20-Oct-2022 - Ludwig-Maximilians-Universität München (LMU)

Light-driven molecular swing

Scientists have used ultrashort laser pulses to make the atoms of molecules vibrate and have gained a precise understanding of the dynamics of energy transfer

When light impinges on molecules, it is absorbed and re-emitted. Advances in ultrafast laser technology have steadily improved the level of detail in studies of such light-matter interactions. FRS, a laser spectroscopy method in which the electric field of laser pulses repeating millions of times per second is recorded with time resolution after passing through the sample, now provides even deeper insights: scientists led by Prof. Dr. Regina de Vivie-Riedle (LMU/Department of Chemistry) and PD Dr. Ioachim Pupeza (LMU/Department of Physics, MPQ) show for the first time in theory and experiment how molecules gradually absorb the energy of the ultrashort light pulse in each individual optical cycle, and then release it again over a longer period of time, thereby converting it into spectroscopically meaningful light. The study elucidates the mechanisms that fundamentally determine this energy transfer. It also develops and verifies a detailed quantum chemical model that can be used in the future to quantitatively predict even the smallest deviations from linear behavior.

A child on a swing sets it in motion with tilting movements of the body, which must be synchronized with the swing movement. This gradually adds energy to the swing, so that the deflection of the swing increases over time. Something similar happens when the alternating electromagnetic field of a short laser pulse interacts with a molecule, only about 100 trillion times faster: when the alternating field is synchronized with the vibrations between the atoms of the molecule, these vibration modes absorb more and more energy from the light pulse, and the vibration amplitude increases. When the exciting field oscillations are over, the molecule continues to vibrate for a while - just like a swing after the person stops the tilting movements. Like an antenna, the slightly electrically charged atoms in motion then radiate a light field. Here, the frequency of the light field oscillation is determined by properties of the molecule such as atomic masses and bond strengths, which allows for an identification of the molecule.

"We can precisely track how a molecule absorbs a little more energy with each subsequent oscillation of the light field", Dr. Ioachim Pupeza, Head of the experiment.

Researchers from the attoworld team at LMU and MPQ, in collaboration with LMU researchers from the Department of Chemistry (Division of Theoretical Femtochemistry), have now distinguished these two constituent parts of the light field - on the one hand, the exciting light pulses, and on the other, the decaying light field oscillations - using time-resolved spectroscopy. In doing so, they investigated the behavior of organic molecules dissolved in water. "While established laser spectroscopy methods usually only measure the spectrum and thus do not allow any information about the temporal distribution of the energy, our method can precisely track how the molecule absorbs a little more energy with each subsequent oscillation of the light field," says Ioachim Pupeza, head of the experiment. That the measurement method allows this temporal distinction is best illustrated by the fact that the scientists repeated the experiment, changing the duration of the exciting pulse but without changing its spectrum. This makes a big difference for the dynamic energy transfer between light and the vibrating molecule: Depending on the temporal structure of the laser pulse, the molecule can then absorb and release energy several times during the excitation.

A supercomputer-based quantum chemical model

In order to understand exactly which contributions are decisive for the energy transfer, the researchers have developed a supercomputer-based quantum chemical model. This can explain the results of the measurements without the aid of measured values. "This allows us to artificially switch off individual effects such as the collisions of the vibrating molecules with their environment, or even the dielectric properties of the environment, and thus elucidate their influence on the energy transfer" explains Martin Peschel, one of the first authors of the study.

In the end, the energy re-emitted during the decaying light field oscillations is decisive for how much information can be obtained from a spectroscopic measurement. The work thus makes a valuable contribution to better understanding the efficiency of optical spectroscopies, for example with regard to molecular compositions of fluids or gases, with the objective of improving it further and further.

Facts, background information, dossiers

  • quantum chemistry

More about LMU

  • News

    Secret structure in the wiring diagram of the brain

    In the brain, our perception arises from a complex interplay of neurons that are connected via synapses. But the number and strength of connections between certain types of neurons can vary. Researchers from the University Hospital Bonn (UKB), the University Medical Center Mainz and the Lud ... more

    Innate immunity: the final touch for antimicrobial defence

    If bacteria enter the body, it often takes just a few minutes for the innate immune system to recognise them as foreign and set the immune defence in motion. Receptors of the immune system that recognise bacterial cell wall components play a central role in this process. An important immune ... more

    Complex patterns: building a bridge from the large to the small

    For many processes important for life such as cell division, cell migration, and the development of organs, the spatially and temporally correct formation of biological patterns is essential. To understand these processes, the principal task consists not in explaining how patterns form out ... more

  • Authors

    Prof. Dr. Thomas Carell

    Thomas Carell graduated in chemistry, completing his doctorate at the Max Planck Institute for Medical Research under the tutelage of Prof. Dr Dr H. A. Staab. Following a research position in the USA, he accepted a position at ETH Zurich, setting up his own research group in the Laboratory ... more

More about MPI für Quantenoptik

  • News

    Pumping up the music of molecules

    Sensitive animal noses can sniff out trace particles, such as volatile organic compounds, in the ambient air. Humans, on the other hand, are developing innovative technologies for this purpose, such as optical spectroscopy. This uses laser light to detect the molecular composition of gases. ... more

    Laser takes pictures of electrons in crystals

    Microscopes of visible light allow us to see tiny objects such living cells and their interior. Yet, they cannot discern how electrons are distributed among atoms in solids. Now researchers around Prof. Eleftherios Goulielmakis of the Extreme Photonics Labs at the University of Rostock and ... more

    A fine sense for molecules

    At the biochemical level, organisms can be thought of as complex collections of different species of molecules. In the course of their metabolism, biological cells synthesize chemical compounds, and modify them in multifarious ways. Many of these products are released into the intercellular ... more

q&more – the networking platform for quality excellence in lab and process

The q&more concept is to increase the visibility of recent research and innovative solutions, and support the exchange of knowledge. In the broad spectrum of subjects covered, the focus is on achieving maximum quality in highly innovative sectors. As a modern knowledge platform, q&more offers market participants one-of-a-kind networking opportunities. Cutting-edge research is presented by authors of international repute. Attractively presented in a high-quality context, and published in German and English, the original articles introduce new concepts and highlight unconventional solution strategies.

> more about q&more

q&more is supported by: