| 1. |
- Alfredsson, Ylfi, et al.
(author)
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Phase and molecular orientation in H2Pc on conducting glass : characterization of two deposition methods
- 2005
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In: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 493:1-2, s. 13-19
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Journal article (peer-reviewed)abstract
- In this study, metal-free phthalocyanine has been deposited on a conducting glass surface by two methods: by spreading the molecular powder directly on the substrate in air and by vapor sublimation under ultra-high vacuum conditions (evaporation). The films have been characterized by means of core level X-ray Photoemission Spectroscopy, X-ray Absorption Spectroscopy (XAS) and Ultra Violet and Visible absorption spectroscopy (UV-Vis). Our results show that the two deposition methods produce molecular overlayers in different polymorphic phases; the UV-Vis measurements indicate that the film obtained by powder deposition is of x-phase type whereas sublimation leads to an α-polymorph structure. The XAS results show that in the powder deposited film the molecules are mainly oriented parallel to the surface. This is opposite to the case of the vapor deposited film, where the molecules mainly are oriented orthogonal to the surface.
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| 2. |
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| 3. |
- Alfredsson, Y, et al.
(author)
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Phase and molecular orientation in metal-free phthalocyanine films on conducting glass: Characterization of two deposition methods
- 2005
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In: Thin Solid Films. - : Elsevier BV. - 0040-6090. ; 493:1-2, s. 13-19
-
Journal article (peer-reviewed)abstract
- in this study, metal-free phthalocyanine has been deposited on a conducting glass surface by two methods: by spreading the molecular powder directly on the substrate in air and by vapor sublimation under ultra-high vacuum conditions (evaporation). The films have been characterized by means of core level X-ray Photoemission Spectroscopy, X-ray Absorption Spectroscopy (XAS) and Ultra Violet and Visible absorption spectroscopy (UV-Vis). Our results show that the two deposition methods produce molecular overlayers in different polymorphic phases; the UV-Vis measurements indicate that the film obtained by powder deposition is of x-phase type whereas sublimation leads to an a-polymorph structure. The XAS results show that in the powder deposited film the molecules are mainly oriented parallel to the surface. This is opposite to the case of the vapor deposited film, where the molecules mainly are oriented orthogonal to the surface.
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| 4. |
- Carabineiro, S A C, et al.
(author)
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Resonant photoemission of N2O on Ir(110)
- 2004
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In: Surface Review and Letters. - 0218-625X. ; 11:4-5, s. 385-389
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Journal article (peer-reviewed)abstract
- The adsorption of N2O on Ir(110) was investigated with high-resolution resonant photoemission at 135 K. The results obtained show evidence of molecular adsorption of N2O, along with some dissociation. It is found that the nitrogen photoemission spectra measured at the terminal and central nitrogen energy positions of the N2O/Ir(110) system are equivalent to those of N2O in the gas phase. In contrast, the oxygen spectrum shows little resemblance to the gas phase oxygen spectrum of N2O. In the nitrogen resonant photoemission spectra one can only discover resonant behavior with constant binding energy peaks. In contrast, the oxygen resonant photoemission spectra shows dominantly Auger behavior with peaks at constant kinetic energies. Both observations reveal that the oxygen is bonded to the Ir surface. A systematic study as a function of coverage and temperature and a comparison with other surfaces differing in structure and composition is needed to understand the variations in the adsorption behavior of N2O On metal surfaces.
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| 5. |
- Chao, Yimin, et al.
(author)
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Evaporation and deposition of alkyl-capped silicon nanocrystals in ultrahigh vacuum
- 2007
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In: Nature Nanotechnology. - : Springer Science and Business Media LLC. - 1748-3395 .- 1748-3387. ; 2:8, s. 486-489
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Journal article (peer-reviewed)abstract
- Nanocrystals are under active investigation because of their interesting size- dependent properties(1,2) and potential applications(3-5). Silicon nanocrystals have been studied for possible uses in optoelectronics(6), and may be relevant to the understanding of natural processes such as lightning strikes(7). Gas-phase methods can be used to prepare nanocrystals, and mass spectrometric techniques have been used to analyse Au-8,(9) and CdSe clusters(10). However, it is difficult to study nanocrystals by such methods unless they are synthesized in the gas phase(11). In particular, pre-prepared nanocrystals are generally difficult to sublime without decomposition. Here we report the observation that films of alkyl-capped silicon nanocrystals evaporate upon heating in ultrahigh vacuum at 200 degrees C, and the vapour of intact nanocrystals can be collected on a variety of solid substrates. This effect may be useful for the controlled preparation of new quantum-confined silicon structures and could facilitate their mass spectroscopic study and size- selection(12).
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| 6. |
- Chao, Y, et al.
(author)
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Reactions and luminescence in passivated Si nanocrystallites induced by vacuum ultraviolet and soft-x-ray photons
- 2005
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In: Applied Physics Reviews. - : AIP Publishing. - 1931-9401. ; 98:4
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Journal article (peer-reviewed)abstract
- Alkyl-modified silicon nanocrystallites are efficient fluorophores which are of interest for fundamental spectroscopic studies and as luminescent probes in biology because of their stability in aqueous media. In this work we have investigated these particles using scanning tunneling microscopy, synchrotron-radiation excited photoemission, and x-ray excited optical luminescence (XEOL). During the course of illumination with 145-eV photons we have monitored the evolution of the Si2p core level and, in samples which have suffered prolonged atmospheric exposure, observed in real time the growth of an extra Si2p component attributed to in situ photoinduced oxidation of the Si nanocrystallites. XEOL reveals that two emission bands are active upon soft-x-ray photon excitation and that photoluminescence intensity decreases with photon exposure, which is attributed to charge trapping within the film.
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| 7. |
- Glover, C. J., et al.
(author)
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Stationary and dispersive features in resonant inelastic soft X-ray scattering at the Ge 3p resonances
- 2009
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In: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048 .- 1873-2526. ; 173:2-3, s. 103-107
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Journal article (peer-reviewed)abstract
- Resonant inelastic soft X-ray scattering at the 3p resonances in crystalline Ge is presented. Both stationary and dispersive features are observed in a wide energy range above as well as below the ionization limits. These observations are in agreement with theoretical predictions based on a two-step model where the initially excited electron has no influence on the emission step. Excess population of states in the conduction band is found, and discussed in terms of attosecond electron dynamics. (c) 2009 Elsevier B.V. All rights reserved.
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| 8. |
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| 9. |
- Kjeldgaard, Lisbeth, et al.
(author)
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Intramolecular vibronic dynamics in molecular solids: C60
- 2005
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In: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 72:20
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Journal article (peer-reviewed)abstract
- Vibronic coupling in solid C60 has been investigated with a combination of resonant photoemission spectroscopy (RPES) and resonant inelastic x-ray scattering (RIXS). Excitation as a function of energy within the lowest unoccupied molecular orbital resonance yielded strong oscillations in intensity and dispersion in RPES, and a strong inelastic component in RIXS. Reconciling these two observations establishes that vibronic coupling in this core hole excitation leads to predominantly inelastic scattering and localization of the excited vibrations on the molecule on a femtosecond time scale. The coupling extends throughout the widths of the frontier valence bands.
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| 10. |
- Käämbre, Tanel, et al.
(author)
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Bulk electronic structure of K3C60 as revealed by soft x-rays
- 2007
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:19, s. 195432-
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Journal article (peer-reviewed)abstract
- We present C 1s x- ray absorption, x- ray emission, and resonant inelastic x- ray scattering (RIXS ) spectra of single- phase crystalline K3C60. The comparison to valence- band photoelectron spectra from the same sample facilitates identification of the contribution from surface and bulk electronic states in the latter. Bulk- sensitive techniques show that the valence bands of K3C60 and pure C-60 are characterized by spectral features of similar width, in agreement with the predictions of band- structure calculations. Symmetry selectivity in the RIXS process allows us to assign peaks in the C 1s absorption spectrum, demonstrating a close correspondence with pure C-60 also in the conduction band. The symmetry selectivity is as pronounced in K3C60 as in pure C-60, indicating that the local C-60 symmetry is not appreciably affected by the K doping, either in the ground state or intermediate state, on the time scale of 6 fs.
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| 11. |
- Macovez, R., et al.
(author)
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Electronic surface reconstruction and correlation in the fcc and dimer phases of RbC60
- 2007
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:19, s. 195424-
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Journal article (peer-reviewed)abstract
- A combined photoemission and x-ray absorption study of a RbC60 crystalline film is presented. We find evidence for an electronic charge reconstruction of the film surface in both the fcc and the dimer phases of RbC60. We confirm the previous conclusion on less crystalline films that the dimer phase is insulating. Several observations, such as the presence of molecular features in the photoemission spectra, indicate that at least partial electron localization occurs in the high-temperature fcc phase. In the fcc phase, the surface consists in a half-charge C60(111) plane and appears weakly metallic, as found for the bulk. In the dimer phase, the charge reconstruction simply implies the presence of neutral C60 in the surface layer. The identification of neutral molecules in the surface layer drastically improves the agreement between calculations of the electronic density of states and photoelectron spectra in both phases.
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| 12. |
- Macovez, Roberto, et al.
(author)
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Hybridization, superexchange, and competing magnetoelastic interactions in TiOBr
- 2007
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 76:20, s. 205111-
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Journal article (peer-reviewed)abstract
- A crystalline sample of TiOBr is probed at room temperature by a combination of electron spectroscopies and the results are compared to theoretical embedded-cluster calculations. Resonant photoemission of the valence band confirms that the lowest binding energy feature arises from the singly occupied Ti 3d orbital. The polarization dependence of this orbital in nonresonant photoemission is consistent with the expected dominant d(y)(2)-z(2) character. The analysis of the Ti L-2,L-3 x-ray absorption spectra confirms the complete splitting of the Ti 3d shell. X-ray absorption and resonant photoemission at the O 1s edge provide direct evidence for hybridization between the transition metal orbitals and the O 2p levels, which leads to superexchange interactions between the Ti ions. The existence of a mixing of O and Ti states and of strong superexchange interactions is supported by calculations of the ground-state electronic and magnetic properties. The calculated superexchange interchain interaction is one fifth in strength of the total magnetic coupling along the chain, and is antiferromagnetic in character. This O-mediated interchain interaction is frustrated in the room temperature phase of TiOBr and thus couples strongly to distortions of the soft lattice. The competition between the interchain magnetoelastic coupling and the spin-Peierls interaction might be at the origin of the complex TiOX phase diagram.
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| 13. |
- Richter, M. C., et al.
(author)
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NiFe2O4 and Fe3O4 studied by XMCD and resonant photoemission
- 2009
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In: The European Physical Journal. Special Topics. - : Springer Science and Business Media LLC. - 1951-6355 .- 1951-6401. ; 169, s. 175-180
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Journal article (peer-reviewed)abstract
- X-ray magnetic circular dichroism and resonant photoemission at the Fe and Ni L-2,L-3 edges have been used to investigate the electronic structure of NiFe2O4 (NFO) thin films. The results, when compared to those obtained on Fe3O4, indicate that in a 12 nm NFO film the Ni atoms occupy mainly B sites, as in bulk NFO, and that a decrease in the thickness of the film results in a modi. cation of the nickel hybridization.
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| 14. |
- Richter, MC, et al.
(author)
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Resonant photoemission at the 2p edge in compounds containing Mn with different valences
- 2006
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In: Synchrotron Radiation and Materials Science - Proceedings of the E-MRS 2005 Symposium O on Synchrotron Radiation and Materials Science (Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms). - : Elsevier BV. - 0168-583X. ; 246:1, s. 184-188
-
Conference paper (peer-reviewed)abstract
- We have investigated the Raman-Auger crossover of the Mn 2p non-radiative decays in bulk MnO. For the highest kinetic energy transition in the Mn 2p3p3p decay, Raman behaviour is observed up to approximate to 3.4 eV above the resonance energy (maximum of the L-3 absorption). In the case of the 2p3p3d and 2p3d3d transitions a similar situation occurs up to approximate to 5 eV above the resonance energy. We compare these results to those obtained on a thin MnO layer deposited on Cu, on bulk Mn and on La0.7Sr0.3MnO3. The results are discussed in terms of the screening dynamics of the Mn 2p core hole and of the metallic/insulating character of the material. (c) 2005 Elsevier B.V. All rights reserved.
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| 15. |
- Schiessling, J, et al.
(author)
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Insulating surface layer on single crystal K3C60
- 2004
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In: European Physical Journal B. Condensed Matter and Complex Systems. - : Springer Science and Business Media LLC. - 1434-6028. ; 41:4, s. 435-438
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Journal article (peer-reviewed)abstract
- Using angle-dependent photoemission spectra of core and valence levels we show that metallic, single crystal K3C60 is terminated by an insulating or weakly-conducting surface layer. We attribute this to the effects of strong intermolecular correlations combined with the average surface charge state. Several controversies on the electronic structure are thereby resolved.
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| 16. |
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| 17. |
- Schiessling, Joachim, et al.
(author)
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Metastable surface phase for NaxC60
- 2007
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In: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 601:18, s. 3933-3936
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Journal article (peer-reviewed)abstract
- NaxC60 thin films with Na concentration 0 <= x <= 3 were investigated using angle-dependent photoelectron spectroscopy. For low doping we observed two distinct fulleride phases coexisting with regions of pristine C-60. One of these fulleride phase is predominantly formed close to the surface and disappears after annealing and further addition of Na. At higher doping a phase with stoichiometry of x = 3 is formed.
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| 18. |
- Schiessling, J, et al.
(author)
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Origin of molecular orbital splitting of C-60 on Al(110)
- 2004
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In: Journal of Physics: Condensed Matter. - 1361-648X. ; 16:36, s. 407-414
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Journal article (peer-reviewed)abstract
- We have investigated the C-60 monolayer on Al(l 10) with angle-dependent photoelectron spectroscopy. We find that orbital components have different angular distributions. Calculations of cross sections of the highest occupied molecular orbital components for free, oriented C-60 are found to describe the experimental data quite well. The observed band splitting is attributed to intramolecular electronic correlations, due to the different coupling of the orbital components to the substrate conduction band.
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| 19. |
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| 20. |
- Schiessling, Joachim, et al.
(author)
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The Role of Charge-Charge Correlations and Covalent Bonding in the Electronic Structure of Adsorbed C-60 : C-60/A1
- 2010
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 114:43, s. 18686-18692
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Journal article (peer-reviewed)abstract
- Aromatic molecules are central components of model systems for molecular electronics, with C-60 one of the most studied. Upon adsorption on (metallic) substrates a splitting of the frontier orbitals is commonly observed, with a strong dependence on substrate material, but little dependence on substrate structure. We report the detailed photoelectron angle dependence of C-60/A1(110) over a wide range of energy, finding a strong remnant molecular character. In particular, certain HOMO-derived suborbitals couple strongly, and others weakly, with the metal, which results in final state charging for those weakly coupled. C Is data correlate well with the assignments made on this basis, as does the comparison of ground state partial densities-of-states (PDOS) to photoelectron spectra. Detailed analysis of the PDOS supports a rough division into surface-near and surface-far components, in agreement with the molecular picture. The component spectral widths are attributed to intramolecular vibrational coupling, which is suggested to aid in the electronic decoupling of certain suborbitals from the substrate, facilitating the observed final state charging.
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| 21. |
- Siller, L., et al.
(author)
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Core and valence exciton formation in x-ray absorption, x-ray emission and x-ray excited optical luminescence from passivated Si nanocrystals at the Si L-2,L-3 edge
- 2009
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In: Journal of Physics: Condensed Matter. - : IOP Publishing. - 1361-648X .- 0953-8984. ; 21:9
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Journal article (peer-reviewed)abstract
- Resonant inelastic x-ray scattering (RIXS), x-ray absorption spectroscopy and x-ray excited optical luminescence (XEOL) have been used to measure element specific filled and empty electronic states over the Si L-2,L-3 edge of passivated Si nanocrystals of narrow size distribution ( diameter 2.2 +/- 0.4 nm). These techniques have been employed to directly measure absorption and luminescence specific to the local Si nanocrystal core. Profound changes occur in the absorption spectrum of the nanocrystals compared with bulk Si, and new features are observed in the nanocrystal RIXS. Clear signatures of core and valence band exciton formation, promoted by the spatial confinement of electrons and holes within the nanocrystals, are observed, together with band narrowing due to quantum confinement. XEOL at 12 K shows an extremely sharp feature at the threshold of orange luminescence (i.e., at similar to 1.56 eV ( 792 nm)) which we attribute to recombination of valence excitons, providing a lower limit to the nanocrystal band gap.
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| 22. |
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| 23. |
- Åhlund, John, et al.
(author)
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The electronic structure of iron phthalocyanine probed by photoelectron and x-ray absorption spectroscopies and density functional theory calculations
- 2006
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 125:3, s. 034709-1-034709-7
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Journal article (peer-reviewed)abstract
- A joint experimental and theoretical work to explain the electronic and geometrical structure of an in situ prepared film of iron phthalocyanine (FePc) on silicon (100) is presented. FePc molecular films have been characterized by core and valence photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS), and the results have been interpreted and simulated by density functional theory (DFT) calculations. C1s and N1s PE spectra have been analyzed by taking into account all chemically nonequivalent C and N atoms in the molecule. In the Fe2p(3/2) spectra it has been possible to resolve two components that can be related to the open shell structure of the molecule. By valence PES and N1s XAS data, the geometrical orientation of the FePc molecules in the film could be determined. Our results indicate that for the FePc on Si(100), the molecules within the film are mainly standing on the surface. The experimental N1s XAS spectra are very well reproduced by the theoretical calculations, which are both angle and atomic resolved, giving a detailed description of the electronic and geometric structure of the FePc film. Furthermore, the asymmetry and the intensity angle variation of the first N1s XAS threshold feature could be explained by the presented DFT calculations as due to the chemical nonequivalence of the N atoms and the symmetry character of the lowest unoccupied molecular orbital.
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