) using synchrotron radiation and by thermal desorption spectroscopy (TDS) with monodeuterated d 1 -phenol. The TDS experiments reveal cleavage of the O-H bond at T K and the formation of surface phenoxide species in the monolayer phase. The ARUPS spectra suggest a molecular ring orientation in proximity to the metal surface, but not necessarily parallel, and surface coordination via the π electrons and possibly also via the O lone pair electrons. The azimuthal anisotropy observed in the ARUPS spectra indicates locally a preferential azimuthal orientation of phenoxide with the O atom pointing in the [1 1 0] direction as specified by mirror plane emission experiments. Interestingly, this azimuthal orientation of phenoxide on Ni(110) is orthogonal to the one determined recently on Pd(110), and possible reasons are discussed.

Evolution of the core and unoccupied orbitals of biphenyl and bithiophene on Cs doping

X‐ray photoelectron spectroscopy (XPS) and near‐edge x‐ray absorption fine structure (NEXAFS) have been used to study the core and unoccupied orbitals of condensed multilayers of biphenyl and bithiophene on Cs dosing. The CK XPS shifts, observed on dosing are understandable in terms of repositioning of the Fermi level due to creation of states in the band gap, and at higher Cs concentrations to the high polarizability of the charge transfer complexes formed. The behavior of the CK XPS satellites on increasing Cs exposure are related to changes observed in the electron energy loss spectra (EELS). The CK NEXAFS of biphenyl and the SL23 NEXAFS of bithiophene indicate that an unoccupied molecular orbital is filled by charge transfer from Cs. In the case of bithiophene, both the SL23 XPS and NEXAFS features shift by 2 eV to lower binding and adsorption energy, respectively. This strong chemical shift suggests significant localization of the transferred charge on the S sites. Comparison of the SL23 NEXAFS of do...

Adsorption of CN on Ni(110): an experimental and theoretical study

Abstract Saturation exposure of the Ni(110) surface to C 2 N 2 at room temperature yields a well ordered c(2 × 2) CN overlayer with four characteristic ionisations that show distinctive angular effects in angle resolved ultraviolet photoemission spectroscopy (ARUPS) using polarised synchrotron radiation. Good azimuthal ordering is observed and application of selection rules indicates that the CN lies parallel to the surface with its molecular axis oriented along the [110] azimuthal direction. Linear combination of Gaussian-type orbitals local density functional (LCGTOLDF) cluster calculations probing various probable adsorption sites and geometries find CN lying flat in the grooves bridging second layer substrate atoms to be the favoured site. Bonding is effectuated mainly by ionic and lateral π interaction. The barrier toward an upright orientation is calculated to be small. The good agreement between the experimental and theoretical results allows a reassessment of previously published photoemission data from other CN adsorption systems.

Valence band photoemission spectra and molecular geometry of biphenyl in condensed and chemisorbed phases

In this paper, three distinct phases of biphenyl on the Pd (110) surface are characterized and studied by angle‐resolved ultraviolet photoemission (ARUPS); a disordered condensed multilayer which desorbs at 218 K, a bilayer that desorbs at 240 K, and a strongly bound monolayer which breaks up at the surface for temperatures greater than 400 K. The multilayer ultraviolet photoemission (UP) spectrum is almost identical to that of the gas phase spectrum and it is inferred that the torsional angle between the phenyl rings in the condensed phase is unaltered from that in the gas phase. Changes observed in the π orbital emissions on the formation of the bilayer indicate a significant reduction in the torsional angle. Application of the selection rules for ARUPS to the strong angular effects observed in the emissions from the highest‐lying π orbitals suggests that the molecules in this second layer have their molecular axes parallel to and molecular planes perpendicular to the surface, that is, edge‐on to the mo...

Electron spectroscopic studies of models for PPP and PT on p- and n-doping

Abstract Condensed layers of sexiphenyl and bithiophene are investigated as models for polyparaphenylene (PPP) and polythiophene (PT). Bipolaron-like gap states are directly observed by ultraviolet photoemission spectroscopy (UPS) on the n-doping of sexiphenyl with Cs. Comparison with the results for Cs dosed biphenyl indicate that the dimer reflects the electronic modifications induced by doping. The in situ dosing of bithiophene with gaseous Br 2 is characterized by thermal desorption mass spectrometry (TDS), UPS and electron energy loss spectroscopy (EELS) and the formation of a Br-bithiophene complex is indicated. Comparison of the EELS spectra with optical absorption results for the p-doping of longer thiophene oligomers suggests that a bipolaron to polaron-like transformation can be achieved by thermal ‘dedoping’.

MODELING OF LATERAL INTERACTIONS IN DENSELY PACKED ADSORBATE OVERLAYERS : HOW FAR DO CHEMISORBED C2N2 MOLECULES TILT ON A NI(110) SURFACE ?

It is proposed to investigate geometric arrangements in chemisorption systems by standardized force fields. This ‘‘molecular modeling’’ technique is particularly simple to apply to densely packed adsorption layers with dominating nonbonding interactions where the substrate essentially acts as a template for interadsorbate distances. The approach is exemplified for Ni(110/c(2×2)C2N2. Tilting of the molecular axis by about 20–30 degrees with respect to the surface (the molecules oriented perpendicularly to the troughs) is deduced, supporting a corresponding interpretation of angular resolved photoemission data.

Electron spectroscopic investigation of biphenyl and Cs: A model system for doping conducting polymers

Abstract Thin layers of biphenyl molecules and Cs on a Pd(110) single crystal substrate have been prepared by evaporation in an ultrahigh vacuum environment. The electronic interactions between Cs atoms and the organic molecules have been studied by electron spectroscopic techniques such as ultraviolet photoelectron spectroscopy (UPS) and electron energy loss spectroscopy (EELS), whereas the energetics of the mixed Cs and biphenyl layers has been investigated by thermal desorption mass spectrometry (TDS). TDS experiments revealed attractive interaction between Cs and biphenyl and the formation of a molecular complex. The electronic structure of the Cs/biphenyl complex is distinctly different from the sum of the individual constituents. New electronic states appear in the region between the molecular HOMO and LUMOs (that is in the band gap in solid state terminology). Two states have been observed in UPS, and excitations involving these new gap states have been detected in EELS. The implications of this very model system for the conducting mechanism in doped polymers will be discussed.