Scientists film rotating carbonyl sulphide molecules

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Scientists film rotating carbonyl sulphide molecules

Scientists used precisely tuned laser light pulses to film the ultra-fast rotation of a molecule. The resulting "molecular film" follows a revolution and a half of carbonyl sulfide (CSF) - a rod-shaped molecule composed of an oxygen atom, a carbon atom and a sulfur atom - occurring within 125 trillion seconds, at high temporal and spatial resolution. The team led by Jochen Küpper of DESY at the Center for Free-Electron Laser Science (CFEL) and Arnaud Rouzée of the Max Born Institute in Berlin present their results in the journal Nature Communications. The CFEL is a cooperation between DESY, the Max Planck Society and the University of Hamburg.

"Molecular physics has long dreamed of capturing the ultra-fast motion of atoms in dynamic processes on film," explains Küpper, who is also a professor at the University of Hamburg. However, this is not simple, as the molecular domain normally requires high-energy radiation with a wavelength in the order of the size of an atom to be able to see the details. The Küpper team therefore adopted a different approach: They used two precisely matched infrared laser light pulses separated by 38 trillionths of a second (picoseconds) to adjust the carbonyl sulfide molecules to rotate rapidly in unison, that is, coherently. They then used another laser pulse of a longer wavelength to determine the position of the molecules at intervals of about 0.2 trillionths of a second each. "As this diagnostic laser pulse destroys the molecules, the experiment had to be restarted for each shot," says Evangelos Karamatskos, the main author of the CFEL study.

In total, scientists took 651 photos covering 1.5 rotation periods of the molecule. Sequentially assembled, the images produced a 125 picosecond film of the molecule's rotation. The carbonyl sulfide molecule takes about 82 trillionths of a second, or 0.00000000000000000000000082 seconds, to make a complete revolution. "But it would be wrong to think that its movement resembles that of a rotating stick," says Küpper. "The processes we observe here are governed by quantum mechanics. At this scale, very small objects such as atoms and molecules behave differently from everyday objects in our environment. The position and momentum of a molecule cannot be determined simultaneously with the greatest precision; you can only define a certain probability of finding the molecule at a precise location at a given time."
The particular characteristics of quantum mechanics can be seen in many of the many images in the film, in which the molecule does not simply point in one direction, but in different directions at the same time, each with a different probability (see for example the 3 o'clock position in the figure). "It is precisely these directions and probabilities that we have experimentally imagined in this study," adds Rouzée. "Because these individual images begin to repeat after about 82 picoseconds, we can deduce the rotation period of a carbonyl sulfide molecule."

Scientists believe that their method can also be used for other molecules and processes, for example to study the internal torsion, i. e. the torsion, of chiral molecules or compounds, those that exist in two mirror forms, just like a person's right and left hands. "We recorded a high-resolution molecular film of the ultra-fast rotation of carbonyl sulfide as part of a pilot project," says Karamatskos, summarizing the experience. "The level of detail we have been able to achieve indicates that our method could be used to produce informative films on the dynamics of other processes and molecules."