D. Batchelor

Electronic relaxation effects in condensed polyacenes: A high-resolution photoemission study

We present a high-resolution photoelectron spectroscopy investigation of condensed films of benzene, naphthalene, anthracene, tetracene, and pentacene. High spectroscopic resolution and a systematic variation of the molecular size allow a detailed analysis of the fine structures. The line shapes of the C 1s main lines are analyzed with respect to the different contributions of inhomogeneous broadening, vibronic coupling, and chemical shifts. The shake-up satellite spectra reveal trends, which give insight into the charge redistribution within the molecule upon photoexcitation. In particular, the shake-up between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) increases in intensity and moves closer toward the C 1s main line if the size of the aromatic system is increased. An explanation is given on the basis of the delocalization of the aromatic system and its capability in screening the photogenerated core hole. A comparison of the HOMO-LUMO shake-up position to the optical band gap gives additional insight into the reorganization of the electronic system upon photoexcitation.

The interaction of CO2 with potassium-promoted Cu(100): adsorption, reactions and radiation induced dissociation of CO2

Abstract The coadsorption of CO 2 and K on Cu(110) has been studied by the methods of photoelectron spectroscopy using synchrotron radiation, high resolution electron energy loss spectroscopy (HREELS) and work function measurements (ΔΦ). Both weakly adsorbed molecular carbon dioxide, chemisorbed carbonate and carbon monoxide are found at 107 K. It was found that the presence of free copper sites is necessary for the adsorption of CO and the large amount of physisorbed CO 2 in comparison with the copper surface completely covered with potassium. A coherent picture of the adsorption and reactions of CO 2 with a potassium modified Cu(110) surface is obtained by combination of the above mentioned spectroscopies with earlier thermally programmed desorption measurements and binding energy determinations of C 1s and O 1s. Irradiation with 125 eV photons causes a transformation of physisorbed CO 2 to chemisorbed CO and O. The kinetics of this photoinduced process was followed via the intensity variations of the O 1s characteristic of physisorbed and reacted CO 2 . The cross-section for the reaction was determined to 3 × 10 7 barn at a photon energy of 125 eV. Both the large cross-section and the photon energy dependence indicate that photon generated secondary electrons play a dominant role in the transformation process.

A high-resolution near-edge x-ray absorption fine structure investigation of the molecular orientation in the pentacene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) pentacene/system

We present x-ray photoemission spectroscopy and highly resolved near-edge x-ray absorption fine structure spectroscopy measurements taken on pentacene thin films of different thicknesses deposited on a spin coated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) substrate. Thin films of pentacene were prepared by using organic molecular beam deposition in situ using strictly controlled evaporation conditions. Our investigations show that pentacene thin films on PEDOT:PSS are characterized by upright standing molecules. Due to the strong dichroic behavior, the calculated values of the molecular orientation give a clear indication not only of the real molecular arrangement in the films but also of a high orientational order. This high degree of molecular orientation order is a characteristic already of the first layer. The films show the tendency to grow on the PEDOT:PSS substrate following an island-fashion mode, with a relatively narrow intermixing zone at the interface between the pentacene and the polymer blend. The peculiarity of the growth of pentacene on PEDOT:PSS is due to the fact that the substrate does not offer any template for the nucleated films and thus exerts a lateral order toward the crystal structure arrangement. Under these conditions, the upright orientation of the molecules in the films minimizes the energy required for the system stability.

Surface microscopy with scanned electron beams

A brief review is given of the methods that are available for studying surfaces on a microscope scale. The use of finely focused scanned electron beams is described in detail. Examples are given of Auger and secondary electron spectroscopy and microscopy, and of diffraction techniques. These examples are largely taken from recent work of the authors on Ag layers on bulk single-crystal Si (111), Si (100) and W (110) surfaces, but applications to other materials and to thin films are also discussed. Future developments are briefly outlined.

Unusual energy shifts in resonant photoemission spectra of organic model molecules

We study the electronic structure of zinc phthalocyanine (ZnPc) and 1,4-octa-decyl substituted zinc phthalocyanine [(Dec)(8)PcZn] thin films (approximately 6-15 nm) using resonant photoemission spectroscopy and X-ray absorption spectroscopy (XAS) at room temperature and at liquid He temperature. From XAS we conclude that the probability amplitude of the lowest unoccupied molecular orbital is located predominantly at the inner C and N atoms of the molecules. Nonlinear energy shifts in resonant photoemission were observed; large shifts are explained by reduced electrical conductivity of inhomogeneously oriented molecules.

Orientation of Differently Substituted Phthalocyanines: First Layers and Thin Films

The molecular orientation of the first layers and thin films of differently substituted phthalocyanines on gold substrates is compared. Due to a relatively strong molecule-substrate interaction the orientation is parallel to the sample surface for all materials in the first layers. At higher film thickness the orientation is determined by the substituents. Materials with short substituents prefer standing orientation, implying the substituents do not inhibit the intermolecular interactions. Molecules with long substituents are not able to build up the same aggregates, they grow disordered in the upper layers.

Validity of the method for quantitative XPS of surface nano-structures: application to Cu/Au/Cu

Abstract The validity of the software packet QUASES TM , developed by Tougaard et al., for quantitative analysis by XPS of nano-scale surface structures is considered. We study the ability of the software to consistently determine the same amount of Au atoms within the surface region of a series of solids where the depth distribution of a fixed number of Au atoms is varied. The sets of samples were produced by first evaporating a few monolayers of gold onto the surface of a polycrystalline copper sample and then evaporating varying amounts of copper on top. A synchrotron radiation source is used to produce the photons. For two series of samples where the measured Au 4d peak intensities vary by a factor of 10, the quantitative amount of gold determined by this formalism is found to be constant to within an RMS deviation of 7% and 15%. When the spectra from the same set of samples using photons of 950 and 750 eV energy are analyzed, the RMS deviation in the total number of gold atoms is ∼ 20%.

Structure of the quasihexagonal close-packed (4 × 4) Na layer on Al(111): a SEXAFS study

Abstract The Al(111)-(4 × 4)-Na structure formed by adsorption of Na on Al(111) at low temperature is studied by surface extended X-ray absorption fine structure (SEXAFS). Analysis of the SEXAFS measurements shows that the adsorbed layer is quasihexagonal close-packed and slightly compressed, with a nearest-neighbour Na-Na bond length of 3.70 ± 0.03 A. Na atoms are adsorbed in 3 different adsorption sites with roughly equal Na-Al bond lengths of 2.8 A but different perpendicular distances from the surface. Thus the Na layer is not strictly planar.

Simulation of LEED patterns from complex surface structures

Abstract A computer simulation technique has been developed which allows a comparison to be made between the LEED pattern from any model structure and an experimental LEED pattern. The model structure is set up as a layered array, each array consisting of about 30×30 scatterers. Two or more layers are included, as necessary, and there are no constraints on the selvedge structure. Incommensurate, domain wall and disordered systems can be readily modelled. The computer program simulates a scattered electron wavefield composed of both single and double scattering contributions. Electron intensities are calculated in a user-determined portion of the two-dimensional k -space. The program is demonstrated by application to several adsorbate-substrate systems.

Unusual chemisorption structure of Na on Al(111): a SEXAFS study

Abstract The adsorption of Na on Al(111) has been studied by surface extended X-ray absorption fine structure (SEXAFS) in the coverage range of θ Na = 0.16-0.7 monolayer at room temperature. For θ Na = 0.16–0.33, Na atoms occupy an unusual 6-fold coordinate site formed by displacing every third ( θ Na = 0.33) Al atom in the first layer of the substrate. The NaAl bond length is 3.31 A, independent of coverage, consistent with metallic bonding. These results have important implications for models of alkali metal adsorption.