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.
Highlights
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
A renaissance of angular-resolved photoemission spectroscopy (ARPES) in the field of organic electronics could be observed[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. This development was mainly driven by the fact that, in opposition to conventional wisdom, the angular dependence of the photoemission current from oriented molecular films can be understood by assuming a plane wave as the final state of the photoemission process
The reciprocal relationship between the ARPES intensity and the spatial distribution of the initial orbital has been utilized to study the hybridization of molecular states with the substrate[6,7], explore intermolecular band dispersions[15,16,25], shed light on the role of intermolecular versus molecular-substrate interactions[12,18,26], determine molecular orientations[10,19,21], reveal the nature of doping-induced states[20], or even enable the reconstruction of molecular orbitals from ARPES data[5,17,22]
Summary
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 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). Within the one-step model of photoemission and the plane wave final state approximation, the measured ARPES intensity is proportional to the square of the absolute value of Fourier transform c~ðkÞ of the initial orbital cðrÞ This relationship between a molecular orbital in real space cðrÞ and in momentum space c~ðkÞ is illustrated in Fig. 1 for the example of the lowest unoccupied molecular orbital (LUMO) of perylene-3,4,9,10tetracarboxylic dianhydride (PTCDA) calculated from density functional theory (DFT).
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