New 2-D spectroscopy methods

July 5, 2018, Julius-Maximilians-Universität Würzburg

In EEI2D spectroscopy (left), two originally separate excitations (yellow arrows) meet. With 2D mass spectrometry (right), ion photoproducts (black arrows) are detected. Credit: Tobias Brixner, JMU
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy. Various methods are known in literature. But generally only the behaviour of a single excitation and its consequences are investigated.

Now physicists and chemists of Julius-Maximilians-Universität Würzburg (JMU) in Bavaria, Germany, have presented two novel principles of in the journal Nature Communications. Both approaches show new developments of the so-called coherent two-dimensional (2-D) . In conventional 2-D spectroscopy, scientists excite a system at a specific frequency and observe what happens at another frequency.

"Instead of starting with one 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.

This method is applicable to numerous physical, chemical, biological or engineering systems, for example, to decode dynamic properties such as of natural light-harvesting systems and artificial dye aggregates.

Investigating ionization with the 2-D scheme

The physicists in Tobias Brixner's team conducted further research by combining coherent 2-D spectroscopy with molecular beams "This has allowed us to investigate ionization with the 2-D scheme for the first time," the professor explains. For this purpose, they used mass spectrometry instead of optical detection and obtained 2-D spectra not only for the parent molecule but simultaneously also for all photoproducts.

"Our main challenge was the fact that particle densities in molecular beams are very low, rendering futile any previous conventional attempts at detecting coherently emitted four-wave-mixing signals," Brixner says. Instead, the researchers observed the ion generated by the sequence of excitation pulses, thereby merging two hitherto separate fields of research, namely 2-D spectroscopy and mass spectrometry.

The physicists used the method exemplarily to identify the ionization pathways of 3d Rydberg states in nitrogen dioxide. In the future, this development will allow studying the influence of the environment on the coherent dynamics in larger molecules.

Explore further: THz spectroscopy could help explain water's anomalies

More information: Jakub Dostál et al. Direct observation of exciton–exciton interactions, Nature Communications (2018). DOI: 10.1038/s41467-018-04884-4

Sebastian Roeding et al. Coherent two-dimensional electronic mass spectrometry, Nature Communications (2018). DOI: 10.1038/s41467-018-04927-w

Related Stories

THz spectroscopy could help explain water's anomalies

June 27, 2018

Liquid water sustains life on earth, but its physical properties remain mysterious among scientific researchers. Recently, a team of Swiss researchers used existing THz spectroscopy techniques to measure liquid water's hydrogen ...

Fast energy transport between unlike partners

October 6, 2016

Chemists from the University of Würzburg have combined different dye molecules in aggregates and thereby observed surprising properties. Their discovery may help to use sunlight more efficiently for the generation of energy.

Fluorescence dyes from the pressure cooker

January 30, 2017

Perylene bisimides are organic pigments with interesting dye properties. While these compounds are red in the solid state, when dissolved, they generate bright yellowish-green solutions under UV irradiation. Aside from their ...

Recommended for you

Insect antibiotic provides new way to eliminate bacteria

November 15, 2018

An antibiotic called thanatin attacks the way the outer membrane of Gram-negative bacteria is built. Researchers at the University of Zurich have now found that this happens through a previously unknown mechanism. Thanatin, ...

Why macrophages rest in healthy tissue

November 15, 2018

ETH scientists have shown that the immune system's macrophages are regulated not only biochemically, but mechanically as well. This could explain why the cells are less active in healthy body tissue.

First microarrayed 3-D neuronal culture platform developed

November 14, 2018

Neuronal development is often regulated by the graded distribution of guidance molecules, which can either attract or repel the neuronal migration or neurite projection when presented in a format of concentration gradients, ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.