Researchers Film Molecule Rotation

Quantum film shows residence probabilities of rotating carbonyl sulfide molecules

Using precisely tuned laser flashes, researchers have filmed the ultra-fast rotation of a molecule. The resulting "molecular film" shows one and a half rotations of carbonyl sulfide (OCS) within 125 billionths of a second, a rod-shaped molecule of one oxygen, one carbon and one sulfur, with high temporal and spatial accuracy. The team led by DESY researcher Jochen Küpper from the Center for Free-Electron Laser Science (CFEL) and Arnaud Rouzée from the Max-Born Institute in Berlin present his work in the journal "Nature Communications". The CFEL is a joint institution of DESY, Max Planck Society and University of Hamburg.

"It is a long-cherished dream in molecular physics to film the ultrafast movements of atoms in dynamic processes," explains Küpper, who is also a professor at the University of Hamburg. That's not easy. Because to be able to recognize details in the realm of molecules, one usually requires high-energy radiation with a wavelength in the order of atoms. Küpper's team therefore took a different approach: The researchers used two timed infrared laser pulses with a distance of 38 billionths of a second (picoseconds) to enable the carbonylsulfide molecules to undergo rapid and simultaneous (coherent) rotation. With another, longer-wave laser pulse, the scientists then determined step by step the position of the molecules after about 0.2 billionths of a second each.

The scientists took a total of 651 images covering one and a half rotation periods of the molecule. Mounted one behind the other, the images yield a 125 picosecond film of molecular rotation. For a full revolution, the carbonyl sulfide molecule needs around 82 billionths of a second, or 0, 000 000 000, 082 seconds. "However, you should not imagine the rotation like a rotating stick," explains Küpper. "Here we look at processes in the realm of quantum mechanics. After that, very small objects such as atoms and molecules behave differently than everyday objects in our environment. The exact position and momentum of a molecule can not be determined at the same time with the highest precision, but at any time only a certain probability of residence,

"The peculiarities of quantum mechanics are shown in many images of the film, where the molecule does not point in a single direction, but simultaneously - with different probabilities - in different directions (see about the 3 o'clock position in the graph). "Exactly these directions and probabilities have been experimentally mapped in this research," adds Rouzée. "The fact that these individual images repeat after approximately 82 picoseconds also indicates how long a period of rotation of a carbonyl sulfide molecule lasts."

According to the researchers, the research method used can also be used in other molecules and processes, for example in the inner torsion of molecules or in chiral compounds, ie those that have two mirror-image forms - similar to the right and left human hands. "As a pilot project, we included a high-resolution molecular film of the ultrafast rotation of carbonyl sulfide," Karamatskos summarizes. "Measured by the level of detail that we were able to demonstrate, our method enabled insightful films to record the dynamics of other processes and molecules."

Apart from DESY, the study also involved the University of Hamburg, the Max Born Institute in Berlin and the Aarhus University in Denmark.