Göttingen Team Images the Full 3D Quantum Wavefunction of a Single Molecule

An interdisciplinary team at the University of Göttingen has imaged the three-dimensional wavefunction of an organic molecule only nanometres in size. A wavefunction is the mathematical description quantum mechanics uses in place of a fixed position, giving the probabilities of properties such as where an electron is and how fast it is moving. Inside molecules these electron wavefunctions are called molecular orbitals, and they govern how a molecule absorbs light and how its chemical reactions unfold. The findings were published in Nature Communications.

"The wavefunction is a fundamental quantity in quantum mechanics, yet it cannot be directly observed or measured," said Professor Stefan Mathias of the University of Göttingen. The team therefore used photoelectron spectroscopy, an indirect technique that measures the momentum of electrons knocked out of the molecule. That yields one half of the wavefunction without physically changing its state; advanced computer algorithms then reconstructed the missing half.

The result is an image of the complete molecular orbital in which features smaller than the spacing between the molecule's own carbon atoms can be told apart. Until now, extending the approach into three dimensions meant long measurements at major synchrotron facilities, which made the technique difficult to use widely and was an especially large obstacle to recording wavefunctions as three-dimensional videos.

"We introduce two powerful new concepts. First, by redesigning the computer algorithm from the ground up, reliable 3D images can now be obtained using much less experimental data. Second, the experiment is based upon a powerful, lab-based soft-X-ray light source that provides ultrashort light pulses," said Dr Matthijs Jansen, a co-leader of the study. "It is the combination of these two techniques that has this remarkable impact."

Dr Wiebke Bennecke, the study's first author, said the method might make stroboscopic videography a reality — building a moving picture from a rapid run of freeze-frames. That would let researchers watch a wavefunction change with femtosecond resolution, a femtosecond being one quadrillionth of a second, and so learn how a molecule adapts to optical, electronic or chemical changes.