Exploring light-driven molecular vibrations

When light hits 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 repeated millions of times per second is recorded in a time-resolved manner after passing through the sample, now provides even deeper insights. Regina de Vivie-Riedle (LMU/Faculty of Chemistry) and PD Dr. Ioachim Pupeza (LMU/Faculty of Physics, MPQ) show for the first time in theory and experiment how molecules gradually absorb the energy of the ultra-short light pulse in each individual light cycle and release it again over a longer period of time, thus converting it into spectroscopically meaningful light around.

The study elucidates the mechanisms that fundamentally determine this energy transfer. In addition, it is developing and verifying a detailed quantum chemical model that will be able to quantitatively predict even the smallest deviations from linear behavior in the future.

A child on a swing sets it in motion by tilting its body, which must be synchronized with the rocking motion. This gradually adds energy to the swing so that the swing’s deflection 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 vibrational modes absorb more and more energy on the light pulse, and the vibration amplitude decreases to.

When the exciting field oscillations are over, the molecule continues to oscillate for a while—like a seesaw after the person has stopped rocking. Like an antenna, the slightly electrically charged atoms then emit a light field as they move. The frequency of the light field oscillation is determined by properties of the molecule such as atomic masses and bond strengths, which enables the molecule to be identified.

Researchers from the attoworld team at MPQ and LMU, in cooperation with LMU researchers from the Faculty of Chemistry (Department for Theoretical Femtochemistry), have now distinguished these two components of the light field – on the one hand the exciting light pulses and on the other hand the decaying light field oscillations – using time-resolved spectroscopy. They investigated the behavior of organic molecules dissolved in water.

“While established laser spectroscopy methods usually only measure the spectrum and therefore do not allow any statement to be made about the temporal distribution of the energy, our method can track exactly how the molecule absorbs a little more energy with each further oscillation of the light field,” says Ioachim Pupeza, head of the experiment.

The fact that the measurement method allows this temporal distinction is best shown by the fact that the scientists repeated the experiment, changing the duration of the exciting pulse but not changing its spectrum. This makes a big difference for the dynamic energy transfer between light and the oscillating molecule: Depending on the temporal structure of the laser pulse, the molecule can absorb and release energy several times during the excitation.

In order to understand exactly which contributions are crucial for the energy transfer, the researchers have developed a supercomputer-based quantum chemical model. This can explain the results of the measurements without the help of measured values. “In this way, we can artificially switch off individual effects such as collisions between the oscillating molecules and their environment or the dielectric properties of the environment and thus elucidate their influence on energy transfer,” explains Martin Peschel, one of the first authors of the study.

The energy radiated again during the decaying light field oscillations is ultimately decisive for how much information can be gained from a spectroscopic measurement. The work thus makes a valuable contribution to a better understanding of the performance of optical spectroscopy, for example with regard to the molecular compositions of liquids or gases, with the aim of continuously improving them.

The research is published in nature communication.