Photochemical reactions in the liquid phase often involve several intermediate products, which in many cases are extremely short-lived and whose individual reaction steps are simply unknown. Our research focuses on such rapid light-induced processes, such as rearrangement or photolysis reactions, which we investigate using time-resolved spectroscopy. We employ a variety of ultrafast spectroscopic techniques to elucidate the photodynamics and underlying reaction mechanisms. For example, the intermediates and end products can be identified using ultrashort laser pulses in the mid-infrared range on the basis of characteristic vibrational signatures. Our experimental set-ups allow us to track the light-induced reaction in real time.
The following animation illustrates a ‘pump-probe experiment’: a laser pulse, typically with a pulse duration in the femtosecond range, triggers the photoreaction. As the reaction proceeds, short-lived intermediates are formed, which can be detected using a second laser pulse that measures the absorption of the intermediate. As the time delay between the two laser pulses can be varied, it is possible to observe directly how long it takes for an intermediate to form and when it reacts further.
In addition, we are investigating whether it is possible to influence the course and product distribution of a reaction in a precisely defined manner, for example by varying the light used in the experiments. To this end, ultrashort laser pulses exhibit remarkable properties, such as very high peak intensities, which open up the possibility of interacting with a molecule multiple times. Furthermore, the laser pulses are not monochromatic, but consist of a multitude of different colours, which can be delayed relative to one another using a pulse shaper. With the aid of such shaped laser pulses, we investigate whether the reaction can be steered in a desired direction, an experimental methodology often referred to as the ‘quantum control approach’.
The photochemical processes we are currently investigating include reaction sequences involving reactive species such as radicals, molecular switching, charge and energy transfer processes, photo-induced structural changes and rearrangements, photolysis, and photoluminescence following an ultrafast intersystem crossing. In all these cases, particular emphasis is placed on the role of the solvent environment and the influence of co-solvents, including with regard to the bimolecular reaction dynamics relevant to photocatalysis.
You can find the latest research highlights from our group in the ‘News’ section.