Solving the problem of distinguishing between single and multiple excitations in laser spectroscopy

The construction of the first laser in the 1960’s ushered in commercial applications with light becoming an integral part of our daily lives. At the same time, this development opened up the scientific field of laser spectroscopy—a technique fundamental to the analysis of materials and the study of fundamental physical phenomena.

Despite all the success, research teams have grappled since the 1970s with the problem that shining a laser on a sample can excite it not just once, but multiple times in each experiment. In this case, the measurement results of single excitations and multiple excitations overlap and usually cannot be separated, which makes it difficult to understand the material.

To remedy this, laser power is usually reduced to the point where multiple excitations are less likely than single excitations. However, it is not completely avoidable and can therefore lead to misinterpretations of the data. Even when multiple excitations are the same as the subject of investigation, it is still difficult to distinguish between two, three, four, or even more excitations.

A complex problem with a simple solution

A team of physicists and physical chemists from Julius-Maximilians-Universität Würzburg (JMU) and the University of Ottawa (Canada) solved this problem decades ago. They present their method in the current issue of the magazine nature. In the experiment conducted in Professor Tobias Brixner’s group in Würzburg, the researchers used the popular method of “transient absorption” to track very rapid changes in different substances that occur in a millionth of a millionth of a second.

While the standard method uses a single laser energy, the researchers used several different powers and collected the data according to a newly derived formula. In this way, they were able to systematically separate the effects from single to six-fold arousals.

“Not long ago, I would not have thought such a distinction was possible, especially with such a simple procedure that any spectroscopic research group could implement and use without much extra effort,” says Brixner.

However, deriving the ‘recipe’ was not a simple thing and required in-depth analysis. Theorist and collaborator Professor Jacob Krech of the University of Ottawa explains, “The interaction of light and matter is very rich, and we’ve shown a beautiful structure hiding within. The fact that this method works for virtually any sample you want to study really surprised us all.”

Applications from photosynthesis to materials science

The new method has a wide range of potential applications. First author Pavel Malo, who was a Brixner postdoctoral fellow at the time of the study and is now a research fellow at Charles University in Prague, explains, “Separating signals from single and multiple excitations is particularly useful for large systems with intense light absorbents, such as natural photosynthetic compounds or materials. membership.”

In the future, the authors plan to extend the method to demonstrate, for example, energy transfer in new photovoltaic materials.