https://phys.org/news/2018-07-d-spectroscopy-methods.html

"Instead of starting with one excitation and analysing its dynamics, we here deploy two excitations into the same system and observe how they interact," says Professor Brixner, Head of the JMU Chair of Physical Chemistry I who is in charge of the research project at the University of Würzburg. This gives direct access, for example, to propagation phenomena (such as energy transport) because signals in the new method arise only if two initially separated excitations move and then meet.

The scientists illustrate the idea of "exciton-exciton-interaction-two-dimensional-(EEI2D)-spectroscopy" using a perylene bisimide-based J-aggregate. "J-aggregates are among the most important classes of supramolecular structures and the class of perylene bisimide dyes is ideally suited for such experiments," Professor Frank Würthner explains; he heads the JMU Chair of Organic Chemistry II and collaborates in the study.