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Orbital Tomography of organic semiconductor films
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 1970´s on, the angular distribution of the photoemitted electrons was thought to be too complex to analyse quantitatively. With our angle resolved UPS (ARUPS) work on conjugated molecules both in ordered thick films and chemisorbed monolayers we have shown that the angular (momentum) distribution of the photocurrent from orbital emissions can be simply understood. The approach we have been developing is becoming known as orbital tomography. It takes an holistic view of the photoemission from the orbitals assuming a plane wave approximation for the final state: The ARUPS data cube is thus essentially a momentum space view of the orbital which ca be related, by a Fourier transform, to the real space electron distribution. In the past few years the potential power of orbital tomography has been demonstrated and it has been shown that it can determine molecular geometries, gain insight into the nature of the surface chemical bond, unambiguously determine the orbital energy ordering in molecular homo- and heterostructures and even reconstruct orbitals in real space.
The global aim of this project is to develop and consolidate orbital tomography and provide a basis whereby it could be even more generally applied.