Prof. Dr. Georg Koller » People

@article{Hurdax2022,

title = {Large Distortion of Fused Aromatics on Dielectric Interlayers Quantified by Photoemission Orbital Tomography},

author = {P. Hurdax and C. S. Kern and T. G. Boné and A. Haags and M. Hollerer and L. Egger and X. Yang and H. Kirschner and A. Gottwald and M. Richter and F. C. Bocquet and S. Soubatch and G. Koller and F. S. Tautz and M. Sterrer and P. Puschnig and M. G. Ramsey},

doi = {10.1021/acsnano.2c08631},

year  = {2022},

date = {2022-01-01},

journal = {ACS Nano},

volume = {16},

pages = {17435-17443},

abstract = {Polycyclic aromatic compounds with fused benzene rings offer an extraordinary versatility as next-generation organic semiconducting materials for nanoelectronics and optoelectronics due to their tunable characteristics, including charge-carrier mobility and optical absorption. Nonplanarity can be an additional parameter to customize their electronic and optical properties without changing the aromatic core. In this work, we report a combined experimental and theoretical study in which we directly observe large, geometry-induced modifications in the frontier orbitals of a prototypical dye molecule when adsorbed on an atomically thin dielectric interlayer on a metallic substrate. Experimentally, we employ angle-resolved photoemission experiments, interpreted in the framework of the photoemission orbital tomography technique. We demonstrate its sensitivity to detect geometrical bends in adsorbed molecules and highlight the role of the photon energy used in experiment for detecting such geometrical distortions. Theoretically, we conduct density functional calculations to determine the geometric and electronic structure of the adsorbed molecule and simulate the photoemission angular distribution patterns. While we found an overall good agreement between experimental and theoretical data, our results also unveil limitations in current van der Waals corrected density functional approaches for such organic/dielectric interfaces. Hence, photoemission orbital tomography provides a vital experimental benchmark for such systems. By comparison with the state of the same molecule on a metallic substrate, we also offer an explanation why the adsorption on the dielectric induces such large bends in the molecule.},

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@article{Yang2022,

title = {Momentum-selective orbital hybridization},

author = {X. Yang and M. Jugovac and G. Zamborlini and V. Feyer and G. Koller and P. Puschnig and S. Soubatch and M. G. Ramsey and F. S. Tautz},

doi = {10.1038/s41467-022-32643-z},

year  = {2022},

date = {2022-01-01},

journal = {Nat. Commun.},

volume = {13},

pages = {5148},

abstract = {When a molecule interacts chemically with a metal surface, the orbitals of the molecule hybridise with metal states to form the new eigenstates of the coupled system. Spatial overlap and energy matching are determining parameters of the hybridisation. However, since every molecular orbital does not only have a characteristic spatial shape, but also a specific momentum distribution, one may additionally expect a momentum matching condition; after all, each hybridising wave function of the metal has a defined wave vector, too. Here, we report photoemission orbital tomography measurements of hybrid orbitals that emerge from molecular orbitals at a molecule-on-metal interface. We find that in the hybrid orbitals only those partial waves of the original orbital survive which match the metal band structure. Moreover, we find that the conversion of the metal’s surface state into a hybrid interface state is also governed by momentum matching constraints. Our experiments demonstrate the possibility to measure hybridisation momentum-selectively, thereby enabling deep insights into the complicated interplay of bulk states, surface states, and molecular orbitals in the formation of the electronic interface structure at molecule-on-metal hybrid interfaces.},

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@article{Haags2021,

title = {Momentum-space imaging of σ-orbitals for chemical analysis},

author = {A. Haags and X. Yang and L. Egger and D. Brandstetter and H. Kirschner and F. C. Bocquet and G. Koller and A. Gottwald and M. Richter and J. M. Gottfried and M. G. Ramsey and P. Puschnig and S. Soubatch and F. S. Tautz},

doi = {10.1126/sciadv.abn0819},

year  = {2022},

date = {2022-01-01},

journal = {Sci. Adv.},

volume = {8},

pages = {eabn0819},

abstract = {Tracing the modifications of molecules in surface chemical reactions benefits from the possibility to image their orbitals. While delocalized frontier orbitals with π character are imaged routinely with photoemission orbital tomography, they are not always sensitive to local chemical modifications, particularly the making and breaking of bonds at the molecular periphery. For such bonds, σ orbitals would be far more revealing. Here, we show that these orbitals can indeed be imaged in a remarkably broad energy range and that the plane wave approximation, an important ingredient of photoemission orbital tomography, is also well fulfilled for these orbitals. This makes photoemission orbital tomography a unique tool for the detailed analysis of surface chemical reactions. We demonstrate this by identifying the reaction product of a dehalogenation and cyclodehydrogenation reaction.},

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tppubtype = {article}

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@article{Saettele2022,

title = {Hexacene on Cu(110) and Ag(110): Influence of the Substrate on Molecular Orientation and Interfacial Charge Transfer},

author = {M. S. Sättele and A. Windischbacher and K. Greulich and L. Egger and A. Haags and H. Kirschner and R. Ovsyannikov and E. Giangrisostomi and A. Gottwald and M. Richter and S. Soubatch and F. S. Tautz and M. G. Ramsey and P. Puschnig and G. Koller and H. F. Bettinger and T. Chassé and H. Peisert},

doi = {10.1021/acs.jpcc.2c00081},

year  = {2022},

date = {2022-01-01},

journal = {J. Phys. Chem. C},

volume = {126},

pages = {5036-5045},

abstract = {Hexacene, composed of six linearly fused benzene rings, is an organic semiconductor material with superior electronic properties. The fundamental understanding of the electronic and chemical properties is prerequisite to any possible application in devices. We investigate the orientation and interface properties of highly ordered hexacene monolayers on Ag(110) and Cu(110) with X-ray photoemission spectroscopy (XPS), photoemission orbital tomography (POT), X-ray absorption spectroscopy (XAS), low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and density functional theory (DFT). We find pronounced differences in the structural arrangement of the molecules and the electronic properties at the metal/organic interfaces for the two substrates. While on Cu(110) the molecules adsorb with their long molecular axis parallel to the high symmetry substrate direction, on Ag(110), hexacene adsorbs in an azimuthally slightly rotated geometry with respect to the metal rows of the substrate. In both cases, molecular planes are oriented parallel to the substrate. A pronounced charge transfer from both substrates to different molecular states affects the effective charge of different C atoms of the molecule. Through analysis of experimental and theoretical data, we found out that on Ag(110) the LUMO of the molecule is occupied through charge transfer from the metal, whereas on Cu(110) even the LUMO+1 receives a charge. Interface dipoles are determined to a large extent by the push-back effect, which are also found to differ significantly between 6A/Ag(110) and 6A/Cu(110).},

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@article{Saettele2020,

title = {Going beyond Pentacene: Photoemission Tomography of a Heptacene Monolayer on Ag(110)},

author = {M. S. Sättele and A. Windischbacher and L. Egger and A. Haags and P. Hurdax and H. Kirschner and A. Gottwald and M. Richter and F. C. Bocquet and S. Soubatch and F. S. Tautz and H. F. Bettinger and H. Peisert and T. Chassé and M. G. Ramsey and P. Puschnig and G. Koller},

doi = {10.1021/acs.jpcc.0c09062},

year  = {2021},

date = {2021-01-01},

journal = {J. Phys. Chem. C},

volume = {125},

pages = {2918-2925},

abstract = {Longer acenes such as heptacene are promising candidates for optoelectronic applications but are unstable in their bulk structure as they tend to dimerize. This makes the growth of well-defined monolayers and films problematic. In this article, we report the successful preparation of a highly oriented monolayer of heptacene on Ag(110) by thermal cycloreversion of diheptacenes. In a combined effort of angle-resolved photoemission spectroscopy and density functional theory (DFT) calculations, we characterize the electronic and structural properties of the molecule on the surface in detail. Our investigations allow us to unambiguously confirm the successful fabrication of a highly oriented complete monolayer of heptacene and to describe its electronic structure. By comparing experimental momentum maps of photoemission from frontier orbitals of heptacene and pentacene, we shed light on differences between these two acenes regarding their molecular orientation and energy-level alignment on the metal surfaces.},

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@article{Egger2020,

title = {Charge-promoted self-metalation of porphyrins on an oxide surface},

author = {L. Egger and M. Hollerer and C. S. Kern and H. Herrmann and P. Hurdax and A. Haags and X. Yang and A. Gottwald and M. Richter and S. Soubatch and F. S. Tautz and G. Koller and P. Puschnig and M. G. Ramsey and M. Sterrer},

doi = {10.1002/anie.202015187},

year  = {2021},

date = {2021-01-01},

journal = {Angew. Chem. Int. Ed.},

volume = {60},

pages = {5078-5082},

abstract = {Metalation and self-metalation reactions of porphyrins on oxide surfaces have recently gained interest. The mechanism of porphyrin self-metalation on oxides is, however, far from being understood. Herein, we show by a combination of results obtained with scanning tunneling microscopy, photoemission spectroscopy, and DFT computations, that the self-metalation of 2H-tetraphenylporphyrin on the surface of ultrathin MgO(001) films is promoted by charge transfer. By tuning the work function of the MgO(001)/Ag(001) substrate, we are able to control the charge and the metalation state of the porphyrin molecules on the surface.},

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@article{Bone2021,

title = {Demonstrating the Impact of the Adsorbate Orientation on the Charge Transfer at Organic-Metal Interfaces},

author = {T. G. Boné and A. Windischbacher and M. S. Sättele and K. Greulich and L. Egger and T. Jauk and F. Lackner and H. F. Bettinger and H. Peisert and T. Chassé and M. G. Ramsey and M. Sterrer and G. Koller and P. Puschnig},

doi = {10.1021/acs.jpcc.1c01306},

year  = {2021},

date = {2021-01-01},

journal = {J. Phys. Chem. C},

volume = {125},

pages = {9129-9137},

abstract = {Charge-transfer processes at molecule–metal interfaces play a key role in tuning the charge injection properties in organic-based devices and thus, ultimately, the device performance. Here, the metal’s work function and the adsorbate’s electron affinity are the key factors that govern the electron transfer at the organic/metal interface. In our combined experimental and theoretical work, we demonstrate that the adsorbate’s orientation may also be decisive for the charge transfer. By thermal cycloreversion of diheptacene isomers, we manage to produce highly oriented monolayers of the rodlike, electron-acceptor molecule heptacene on a Cu(110) surface with molecules oriented either along or perpendicular to the close-packed metal rows. This is confirmed by scanning tunneling microscopy (STM) images as well as by angle-resolved ultraviolet photoemission spectroscopy (ARUPS). By utilizing photoemission tomography momentum maps, we show that the lowest unoccupied molecular orbital (LUMO) is fully occupied and also, the LUMO + 1 gets significantly filled when heptacene is oriented along the Cu rows. Conversely, for perpendicularly aligned heptacene, the molecular energy levels are shifted significantly toward the Fermi energy, preventing charge transfer to the LUMO + 1. These findings are fully confirmed by our density functional calculations and demonstrate the possibility to tune the charge transfer and level alignment at organic–metal interfaces through the adjustable molecular alignment.},

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@article{Haags2020,

title = {Kekulene: On-Surface Synthesis, Orbital Structure, and Aromatic Stabilization},

author = {A. Haags and A. Reichmann and Q. Fan and L. Egger and H. Kirschner and T. Naumann and S. Werner and T. Vollgraff and J. Sundermeyer and L. Eschmann and X. Yang and D. Brandstetter and F. C. Bocquet and G. Koller and A. Gottwald and M. Richter and M. G. Ramsey and M. Rohlfing and P. Puschnig and J. M. Gottfried and S. Soubatch and F. S. Tautz},

doi = {10.1021/acsnano.0c06798},

year  = {2020},

date = {2020-01-01},

journal = {ACS Nano},

volume = {14},

pages = {15766-15775},

abstract = {We revisit the question of kekulene’s aromaticity by focusing on the electronic structure of its frontier orbitals as determined by angle-resolved photoemission spectroscopy. To this end, we have developed a specially designed precursor, 1,4,7(2,7)-triphenanthrenacyclononaphane-2,5,8-triene, which allows us to prepare sufficient quantities of kekulene of high purity directly on a Cu(111) surface, as confirmed by scanning tunneling microscopy. Supported by density functional calculations, we determine the orbital structure of kekulene’s highest occupied molecular orbital by photoemission tomography. In agreement with a recent aromaticity assessment of kekulene based solely on C–C bond lengths, we conclude that the π-conjugation of kekulene is better described by the Clar model rather than a superaromatic model. Thus, by exploiting the capabilities of photoemission tomography, we shed light on the question which consequences aromaticity holds for the frontier electronic structure of a π-conjugated molecule.},

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@article{Hurdax2020a,

title = {Controlling the electronic and physical coupling on dielectric thin films},

author = {P. Hurdax and M. Hollerer and L. Egger and G. Koller and X. Yang and A. Haags and S. Soubatch and F. S. Tautz and M. Richter and A. Gottwald and P. Puschnig and M. Sterrer and M. G. Ramsey},

doi = {10.3762/bjnano.11.132},

year  = {2020},

date = {2020-01-01},

journal = {Beilstein J. Nanotechnol.},

volume = {11},

pages = {1492-1503},

abstract = {Ultrathin dielectric/insulating films on metals are often used as decoupling layers to allow for the study of the electronic properties of adsorbed molecules without electronic interference from the underlying metal substrate. However, the presence of such decoupling layers may effectively change the electron donating properties of the substrate, for example, by lowering its work function and thus enhancing the charging of the molecular adsorbate layer through electron tunneling. Here, an experimental study of the charging of para-sexiphenyl (6P) on ultrathin MgO(100) films supported on Ag(100) is reported. By deliberately changing the work function of the MgO(100)/Ag(100) system, it is shown that the charge transfer (electronic coupling) into the 6P molecules can be controlled, and 6P monolayers with uncharged molecules (Schottky–Mott regime) and charged and uncharged molecules (Fermi level pinning regime) can be obtained. Furthermore, it was found that charge transfer and temperature strongly influence the orientation, conformation, and wetting behavior (physical coupling) of the 6P layers on the MgO(100) thin films.},

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pubstate = {published},

tppubtype = {article}

}

@article{Egger2018,

title = {Can photoemission tomography be useful for small, strongly-interacting adsorbate systems?},

author = {L. Egger and B. Kollmann and P. Hurdax and D. Lüftner and X. Yang and S. Weiß and A. Gottwald and M. Richter and G. Koller and S. Soubatch and F. S. Tautz and P. Puschnig and M. G. Ramsey},

doi = {10.1088/1367-2630/ab0781},

year  = {2019},

date = {2019-01-01},

journal = {New J. Phys.},

volume = {21},

pages = {043003},

abstract = {Molecular orbital tomography, also termed photoemission tomography, which considers the final state as a simple plane wave, has been very successful in describing the photoemisson distribution of large adsorbates on noble metal surfaces. Here, following a suggestion by Bradshaw and Woodruff (2015 New J. Phys. 17 013033), we consider a small and strongly-interacting system, benzene adsorbed on palladium (110), to consider the extent of the problems that can arise with the final state simplification. Our angle-resolved photoemission experiments, supported by density functional theory calculations, substantiate and refine the previously determined adsorption geometry and reveal an energetic splitting of the frontier π-orbital due to a symmetry breaking which has remained unnoticed before. We find that, despite the small size of benzene and the comparably strong interaction with palladium, the overall appearance of the photoemission angular distributions can basically be understood within a plane wave final state approximation and yields a deeper understanding of the electronic structure of the interface. There are, however, noticeable deviations between measured and simulated angular patterns which we ascribe to molecule-substrate interactions and effects beyond a plane-wave final state description.},

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tppubtype = {article}

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@article{Yang2019,

title = {Identifying surface reaction intermediates with photoemission tomography},

author = {X. Yang and L. Egger and P. Hurdax and H. Kaser and D. Lüftner and F. C. Bocquet and G. Koller and A. Gottwald and P. Tegeder and M. Richter and M. G. Ramsey and P. Puschnig and S. Soubatch and F. S. Tautz},

doi = {10.1038/s41467-019-11133-9},

year  = {2019},

date = {2019-01-01},

journal = {Nat. Commun.},

volume = {10},

pages = {3189},

abstract = {The determination of reaction pathways and the identification of reaction intermediates are key issues in chemistry. Surface reactions are particularly challenging, since many methods of analytical chemistry are inapplicable at surfaces. Recently, atomic force microscopy has been employed to identify surface reaction intermediates. While providing an excellent insight into the molecular backbone structure, atomic force microscopy is less conclusive about the molecular periphery, where adsorbates tend to react with the substrate. Here we show that photoemission tomography is extremely sensitive to the character of the frontier orbitals. Specifically, hydrogen abstraction at the molecular periphery is easily detected, and the precise nature of the reaction intermediates can be determined. This is illustrated with the thermally induced reaction of dibromo-bianthracene to graphene which is shown to proceed via a fully hydrogenated bisanthene intermediate. We anticipate that photoemission tomography will become a powerful companion to other techniques in the study of surface reaction pathways.},

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tppubtype = {article}

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@article{Yang2019a,

title = {Coexisting Charge States in a Unary Organic Monolayer Film on a Metal},

author = {X. Yang and L. Egger and J. Fuchsberger and M. Unzog and D. Lüftner and F. Hajek and P. Hurdax and M. Jugovac and G. Zamborlini and V. Feyer and G. Koller and P. Puschnig and F. S. Tautz and M. G. Ramsey and S. Soubatch},

doi = {10.1021/acs.jpclett.9b02231},

year  = {2019},

date = {2019-01-01},

journal = {J. Phys. Chem. Lett.},

volume = {10},

pages = {6438-6445},

abstract = {The electronic and geometric structures of tetracene films on Ag(110) and Cu(110) have been studied with photoemission tomography and compared to that of pentacene. Despite similar energy level alignment of the two oligoacenes on these surfaces revealed by conventional ultraviolet photoelectron spectroscopy, the momentum-space resolved photoemission tomography reveals a significant difference in both structural and electronic properties of tetracene and pentacene films. Particularly, the saturated monolayer of tetracene on Ag(110) is found to consist of two molecular species that, despite having the same orientation, are electronically very different—while one molecule remains neutral, another is charged because of electron donation from the substrate.},

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tppubtype = {article}

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@article{Yang2018,

title = {On the decoupling of molecules at metal surfaces},

author = {X. Yang and I. Krieger and D. Lüftner and S. Weiß and T. Heepenstrick and M. Hollerer and P. Hurdax and G. Koller and M. Sokolowski and P. Puschnig and M. G. Ramsey and F. S. Tautz and S. Soubatch},

doi = {10.1039/C8CC03334J},

year  = {2018},

date = {2018-01-01},

urldate = {2018-01-01},

journal = {Chem. Commun.},

volume = {54},

pages = {9039-9042},

abstract = {We report a method to achieve physical and electronic decoupling of organic molecules from a metal surface. Oxygen adsorbed on the Cu(100) surface immobilizes the surface electrons in the Cu–O covalent bonds. This results in electronic surface hardening and prevents charge transfer from the metal into perylene-tetracarboxylic dianhydride molecules subsequently deposited on this surface.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Puschnig2016,

title = {Energy ordering of molecular orbitals},

author = {P. Puschnig and A. D. Boese and M. Willenbockel and M. Meyer and D. Lüftner and E. M. Reinisch and T. Ules and G. Koller and S. Soubatch and M. G. Ramsey and F. S. Tautz},

doi = {10.1021/acs.jpclett.6b02517},

year  = {2017},

date = {2017-01-01},

journal = {J. Phys. Chem. Lett.},

volume = {8},

pages = {208-213},

abstract = {Orbitals are invaluable in providing a model of bonding in molecules or between molecules and surfaces. Most present-day methods in computational chemistry begin by calculating the molecular orbitals of the system. To what extent have these mathematical objects analogues in the real world? To shed light on this intriguing question, we employ a photoemission tomography study on monolayers of 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) grown on three Ag surfaces. The characteristic photoelectron angular distribution enables us to assign individual molecular orbitals to the emission features. When comparing the resulting energy positions to density functional calculations, we observe deviations in the energy ordering. By performing complete active space calculations (CASSCF), we can explain the experimentally observed orbital ordering, suggesting the importance of static electron correlation beyond a (semi)local approximation. On the other hand, our results also show reality and robustness of the orbital concept, thereby making molecular orbitals accessible to experimental observations.},

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pubstate = {published},

tppubtype = {article}

}

@article{Hollerer2017,

title = {Charge Transfer and Orbital Level Alignment at Inorganic/Organic Interfaces: The Role of Dielectric Interlayers},

author = {M. Hollerer and D. Lüftner and P. Hurdax and T. Ules and S. Soubatch and F. S. Tautz and G. Koller and P. Puschnig and M. Sterrer and M. G. Ramsey},

doi = {10.1021/acsnano.7b02449},

year  = {2017},

date = {2017-01-01},

journal = {ACS Nano},

volume = {11},

pages = {6252-6260},

abstract = {It is becoming accepted that ultrathin dielectric layers on metals are not merely passive decoupling layers, but can actively influence orbital energy level alignment and charge transfer at interfaces. As such, they can be important in applications ranging from catalysis to organic electronics. However, the details at the molecular level are still under debate. In this study, we present a comprehensive analysis of the phenomenon of charge transfer promoted by a dielectric interlayer with a comparative study of pentacene adsorbed on Ag(001) with and without an ultrathin MgO interlayer. Using scanning tunneling microscopy and photoemission tomography supported by density functional theory, we are able to identify the orbitals involved and quantify the degree of charge transfer in both cases. Fractional charge transfer occurs for pentacene adsorbed on Ag(001), while the presence of the ultrathin MgO interlayer promotes integer charge transfer with the lowest unoccupied molecular orbital transforming into a singly occupied and singly unoccupied state separated by a large gap around the Fermi energy. Our experimental approach allows a direct access to the individual factors governing the energy level alignment and charge-transfer processes for molecular adsorbates on inorganic substrates.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Lueftner2017,

title = {Understanding the photoemission distribution of strongly interacting two-dimensional overlayers},

author = {D. Lüftner and S. Weiß and X. Yang and P. Hurdax and V. Feyer and A. Gottwald and G. Koller and S. Soubatch and P. Puschnig and M. G. Ramsey and F. S. Tautz},

doi = {10.1103/PhysRevB.96.125402},

year  = {2017},

date = {2017-01-01},

journal = {Phys. Rev. B},

volume = {96},

pages = {125402},

abstract = {Photoemission tomography (PT), the analysis of the photoemission intensity distribution within the plane wave final-state approximation, is being established as a useful tool for extracting the electronic and geometric structure of weakly interacting organic overlayers. Here we present a simple method for extending PT, which until now has been based on the calculations of isolated molecules. By including the substrate and a damped plane-wave final state, we are able to simulate the photoemission intensity distribution of two-dimensional molecular overlayers with both strong intermolecular and molecule-substrate interactions, here demonstrated for the model system 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) on Cu(100). It is shown that the interaction and hybridization of the lowest unoccupied molecular orbital of PTCDA with substrate states leads to its occupation and the formation of a strongly dispersing intermolecular band, whose experimental magnitude of 1.1 eV and k-space periodicity is well reproduced theoretically.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Reinisch2016,

title = {Layer-resolved photoemission tomography: The $p$-sexiphenyl bilayer upon Cs doping},

author = {E. M. Reinisch and P. Puschnig and T. Ules and M. G. Ramsey and G. Koller},

doi = {10.1103/PhysRevB.93.155438},

year  = {2016},

date = {2016-04-01},

journal = {Phys. Rev. B},

volume = {93},

issue = {15},

pages = {155438},

abstract = {The buried interface between a molecular thin film and the metal substrate is generally not accessible to the photoemission experiment. With the example of a sexiphenyl (6P) bilayer on Cu we show that photoemission tomography can be used to study the electronic level alignment and geometric structure, where it was possible to assign the observed orbital emissions to the individual layers. We further study the Cs doping of this bilayer. Initial Cs exposure leads to a doping of only the first interface layer, leaving the second layer unaffected except for a large energy shift. This result shows that it is in principle possible to chemically modify just the interface, which is important to issues like tuning of the energy level alignment and charge transfer to the interface layer. Upon saturating the film with Cs, photoemission tomography shows a complete doping (6p4−) of the bilayer, with the molecular geometry changing such that the spectra become dominated by σ-orbital emissions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Koller2016,

title = {Elektronenorbitale in 3D - Photoelektronen-tomographische Bilder von Molekülorbitalen},

author = {G. Koller and P. Puschnig and A. Gottwald and F. S. Tautz},

doi = {10.1002/piuz.201601442},

year  = {2016},

date = {2016-01-01},

urldate = {2016-01-01},

journal = {Physik in unserer Zeit},

volume = {47},

pages = {192-198},

abstract = {Die winkelaufgelöste Photoelektronen-Spektroskopie, Photoelektronen-Tomographie genannt, erlaubt die Rekonstruktion von Molekülorbitalen in drei Dimensionen. Dazu werden auf einer Metalloberfläche angeordnete Moleküle mit extremem ultravioletten Licht bestrahlt und die Winkel- und Energieverteilung der über den photoelektrischen Effekt herausgelösten Elektronen gemessen. Die Ergebnisse sind ein weiterer Beleg für das Konzept der Molekülorbitale, denen der Orbitaltheoretiker Kenichi Fukui 1977 eine “irgendwie unwirkliche Natur” zuschrieb. Anders als zum Beispiel Rastersonden-Methoden funktioniert die Photoelektronen-Tomographie auch bei Zimmertemperatur. Sie kann zudem Orbitale organischer Moleküle auf reaktiven Substraten abbilden. In Zukunft könnte sie auch 3D-Bilder von dynamischen Veränderungen in Orbitalen, zum Beispiel während chemischen Reaktionen, liefern.},

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pubstate = {published},

tppubtype = {article}

}

@article{Ules2016,

title = {Continuous or discrete: Tuning the energy level alignment of organic layers with alkali dopants},

author = {T. Ules and D. Lüftner and E. M. Reinisch and G. Koller and P. Puschnig and M. G. Ramsey},

doi = {10.1103/PhysRevB.94.205405},

year  = {2016},

date = {2016-01-01},

journal = {Phys. Rev. B},

volume = {94},

pages = {205405},

abstract = {This paper investigates the effects of cesium (Cs) deposited on pentacene (5A) and sexiphenyl (6P) monolayers on the Ag(110) substrate. The process of doping and the energy level alignment are studied quantitatively and contrasted. While ultimately for both molecules lowest unoccupied molecular orbital (LUMO) filling on charge transfer upon Cs dosing is observed, the doping processes are tellingly different. In the case of 5A, hybrid molecule-substrate states and doping states coexist at lowest Cs exposures, while for 6P doping states appear only after Cs has completely decoupled the monolayer from the substrate. With the support of density functional theory calculations, this different behavior is rationalized by the local character of electrostatic potential changes induced by dopants in relation to the spatial extent of the molecules. This also has severe effects on the energy level alignment, which for most dopant/molecule systems cannot be considered continuous but discrete.},

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pubstate = {published},

tppubtype = {article}

}

@article{Willenbockel2014,

title = {The interplay between interface structure, energy level alignment and chemical bonding strength at organic-metal interfaces},

author = {M. Willenbockel and D. Lüftner and B. Stadtmüller and G. Koller and C. Kumpf and S. Soubatch and P. Puschnig and M. G. Ramsey and F. S. Tautz},

doi = {10.1039/C4CP04595E},

year  = {2015},

date = {2015-01-01},

journal = {Phys. Chem. Chem. Phys.},

volume = {17},

pages = {1530-1548},

abstract = {What do energy level alignments at metal–organic interfaces reveal about the metal–molecule bonding strength? Is it permissible to take vertical adsorption heights as indicators of bonding strengths? In this paper we analyse 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on the three canonical low index Ag surfaces to provide exemplary answers to these questions. Specifically, we employ angular resolved photoemission spectroscopy for a systematic study of the energy level alignments of the two uppermost frontier states in ordered monolayer phases of PTCDA. Data are analysed using the orbital tomography approach. This allows the unambiguous identification of the orbital character of these states, and also the discrimination between inequivalent species. Combining this experimental information with DFT calculations and the generic Newns–Anderson chemisorption model, we analyse the alignments of highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) with respect to the vacuum levels of bare and molecule-covered surfaces. This reveals clear differences between the two frontier states. In particular, on all surfaces the LUMO is subject to considerable bond stabilization through the interaction between the molecular π-electron system and the metal, as a consequence of which it also becomes occupied. Moreover, we observe a larger bond stabilization for the more open surfaces. Most importantly, our analysis shows that both the orbital binding energies of the LUMO and the overall adsorption heights of the molecule are linked to the strength of the chemical interaction between the molecular π-electron system and the metal, in the sense that stronger bonding leads to shorter adsorption heights and larger orbital binding energies.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Weiss2015,

title = {Exploring three-dimensional orbital imaging with energy-dependent photoemission tomography},

author = {S. Weiß and D. Lüftner and T. Ules and E. M. Reinisch and H. Kaser and A. Gottwald and M. Richter and S. Soubatch and G. Koller and M. G. Ramsey and F. S. Tautz and P. Puschnig},

doi = {10.1038/ncomms9287},

year  = {2015},

date = {2015-01-01},

journal = {Nat. Commun.},

volume = {6},

pages = {8287},

abstract = {Recently, it has been shown that experimental data from angle-resolved photoemission spectroscopy on oriented molecular films can be utilized to retrieve real-space images of molecular orbitals in two dimensions. Here, we extend this orbital tomography technique by performing photoemission initial state scans as a function of photon energy on the example of the brickwall monolayer of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on Ag(110). The overall dependence of the photocurrent on the photon energy can be well accounted for by assuming a plane wave for the final state. However, the experimental data, both for the highest occupied and the lowest unoccupied molecular orbital of PTCDA, exhibits an additional modulation attributed to final state scattering effects. Nevertheless, as these effects beyond a plane wave final state are comparably small, we are able, with extrapolations beyond the attainable photon energy range, to reconstruct three-dimensional images for both orbitals in agreement with calculations for the adsorbed molecule.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ramsey2015,

title = {Orbital tomography: molecular band maps, momentum maps and the imaging of real space orbitals of adsorbed molecules},

author = {H. Offenbacher and D. Lüftner and T. Ules and E. M. Reinisch and G. Koller and P. Puschnig and M. G. Ramsey},

doi = {10.1016/j.elspec.2015.04.023},

year  = {2015},

date = {2015-01-01},

journal = {J. Elec. Spec. Relat. Phenom.},

volume = {204A},

pages = {92-101},

abstract = {The frontier orbitals of molecules are the prime determinants of their chemical, optical and electronic properties. Arguably, the most direct method of addressing the (filled) frontier orbitals is ultra-violet photoemission spectroscopy (UPS). Although UPS is a mature technique from the early 1970s on, the angular distribution of the photoemitted electrons was thought to be too complex to be analysed quantitatively. Recently angle resolved UPS (ARUPS) work on conjugated molecules both, in ordered thick films and chemisorbed monolayers, has shown that the angular (momentum) distribution of the photocurrent from orbital emissions can be simply understood. The approach, based on the assumption of a plane wave final state is becoming known as orbital tomography. Here we will demonstrate, with selected examples of pentacene (5A) and sexiphenyl (6P), the potential of orbital tomography. First it will be shown how the full angular distribution of the photocurrent (momentum map) from a specific orbital is related to the real space orbital by a Fourier transform. Examples of the reconstruction of 5A orbitals will be given and the procedure for recovering the lost phase information will be outlined. We then move to examples of sexiphenyl where we interrogate the original band maps of thick sexiphenyl in the light of our understanding of orbital tomography that has developed since then. With comparison to theoretical simulations of the molecular band maps, the molecular conformation and orientation will be concluded. New results for the sexiphenyl monolayer on Al(1 1 0) will then be presented. From the band maps it will be concluded that the molecule is planarised and adopts a tilted geometry. Finally the momentum maps down to HOMO-11 will be analysed and real space orbitals reconstructed.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Ules2014,

title = {Orbital Tomography of Hybridized and Dispersing Molecular Overlayers},

author = {T. Ules and D. Lüftner and E. M. Reinisch and G. Koller and P. Puschnig and M. G. Ramsey},

doi = {10.1103/PhysRevB.90.155430},

year  = {2014},

date = {2014-01-01},

journal = {Phys. Rev. B},

volume = {90},

pages = {155430},

abstract = {With angle-resolved photoemission experiments and ab initio electronic structure calculations, the pentacene monolayers on Ag(110) and Cu(110) are compared and contrasted, allowing the molecular orientation to be determined and an unambiguous assignment of emissions to specific orbitals to be made. On Ag(110), the orbitals remain essentially isolated-molecule-like, while strong substrate-enhanced dispersion and orbital modification are observed upon adsorption on Cu(110). We show how the photoemission intensity of extended systems can be simulated and that it behaves essentially like that of the isolated molecule modulated by the band dispersion due to intermolecular interactions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Stadtmuller2013,

title = {Unexpected interplay of bonding height and energy level alignment at heteromolecular hybrid interfaces},

author = {B. Stadtmüller and D. Lüftner and M. Willenbockel and E. M. Reinisch and T. Sueyoshi and G. Koller and S. Soubatch and M. G. Ramsey and P. Puschnig and F. S. Tautz and C. Kumpf},

doi = {10.1038/ncomms4685},

year  = {2014},

date = {2014-01-01},

journal = {Nat. Commun.},

volume = {5},

pages = {3685},

abstract = {Although geometric and electronic properties of any physical or chemical system are always mutually coupled by the rules of quantum mechanics, counterintuitive coincidences between the two are sometimes observed. The coadsorption of the organic molecules 3,4,9,10-perylene tetracarboxylic dianhydride and copper-II-phthalocyanine on Ag(111) represents such a case, since geometric and electronic structures appear to be decoupled: one molecule moves away from the substrate while its electronic structure indicates a stronger chemical interaction, and vice versa for the other. Our comprehensive experimental and ab-initio theoretical study reveals that, mediated by the metal surface, both species mutually amplify their charge-donating and -accepting characters, respectively. This resolves the apparent paradox, and demonstrates with exceptional clarity how geometric and electronic bonding parameters are intertwined at metal–organic interfaces.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Reinisch2013,

title = {Development and character of gap states on alkali doping of molecular films},

author = {E. M. Reinisch and T. Ules and P. Puschnig and S. Berkebile and M. Ostler and T. Seyller and M. G. Ramsey and G. Koller},

doi = {10.1088/1367-2630/16/2/023011},

year  = {2014},

date = {2014-01-01},

journal = {New J. Phys.},

volume = {16},

pages = {023011},

abstract = {Here we study the alkali metal induced effects on an ordered and aligned sexiphenyl monolayer on Cu(110) with angle-resolved UV spectroscopy (ARUPS). The caesium (Cs) induced gap states could clearly be identified by orbital tomography, a method based on ARUPS, which allows both the orbital character of these states and the molecular orientation to be determined. We show that with increasing alkali metal dose, doping proceeds in three distinct steps. Initially, Cs decouples the molecular monolayer from the substrate, with emptying of the lowest unoccupied molecular orbital (LUMO) that had been filled on hybridization with the substrate. Further Cs exposure refills the LUMO. Finally a filling of the LUMO+1 by charge transfer from the alkali metal occurs. Remarkably, although long range order is not preserved and the molecular planes tilt away from the surface, the molecules remain aligned parallel to the $[1 bar 1 0]$ azimuth during the whole doping process.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Luftner2013,

title = {Imaging the wave functions of adsorbed molecules},

author = {D. Lüftner and T. Ules and E. M. Reinisch and G. Koller and S. Soubatch and F. S. Tautz and M. G. Ramsey and P. Puschnig},

doi = {10.1073/pnas.1315716110},

year  = {2014},

date = {2014-01-01},

journal = {PNAS},

volume = {111},

number = {2},

pages = {605-610},

abstract = {In quantum mechanics, the electrons in a molecule are described by a mathematical object termed the wave function or molecular orbital. This function determines the chemical and physical properties of matter and consequently there has been much interest in measuring orbitals, despite the fact that strictly speaking they are not quantum-mechanical observables. We show how the amplitude and phase of orbitals can be measured in good agreement with wave functions from ab initio calculations. Not only do such measurements allow wave functions of complex molecules and nanostructures to be determined, they also open up a window into critical discussions of theoretical orbital concepts.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Willenbockel2012,

title = {Energy offsets within a molecular monolayer: The influence of the molecular environment},

author = {M. Willenbockel and B. Stadtmüller and K. Schönauer and F. C. Bocquet and D. Lüftner and E. M. Reinisch and T. Ules and G. Koller and C. Kumpf and S. Soubatch and P. Puschnig and M. G. Ramsey and F. S. Tautz},

doi = {10.1088/1367-2630/15/3/033017},

year  = {2013},

date = {2013-01-01},

journal = {New J. Phys.},

volume = {15},

pages = {033017},

abstract = {The compressed 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) herringbone monolayer structure on Ag(110) is used as a model system to investigate the role of molecule–molecule interactions at metal–organic interfaces. By means of the orbital tomography technique, we can not only distinguish the two inequivalent molecules in the unit cell but also resolve their different energy positions for the highest occupied and the lowest unoccupied molecular orbitals. Density functional theory calculations of a freestanding PTCDA layer identify the electrostatic interaction between neighboring molecules, rather than the adsorption site, as the main reason for the molecular level splitting observed experimentally.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Stadtmuller2012a,

title = {Orbital tomography for highly symmetric adsorbate systems},

author = {B. Stadtmüller and M. Willenbockel and E. M. Reinisch and T. Ules and F. C. Bocquet and S. Soubatch and P. Puschnig and G. Koller and M. G. Ramsey and F. S. Tautz and C. Kumpf},

doi = {10.1209/0295-5075/100/26008},

year  = {2012},

date = {2012-01-01},

journal = {Europhys. Lett.},

volume = {100},

pages = {26008},

abstract = {Orbital tomography is a new and very powerful tool to analyze the angular distribution of a photoemission spectroscopy experiment. It was successfully used for organic adsorbate systems to identify (and consequently deconvolute) the contributions of specific molecular orbitals to the photoemission data. The technique was so far limited to surfaces with low symmetry like fcc(110) oriented surfaces, owing to the small number of rotational domains that occur on such surfaces. In this letter we overcome this limitation and present an orbital tomography study of a 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) monolayer film adsorbed on Ag(111). Although this system exhibits twelve differently oriented molecules, the angular resolved photoemission data still allow a meaningful analysis of the different local density of states and reveal different electronic structures for symmetrically inequivalent molecules. We also discuss the precision of the orbital tomography technique in terms of counting statistics and linear regression fitting algorithm. Our results demonstrate that orbital tomography is not limited to low-symmetry surfaces, a finding which makes a broad field of complex adsorbate systems accessible to this powerful technique.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Puschnig2011,

title = {Orbital tomography: Deconvoluting photoemission spectra of organic molecules},

author = {P. Puschnig and E. M. Reinisch and T. Ules and G. Koller and S. Soubatch and M. Ostler and L. Romaner and F. S. Tautz and C. Ambrosch-Draxl and M. G. Ramsey},

doi = {10.1103/PhysRevB.84.235427},

year  = {2011},

date = {2011-01-01},

journal = {Phys. Rev. B},

volume = {84},

pages = {235427},

abstract = {We study the interface of an organic monolayer with a metallic surface, i.e., PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) on Ag(110), by means of angle-resolved photoemission spectroscopy (ARPES) and ab initio electronic structure calculations. We present a tomographic method that uses the energy and momentum dependence of ARPES data to deconvolute spectra into individual orbital contributions beyond the limits of energy resolution. This provides an orbital-by-orbital characterization of large adsorbate systems without the need to invoke a sophisticated theory of photoemission, allowing us to directly estimate the effects of bonding on individual orbitals. Moreover, these experimental data serve as a most stringent test necessary for the further development of ab initio electronic structure theory.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Berkebile2011,

title = {A momentum space view of the surface chemical bond},

author = {S. Berkebile and T. Ules and P. Puschnig and L. Romaner and G. Koller and A. J. Fleming and K. Emtsev and T. Seyller and C. Ambrosch-Draxl and F. P. Netzer and M. G. Ramsey},

doi = {10.1039/C0CP01458C},

year  = {2011},

date = {2011-01-01},

journal = {Phys. Chem. Chem. Phys.},

volume = {13},

pages = {3604-3611},

abstract = {Well-ordered and oriented monolayers of conjugated organic molecules can offer new perspectives on surface bonding. We will demonstrate the importance of the momentum distribution, or symmetry, of the adsorbate molecules' π orbitals in relation to the states available for hybridization at the metal surface. Here, the electronic band structure of the first monolayer of sexiphenyl on Cu(110) has been examined in detail with angle-resolved ultraviolet photoemission spectroscopy over a large momentum range and will be compared to measurements of a multilayer thin film and to density functional calculations. In the monolayer, the one-dimensional intramolecular band structure can still be recognized, allowing an accurate determination of orbital modification upon bonding and the relative energetic positions of the electronic levels. It is seen that the character of the molecular π orbitals is largely maintained despite strong mixing between Cu and molecular states and that the lowest unoccupied molecular orbital (LUMO) is filled by hybridization with Cu s,p states rather than through a charge transfer process. It is also shown that the momentum distribution of the substrate states involved and the periodicity of the molecular overlayer play a large role in the final E(k) distribution of the hybrid states. The distinct momentum distribution of the LUMO, interacting with the Cu substrate s,p valence bands around the gap in the surface projection of the bulk band structure, make this system a particularly illustrative example of momentum resolved hybridization. This system demonstrates that, for hybridization to occur, not only do states require overlap in energy and space, but also in momentum.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Puschnig2009a,

title = {Reconstruction of Molecular Orbital Densities from Photoemission Data},

author = {P. Puschnig and S. Berkebile and A. J. Fleming and G. Koller and K. Emtsev and T. Seyller and J. D. Riley and C. Ambrosch-Draxl and F. P. Netzer and M. G. Ramsey},

doi = {10.1126/science.1176105},

year  = {2009},

date = {2009-01-01},

journal = {Science},

volume = {326},

pages = {702-706},

abstract = {Photoemission spectroscopy is commonly applied to study the band structure of solids by measuring the kinetic energy versus angular distribution of the photoemitted electrons. Here, we apply this experimental technique to characterize discrete orbitals of large π-conjugated molecules. By measuring the photoemission intensity from a constant initial-state energy over a hemispherical region, we generate reciprocal space maps of the emitting orbital density. We demonstrate that the real-space electron distribution of molecular orbitals in both a crystalline pentacene film and a chemisorbed p-sexiphenyl monolayer can be obtained from a simple Fourier transform of the measurement data. The results are in good agreement with density functional calculations.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Berkebile2009,

title = {Angle-resolved photoemission of chain-like molecules: the electronic band structure of sexithiophene and sexiphenyl},

author = {S. Berkebile and G. Koller and P. Puschnig and C. Ambrosch-Draxl and F. P. Netzer and M. G. Ramsey},

doi = {10.1007/s00339-008-5034-9},

year  = {2009},

date = {2009-01-01},

journal = {Appl. Phys. A},

volume = {95},

pages = {101-105},

abstract = {Here we report the electronic π-band structure of sexithiophene obtained from 6T(010) oriented films. The angle-resolved valence band photoemission results taken parallel and perpendicular to the molecular axis are compared to those of sexiphenyl and interpreted in terms of intra- and inter-molecular band dispersion. We show that the strong photoemission intensity variations with emission angle parallel to the molecular axis are well reproduced by the Fourier transforms of the molecular orbitals of the isolated molecules. These results imply that ARUPS can yield quite detailed information about molecular geometry, both in terms of molecular orientation and internal structure.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Berkebile2009a,

title = {The electronic structure of pentacene revisited},

author = {S. Berkebile and G. Koller and A. J. Fleming and P. Puschnig and C. Ambrosch-Draxl and K. Emtsev and T. Seyller and J. D. Riley and M. G. Ramsey},

doi = {10.1016/j.elspec.2009.04.001},

year  = {2009},

date = {2009-01-01},

urldate = {2009-01-01},

journal = {J. Electron. Spectrosc. Relat. Phenom.},

volume = {174},

pages = {22-27},

abstract = {Recently, there have been reports of the valence band photoemission of pentacene films grown on various substrates with particular emphasis on the highest occupied molecular orbital (HOMO) and its dispersion. In various works, evidence for HOMO band dispersion as high as 0.5eV, even for polycrystalline films, has been presented. In apparent contradiction to these results, we have previously reported a band dispersion of only 50meV, measured on a well characterised film with a single polymorph and single crystalline orientation, 5A(022). Here, we first present the two-dimensional momentum distribution of the HOMO of a 5A(022) film. Then the development of the valence band spectra for films grown at room temperature and low temperature are compared, and we show that morphological aspects can lead to the apparent observation of high HOMO dispersion. Finally, with the aid of the two-dimensional momentum distribution of the HOMO, we show that a reasonably large dispersion (0.25eV) does indeed exist in 5A(022).},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Berkebile2008,

title = {Electronic band structure of pentacene: An experimental and theoretical study},

author = {S. Berkebile and P. Puschnig and G. Koller and M. Oehzelt and F. P. Netzer and C. Ambrosch-Draxl and M. G. Ramsey},

doi = {10.1103/PhysRevB.77.115312},

year  = {2008},

date = {2008-01-01},

journal = {Phys. Rev. B},

volume = {77},

pages = {115312},

abstract = {The intermolecular and intramolecular dispersions of pentacene are measured by angle resolved photoemission spectroscopy using a uniaxially aligned crystalline thin film. The band structure perpendicular to the molecules displays a small dispersion in agreement with density functional theory (DFT). Parallel to the molecules, two π bands consisting of five and six orbitals are clearly observed. In these intramolecular bands, the orbital emissions are shown to be in agreement with calculations of the photoemission intensities based on DFT both in terms of position and width in momentum space.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Koller2007,

title = {Intra- and Intermolecular Band Dispersion in an Organic Crystal},

author = {G. Koller and S. Berkebile and M. Oehzelt and P. Puschnig and C. Ambrosch-Draxl and F. P. Netzer and M. G. Ramsey},

doi = {10.1126/science.1143239},

year  = {2007},

date = {2007-01-01},

journal = {Science},

volume = {317},

pages = {351-355},

abstract = {The high crystallinity of many inorganic materials allows their band structures to be determined through angle-resolved photoemission spectroscopy (ARPES). Similar studies of conjugated organic molecules of interest in optoelectronics are often hampered by difficulties in growing well-ordered and well-oriented crystals or films. We have grown crystalline films of uniaxially oriented sexiphenyl molecules and obtained ARPES data. Supported by density-functional calculations, we show that, in the direction parallel to the principal molecular axis, a quasi–one-dimensional band structure of a system of well-defined finite size develops out of individual molecular orbitals. In contrast, perpendicular to the molecules, the band structure reflects the periodicity of the molecular crystal, and continuous bands with a large dispersion were observed.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}