![]() ![]() 1, 3), was excluded in a previous study based on the analysis of frontier π orbitals ( 12), the discrimination between bisanthene ( Fig. 1, 2) ( 33), or even all carbon atoms along the zig-zag edge of bisanthene ( Fig. While a direct chemical bonding between the underlying substrate atoms and the carbon atoms in the 10,10′ positions ( Fig. DBBA undergoes catalytic dehalogenation and cyclodehydrogenation reactions at 525 K ( 12, 33), during which the adsorbate planarizes and the π conjugation expands to the entire fused carbon backbone, leading to the formation of a bisanthene-like species that is adsorbed flat on the surface-possible products in agreement with STM and XPS ( 12, 33) are shown in Fig. Our model system is therefore a promising candidate to achieve our conceptual goal of proving momentum space imaging of σ orbitals for chemical analysis and, at the same time, to address an actual scientific problem in the field of on-surface dehalogenation and cyclodehydrogenation reactions. The size of the molecular products suppresses photoelectron scattering by atoms of the metal substrate ( 32) because many atoms of the molecule are typically out of registry with the substrate, thus safeguarding the PWA as far as possible. Because the products of this reaction have an influence on the structural quality of the targeted nanostructures ( 28), the analytical discrimination between these competing reactions and products is important, and we used it as a test case for POT on σ orbitals. ![]() The initially dehalogenated positions of the nonplanar precursor molecule DBBA not only can directly engage in an Ullmann-type C─C coupling ( 30, 31) but also can become metalated or hydrogenated. DBBA and similar molecules are often used to create carbon-based nanostructures by on-surface synthesis ( 27– 29). Our model is the on-surface dehalogenation and cyclodehydrogenation of 10,10′-dibromo-9,9′-bianthracene (DBBA). Therefore, the applicability of the PWA to the final state of the photoemission needs to be questioned in this case. If, on the other hand, the photoemission occurs from several different atomic orbital types, for example, the carbon 2s, 2p x, and 2p y-orbitals of which σ orbitals consist, then the PWA may become less accurate because the contributions of the different atomic orbitals do not any more assemble to the simple PW final state that essentially Fourier transforms the initial state orbital. However, this equivalence relies on photoemission from only one type of atomic orbital. Under these circumstances, the PWA is equivalent to the independent atomic center (IAC) approximation ( 7, 25) in which the photoelectron wave function is described in terms of unbound solutions of the atomic Schrödinger equation with energy E kin summed over all atoms of the molecule and taken in the asymptotic limit for infinite distances of the detector from the photoemitter (see Materials and Methods for details) ( 26). For π-conjugated organic molecules, these originate from linear combinations of only 2p z-orbitals on identical (all carbon) atoms. So far, the PWA has only been confirmed experimentally for π orbitals close to E F. ![]() īecause it is based on ARPES, POT per se is not limited to a particular binding energy range if the photon energy is large enough. This is valid if the final state of the photoelectron after photoemission can be represented by a plane wave (PW). There is overwhelming evidence that, for π orbitals in conjugated molecules, this relation is a straightforward Fourier transform ( 24, 25). The measured angular distribution of photoelectrons is governed by the spatial distribution of electrons in the initial state-the orbital. In contrast, in POT, photoelectrons are collected in the half-space above the sample surface on which the adsorbed molecules are fixed in space ( 7, 17– 19). While it has long been recognized that angle-resolved photoelectron spectroscopy (ARPES) can image orbitals in the molecular frame, the alignment of molecules in the gas phase is a challenge ( 21– 23). For example, valence spectra have been deconvolved model-free into orbital projected densities of states (pDOS) ( 15, 16), orbitals have been reconstructed from experimental data in two dimensions (2D) and 3D ( 17– 19), and orbital patterns in momentum space have been recorded with femtosecond time resolution ( 20). ![]() Photoemission orbital tomography (POT) ( 7) is a recent technique for the orbital analysis and orbital imaging of molecules on surfaces. ![]()
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