How Fast Electron Spins Dance

Chemists study the interaction of metal compounds and light

Metal compounds show fascinating behavior in their interaction with light, which is used for example in light-emitting diodes, solar cells, quantum computers and even in cancer therapy. In many cases, the electron spin, a kind of self-rotation of the electrons, plays a special role. The chemists Sebastian Mai and Leticia González from the Faculty of Chemistry at the University of Vienna succeeded in simulating those extremely fast spin-folding processes on the computer, which are triggered by light absorption of metal compounds. The study appears in the journal "Chemical Science".

When light falls on molecules, in many cases a so-called "photoinduced" reaction is triggered. This can be thought of as an interplay of electron motion and nuclear motion. By absorbing the light, the electrons are first "energized" energetically, weakening, for example, bonds. Then the much heavier atomic nuclei start to move. If the cores are in an appropriate constellation at a later time, the electrons can change from one orbit to another. Due to the physical effect of the "spin-orbit coupling", the electron spin can also "fold over".

Due to this change of motion, spin-flip processes in molecules usually take a relatively long time. Computer simulations have shown that this is not the case with some metal compounds. For example, in the investigated rhenium complex the spin-folding process takes place within ten femtoseconds, although in such a short time the atomic nuclei practically do not move - even light drops just three thousandths of a millimeter in this time span. This knowledge is especially useful for the precise control of electron spin, such as quantum computers.

Investigation is based on enormous computing power with supercomputers
One of the biggest challenges of the investigation was the large amount of computation required for the computer simulations. While very accurate simulations can be performed on small organic molecules at a moderate cost today, metal compounds pose a much greater challenge. This is due, for example, to the large number of atoms, electrons, and solvent molecules that must be considered; but also because the electron spin can only be correctly simulated by equations from the theory of relativity. All in all, the scientists from the Institute for Theoretical Chemistry had to spend nearly one million computer hours on the Austrian supercomputer "Vienna Scientific Cluster".