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Scientists Image the Ultrafast Motion of a Rotating Molecule

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HAMBURG, Germany, Aug. 6, 2019 — Scientists at the Center for Free-Electron Laser Science (CFEL) and the Max Born Institute (MBI) have used precisely tuned pulses of laser light to film the ultrafast rotation of a molecule. The resulting “molecular movie” tracks one and one-half revolutions of carbonyl sulfide (OCS), a rod-shaped molecule consisting of one oxygen, one carbon, and one sulfur atom. The revolutions take place within 125 trillionths of a second, at a high temporal and spatial resolution.

Professor Jochen Küpper, from CFEL and the University of Hamburg, led the research with MBI’s Arnaud Rouzée. Küpper said that one of the challenges of capturing the ultrafast motion of atoms on film is that high-energy radiation with a wavelength about the size of an atom is typically needed to see details at the molecular level.

These are the different stages of the molecule's periodic rotation repeat after about 82 picoseconds. Courtesy of DESY, Evangelos Karamatskos/Britta Liebaug.
These are the different stages of the molecule's periodic rotation repeat after about 82 picoseconds. Courtesy of DESY, Evangelos Karamatskos/Britta Liebaug.

His team took a different approach. The researchers used two pulses of infrared laser light that were precisely tuned to each other and that were separated by 38 trillionths of a second (picoseconds), to set the OCS molecules spinning coherently. They then used a laser pulse with a longer wavelength to determine the position of the molecules at intervals of around 0.2 trillionths of a second each. “Since this diagnostic laser pulse destroys the molecules, the experiment had to be restarted again for each snapshot,” researcher Evangelos Karamatskos said.

Altogether, the scientists took 651 pictures covering one and one-half periods of rotation of the molecule. Assembled sequentially, the pictures produced a 125-picosecond film of the molecule’s rotation. The OCS molecule takes about 82 trillionths of a second to complete one whole revolution. “It would be wrong to think of its motion as being like that of a rotating stick, though,” Küpper said. “The processes we are observing here are governed by quantum mechanics. On this scale, very small objects like atoms and molecules behave differently from the everyday objects in our surroundings. The position and momentum of a molecule cannot be determined simultaneously with the highest precision; you can only define a certain probability of finding the molecule in a specific place at a particular point in time.”

These are steps of the molecule's rotation, recorded with an average gap of seven picoseconds each. Courtesy of DESY, Evangelos Karamatskos.
These are steps of the molecule's rotation, recorded with an average gap of seven picoseconds each. Courtesy of DESY, Evangelos Karamatskos.

The peculiar features of quantum mechanics can be seen in several of the movie’s images, in which the molecule does not simply point in one direction, but in various different directions at the same time — each with a different probability (see, for example, the 3 o’clock position in the image of periodic rotation). “It is precisely those directions and probabilities that we imaged experimentally in this study,” Rouzée said. “From the fact that these individual images start to repeat after about 82 picoseconds, we can deduce the period of rotation of a carbonyl sulfide molecule.”

The scientists believe that their method could be used for other molecules and processes, for example to study the internal twisting, that is, torsion, of molecules or chiral compounds. “We recorded a high-resolution molecular movie of the ultrafast rotation of carbonyl sulfide as a pilot project,” Karamatskos said. “The level of detail we were able to achieve indicates that our method could be used to produce instructive films about the dynamics of other processes and molecules.”

CFEL is a collaboration of Deutsches Elektronen-Synchrotron (DESY), the Max Planck Society, and the University of Hamburg. In addition to CFEL and MBI, the University of Aarhus was also involved in the project.

The research was published in Nature Communications (https://doi.org/10.1038/s41467-019-11122-y). 



The movie, assembled from the individual snapshots, covers about 1.5 rotational periods of the molecule. Courtesy of DESY, Evangelos Karamatskos.

Photonics.com
Aug 2019
Research & TechnologyeducationEuropeimagingSensors & Detectorsopticsoptical tweezersatomic and molecular interactions with photonslaserspulsed lasersTunable Lasersultrafast lasers

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