02.12.2015
Fig. 1: Electron orbitals of a PTCDA-Molecule (left) and 3-dimensional momentum distribution of the photoelectrons after photoemission (right). In the case of (almost) free photoelectrons, the relation is expressed by a Fourier Transform (FT).
The three-dimensional distribution of electrons inside molecules has been made visible for the first time at PTB's electron storage ring Metrology Light Source (MLS) in Berlin-Adlershof by means of electron spectroscopy. For this purpose, molecules which were aligned on a metallic surface were irradiated with shortwave synchrotron radiation, and the angle and energy distribution of the electrons that were detached by the photoelectric effect was measured. This procedure, which is called orbital tomography, was developed at TU Graz (Austria) and at Forschungszentrum Jülich (Germany); in cooperation with PTB, it was, for the first time, successfully extended to three dimensions. These results could be achieved especially by a precise radiometric characterization of the exciting radiation and were published in the scientific journal 
Nature Communications in October 2015.
The measurements were carried out with an electrostatic toroidal electron spectrometer and with monochromatized undulator radiation in the photon energy range from 14 eV to 55 eV. At the MLS beamline used, the photon flux of the exciting radiation can be determined with relative uncertainties on the order of one percent. This allowed datasets to be exactly normalized at various photon energies and photon fluxes for the first time. Thus, it was not only possible to measure the relative photoelectron intensities as a function of the direction of the electron impulse in two dimensions via the angular distribution, but also extended to the third dimension as a function of the absolute impulse value by varying the photon energy – and thus the electron energy. From the three-dimensional impulse distribution of the photoelectrons thus obtained, the three-dimensional local distribution of the electrons of the original molecule orbital could be determined numerically – directly and without modeling.
The fundamental findings on the charge distribution and on the orientation of individual molecules, which were obtained by means of metrological procedures, is highly relevant for the development of functional surfaces, e.g. of organic semiconductor materials on metallic surfaces, which open up new perspectives for photovoltaic components with increased efficiency. In turn, orbital tomography represents a very interesting metrological approach to quantitative electron spectroscopy, since this method allows reliable uncertainty budgets.
It is planned to continue this cooperation in the future to further develop orbital tomography for the quantitative characterization of electronic properties of organic semiconductors and of solar cells.
Contact:
A. Gottwald, 7.13, e-mail: 
Alexander.Gottwald(at)ptb.de
M. Richter, 7.1, e-mail: Mathias.Richter(at)ptb.de