https://www.sciencedaily.com/releases/2018/07/180705100411.htm > Now physicists and chemists of Julius-Maximilians-Universität Würzburg (JMU) in Bavaria, Germany, have presented two novel principles of optical spectroscopy in the journal Nature Communications. Both approaches show new developments of the so-called coherent two-dimensional (2D) spectroscopy. In conventional 2D spectroscopy, scientists excite a system at a specific frequency and observe what happens at another frequency. > "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.