When light strikes molecules, it is absorbed and re-emitted. Advances in ultra-fast laser technology have steadily improved the level of detail in studies of such interactions between light and matter.
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 they pass 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 the energy of the ultrashort pulse of light in each individual optical cycle and then release it again over a longer period, 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 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 vibrational 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 – much like a swing after the person stops the tilting movements. Like an antenna, the slightly electrically charged atoms in motion emit 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, allowing identification of the molecule.
Researchers from the attoworld team of MPQ and LMU, 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: the exciting pulses of light on the one hand and the oscillations on the other. of the decreasing light field – 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 information about the temporal distribution of the energy, our method can track exactly how the molecule absorbs a little more energy with each subsequent oscillation of the light field,” said 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 light and the vibrating molecule: depending on the temporal structure of the laser pulse, the molecule can absorb and release energy several times during the excitation.
To understand exactly which contributions determine 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. “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, thus clarifying their influence on the energy transfer,” explains Martin Peschel, a researcher. of the study’s first authors.
Ultimately, the energy that is re-emitted during the decreasing light field oscillations determines how much information can be obtained from a spectroscopic measurement. The work thus makes a valuable contribution to a better understanding of the efficiency of optical spectroscopies, for example with regard to molecular compositions of liquids or gases, with the aim of continuously improving them.
The research was published in nature communication.
Amplification of the radiation of molecules after excitation to improve molecular laser spectroscopy
Martin T. Peschel et al, Sub-optical cycle light-matter energy transfer in molecular vibration spectroscopy, nature communication (2022). DOI: 10.1038/s41467-022-33477-5
Quote: Light-driven molecular wobble study (2022, Oct. 18) retrieved Oct. 18, 2022 from https://phys.org/news/2022-10-exploring-light-driven-molecular.html
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