We study the interface of an organic monolayer with a metallic surface, i.e., PTCDA (3,4,9,10-perylenetetracarboxylic-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.

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.

The basis for a quantum-mechanical description of matter is electron wave functions. For atoms and molecules, their spatial distributions and phases are known as orbitals. Although orbitals are very powerful concepts, experimentally only the electron densities and -energy levels are directly observable. Regardless whether orbitals are observed in real space with scanning probe experiments, or in reciprocal space by photoemission, the phase information of the orbital is lost. Here, we show that the experimental momentum maps of angle-resolved photoemission from molecular orbitals can be transformed to realspace orbitals via an iterative procedure which also retrieves the lost phase information. This is demonstrated with images obtained of a number of orbitals of the molecules pentacene (C 22 H 14 ) and perylene-3,4,9,10-tetracarboxylic dianhydride (C 24 H 8 O 6 ), adsorbed on silver, which are in excellent agreement with ab initio calculations. The procedure requires no a priori knowledge of the orbitals and is shown to be simple and robust.photoemission spectroscopy | surface science | organic molecules | density functional theory A s the electronic, optical, and chemical properties of nanostructures are defined by their electronic orbitals, in the last decades experimentalists have striven to image them. This is despite the fact that orbitals are not, strictly speaking, quantummechanical observables. Molecules are arguably the best-defined nanostructures, and for simple diatomic molecules, such as N 2 , both the amplitude and the phase of the highest occupied molecular orbital (HOMO) in three-dimensional space have been recovered (1). This tomographic reconstruction requires higher harmonics generated from intense femtosecond laser pulses, focused on a series of molecular alignments, together with theoretical modeling (1). Although offering the exciting prospect of imaging orbitals on the time scale of chemical reactions, being both complex and only appropriate for simple molecules and orbitals, the technique is not generally applicable for the task of orbital reconstruction. Alternatively, scanning probe techniques offer real-space imaging of large molecules with submolecular resolution on surfaces. Although great advances have been made with understanding and controlling scanning probe tips, so-called "tip functionalization" (2), tips are still a factor of uncertainty. With an appropriate tip, the correct nodal structure of orbitals can be directly observed. Moreover, with tip molecules of p-wave structure the relative phase of the sample wave function may be inferred (3). Unfortunately, as the wave functions of the substrate generally spill out beyond the adsorbed molecules, decoupling layers such as NaCl are necessary to avoid direct tunneling into the substrate.The full angle dependence of valence band UV photoelectron spectroscopy from molecular films has been shown to contain rich information on the orbital structure (4, 5). In the past few years a number of studies on molecular films have demonstrated a...

With angle resolved photoemission experiments and \emph{ab-initio} electronic structure calculations, the pentacene monolayers on Ag(110) and Cu(110) are compared and contrasted allowing the molecular orientation 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