Scientists from LMU and the Max Planck Institute of Quantum Optics (MPQ) used ultrashort laser pulses to vibrate the atoms of the molecules and gained a precise understanding of the dynamics of the energy transfer that occurs in the process.
When light hits the molecules, it is absorbed and re-emitted. Advances in ultrafast laser technology have consistently 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 a temporal resolution after passing through the sample, now provides even more 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 experience how molecules gradually absorb the energy of the light pulse ultrashort in each single optical cycle, then releasing again for a longer period of time, thus converting it into spectroscopically significant light. The study clarifies the mechanisms that fundamentally determine this energy transfer. It also develops and tests 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 swinging 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 gives the light pulse and the amplitude of the vibration increases. When the exciting swings of the field are over, the molecule continues to vibrate for a while, just like a wobble after the person has stopped the tilt movements. Like an antenna, the slightly electrically charged atoms in motion then radiate a light field. Here, the frequency of the oscillation of the light field is determined by the properties of the molecule such as atomic masses and binding forces, which allow an identification of the molecule.
Researchers from the MPQ and LMU attiworld team, 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 pulses of light, and on the other, the oscillations of the decaying light field, using time-resolved spectroscopy. In doing so, they studied the behavior of organic molecules dissolved in water. “While established laser spectroscopy methods usually 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 little more energy with each subsequent oscillation of the light field, “says Ioachim Pupeza, head of the experiment. That the measurement method allows for 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 the light and the vibrating molecule: depending on the time structure of the laser pulse, the molecule can then absorb and release energy multiple times during excitation.
To understand exactly which contributions are decisive for energy transfer, the researchers developed a supercomputer-based quantum chemical model. This can explain the measurement results without the aid of measured values. “This allows us to artificially deactivate individual effects such as collisions of vibrating molecules with their environment, or even the dielectric properties of the environment, and thus clarify their influence on energy transfer,” explains Martin Peschel, one of the first authors. of study.
Ultimately, 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. The work therefore provides a valuable contribution to better understand the efficiency of optical spectroscopies, for example as regards the molecular compositions of fluids or gases, with the aim of improving it more and more.
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