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Orbital tomography

Angle-resolved electron spectroscopy – direct reconstruction of molecule orbitals in three dimensions

PTB-News 3.2015
01.10.2015
Especially interesting for

fundamental research

quantitative surface analysis

The three-dimensional distribution of electrons inside molecules has been made visible for the first time by means of electron spectroscopy. For this purpose, molecules which were aligned on a metallic surface were irradiated with vacuum-ultraviolet 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 they justify to a large extent the assumption of free photo electrons for this procedure – which is controversial in the scientific community.

Electron orbital of a PTCDA molecule (left) and three-dimensional impulse distribution of the electrons after the photoemission (right). Assuming that there are free photoelectrons, the relation mainly consists in a Fourier transform (FT). The numbers 1 to 4 designate the four most important lobes of the distributi-on in the momentum space.

The measurements were carried out at the Metrology Light Source (MLS) of PTB in Berlin-Adlershof with an electrostatic toroidal electron spectrometer and with monochromatized undulator radiation in the photon energy range from 14  eV to 55 eV. The electron spectrometer allows the angle- and energy-resolved measurement of the electrons detached by the photo emission process in the whole semi-infinite space above the sample. At the MLS beamline used, the relative fractions of higher spectral grating diffraction orders (i. e. photons with an energy which is different from the set energy) are reduced down to less than 1 % by filters, so that it was possible to determine the photon flux of the exciting radiation by means of calibrated semiconductor photodiodes with relative uncertainties in the order of 1 %. This allowed the datasets which were measured at various photon energies and photon fluxes to be standardized 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 across the angular distribution, but also extended to the third dimension as a function of the 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. In the case of this “forward pass” via a Fourier transform, it was assumed that not the quantum-mechanical final state of the photoelectrons after the emission, but solely their molecular initial state before the emission determines the impulse distribution. The results seem to confirm the validity of the assumption of free photoelectrons that are, to a large extent, not influenced by the residual molecules: especially the evolution of the integral photoelectron yield as a function of the photo energy showed only small deviations from the theoretical predictions for a free photoelectron.

The fundamental findings on the charge distribution (and thus also 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. With the established indirect methods, this is possible to a limited extent only, since the theoretical multi-particle models which are required for this purpose are very difficult to validate.

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.

Ansprechpartner

Alexander Gottwald
Department 7.1 Radiometry with Synchrotron Radiation
Phone: +49 (0)30 3481-7130
E-mail: alexander.gottwald(at)ptb.de

Scientific publication

S. Weiß, D. Lüftner, T. Ules, E. M. Reinisch, H. Kaser, A. Gottwald, M. Richter, S. Soubatch, G. Koller, M. G. Ramsey, F. S. Tautz, P. Puschnig: Exploring
three-dimensional orbital imaging with energy dependent photoemission tomography. Nature Communications 6, 8287 (2015)
DOI 10.1038/NCOMMS9287