Scientists capture molecular rotation

German scientists produced this “molecular movie” by using precisely tuned pulses of laser light to film the ultrafast rotation of a molecule. 

It tracks one-and-a-half revolutions of carbonyl sulphide (OCS) – a rod-shaped molecule consisting of one oxygen, one carbon and one sulphur atom – taking place within 125 picoseconds (trillionths of a second) at a high temporal and spatial resolution. 

The work was led by Jochen Küpper from the Centre for Free-Electron Laser Science, in Hamburg, and Arnaud Rouzée from the Max Born Institute in Berlin. It is described in a paper published in the journal Nature Communications. 

Küpper says molecular physics has “long dreamed” of capturing the ultrafast motion of atoms during dynamic processes on film. It’s been tricky, however, because in the realm of molecules you normally need high-energy radiation with a wavelength of the order of the size of an atom to be able to see details.

So he and his colleagues took a different approach. They used two pulses of infrared laser light precisely tuned to each other and separated by 38 picoseconds to set the carbonyl sulphide molecules spinning rapidly in unison. 

They then used a further laser pulse, having 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,” says principal author Evangelos Karamatskos.

Altogether, the scientists took 651 pictures covering one and a half periods of rotation of the molecule. Assembled sequentially, these produced the 125-picosecond film of the molecule´s rotation. 

The carbonyl sulphide molecule takes about 82 picosecond to complete one revolution. 

“It would be wrong to think of its motion as being like that of a rotating stick, though,” says Küpper. “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.”

The team believes its approach – and success – could have wider application. 

“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,” says Karamatskos.