Optical Centrifuges for Superrotors

Optical centrifuges are made of corkscrew-shaped laser pulses that can make molecules rotate extremely fast. (Source: DESY)

Using corkscrew-shaped laser pulses, scientists at Deutsches Elek­tronen-Syn­chrotron DESY in Hamburg have devised a sophis­ticated optical centrifuge that can make molecules rotate rapidly about a desired molecular axis. The inno­vative method opens up new ways to control and study super fast spinning molecules, called super­rotors. Until now, optical centri­fuges can make molecules rotate about one specific axis only. The new scheme lets scientists select between two axes.

Alec Owens, Andrey Yachmenev and Jochen Küpper from the Con­trolled Molecule Imaging (CMI) Group at the Center for Free-Electron Laser Science (CFEL) developed their optical centri­fuges with rotating laser pulses. They can make molecules spin faster than ten trillion times per second. These molecular super­rotors have revealed unexpected beha­viours and are interesting quan­tities for studies of scattering, spectro­scopy, and dynamics. Super­rotors can bring in significant energy into colli­sions and, at the same time, behave like tiny little gyro­scopes that are more resistant to colli­sions and reorien­tation.

“Control­ling ultrafast rotational motion of molecules has seen tremendous progress in recent years thanks to the develop­ment of inno­vative techniques in strong-field laser physics,” says Owens, who is also affiliated with the Hamburg Center for Ultra­fast Imaging CUI. Existing optical centri­fuges always trap molecules along the axis where the electric charges of the molecule can most easily be shaken around to form a dipole. The molecule’s axis then follows the rotating laser field and starts spinning faster and faster.

The CMI team now deve­loped a method that uses a corkscrew laser that is repeatedly turned on and off. This modified optical centri­fuge allows the axis about which the molecule rotates to be selected. Using the example of hydrogen sulfide, the study demon­strates how the rotation of asym­metric molecules can be controlled, choosing between two distinct mole­cular axes. Turning the corkscrew laser on and off can be achieved by adding a modi­fying pulse envelope that is super­imposed to the laser field. This way the molecules can be excited along different pathways of rotational states, ultimately leading to rotation about one of two different axes.

“Such a scheme to control the angular momentum alignment of a molecule will be useful in studies of molecule-molecule or molecule-surface scat­tering, where you can change the outcome and study the stereo­dynamics of a scattering event by control­ling the rotation axis,” explains Yachmenev. Equally bene­ficial is the large amount of energy asso­ciated with super­rotors, which can be controlled by changing the dura­tion of the optical centri­fuge pulse. (Source: DESY)

Reference: A. Owens et al.: Coherent Control of the Rotation Axis of Molecular Superrotors, J. Phys. Chem. Lett. 94206 (2018); DOI: 10.1021/acs.jpclett.8b01689

Links: Controlled Molecule Imaging Group CMI, Center for Free-Electron Laser Science CFEL, Hamburg, Germany • Hamburg Center for Ultrafast Imaging CUI, University of Hamburg, Hamburg, Germany

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