Scientists from LMU and the Max Planck Institute for Quantum Optics (MPQ) have used ultrashort laser pulses to vibrate atoms in molecules and have achieved a precise understanding of the energy transfer dynamics that take place in the process
When light hits the molecules, it is absorbed and re-emitted. Advances in ultrafast laser technology have steadily improved the level of detail in studies of these light-matter interactions. FRS, a method of laser spectroscopy in which the electric field of laser pulses repeated millions of times per second is recorded with time resolution after passing through the sample, now offers 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 energy from the ultrashort light pulse in each individual optical cycle, then release it again over a longer period of time , thus turning it into spectroscopically significant 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 movement of the swing. This gradually adds energy to the swing, so the swing 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 in the molecule, these vibrational modes absorb more and more energy of the light pulse and the amplitude of the vibration increases. When the exciting oscillations of the field stop, the molecule continues to vibrate for a while, like a swing after the person stops the tilting movements. Like an antenna, lightly electrically charged atoms in motion radiate a field of light. Here, the frequency of oscillation of the light field is determined by the properties of the molecule, such as atomic masses and bond strengths, which allow an identification of the molecule.
Researchers from the MPQ and LMU attoworld team, in collaboration with LMU researchers from the Department of Chemistry (Division of Theoretical Femtochemistry), have now distinguished these two constitutive parts of the light field: on the one hand, the exciting light pulses and on the other, the oscillations of the decaying light field, using time-resolved spectroscopy. In doing so, they investigated the behavior of organic molecules dissolved in water. “While established laser spectroscopy methods typically only measure the spectrum and therefore do not allow any information about the temporal distribution of energy, our method can accurately track how the molecule absorbs a a little more energy with each subsequent oscillation of the light field,” he 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 not changing its spectrum. This makes a big difference to the dynamic energy transfer between the light and the vibrating molecule: depending on the temporal structure of the laser pulse, the molecule can absorb and release energy several times during excitation.
To understand exactly which contributions are decisive for the energy transfer, the researchers have developed a quantum chemical model based on supercomputers. This can explain the results of measurements without the help of measured values. “This allows us to artificially disable 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 energy transfer,” explains Martin Peschel, one of the first authors. of the study
In the end, the energy re-emitted during the oscillations of the decaying light field is decisive for how much information can be obtained from a spectroscopic measurement. Thus, the work makes a valuable contribution to better understand the efficiency of optical spectroscopies, for example regarding the molecular compositions of fluids or gases, with the aim of improving it more and more.
Reference:
- Martin T. Peschel, Maximilian Högner, Theresa Buberl, Daniel Keefer, Regina de Vivie-Riedle, Ioachim Pupeza. Suboptic cycle light-matter energy transfer in molecular vibrational spectroscopy. Nature Communications, 2022; 13 (1) DOI: 10.1038/s41467-022-33477-5
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