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Molecules in a gas – any gas – are darting about randomly in all directions. They are also spinning randomly, which poses difficulties for researchers trying to understand the detailed properties of their interaction with light. Is there any way to make molecules all point in the same direction, even for a brief moment?
That was the challenge posed by the EU-funded COSMODET project. ‘Our project set out to explore the ability to orient molecules in a specific chosen direction in space,’ says project coordinator Sharly Fleischer, a physical chemist at Tel Aviv University.
For about 20 years scientists have known how to align molecules in a gas by directing short pulses of laser light at them. Though this technique can align all the molecules vertically, for example, it can’t distinguish between up and down. ‘Short optical pulses don't distinguish heads from tails, so for each molecule pointing up there’s another molecule pointing down,’ says Fleischer.
Terahertz radiation
But for many types of molecule it has proved possible to make them all point in the same direction – become ‘oriented’ rather than aligned – with flashes of electromagnetic radiation at terahertz frequencies. Fleischer worked on this technique as a post-doc at MIT.
As the two methods exploit different principles, the COSMODET team explored the possibilities of using them in concert. By exposing a gas to bursts of infrared laser light and terahertz waves, they found they now had two different ‘rotational handles’ over the angular direction of the molecules.
‘The terahertz method is especially promising for orienting asymmetric molecular rotors,’ Fleischer says. Much of the work was done on sulfur dioxide, one of the simplest such molecules, but he believes the same techniques could be used to orient larger asymmetric molecules such as proteins or other complex biomolecules. Terahertz radiation is also less likely to damage delicate molecule.
Along the way, the group demonstrated an effect that had been previously overlooked. Once oriented, the molecules act like microscopic antennas, radiating away the energy imparted to them by the incident pulse of radiation. ‘This is a very important feature because this is a general phenomenon which should exist in each and every case where resonant interaction is induced,’ says Fleischer.
Benefits for spectroscopy
‘COSMODET was a project of pure science research,’ Fleischer says. ‘There are no commercial endeavours or patents arising from our work. However, we expect that researchers in tangential fields will utilise and implement our findings and methods. For example, the ability to explore an ensemble of gas molecules with a certain angular distribution is at the heart of current spectroscopic techniques that wish to avoid the angular averaging that is inherent in spectroscopy of unordered media.’
One promising application is X-ray diffraction, a technique that has long been used to probe the structure of crystalline solids. If the COSMODET methods could be used to orient the molecules in a gas then it would – for a brief moment – enable the investigation of the molecular structure by ultrafast X-ray diffraction.
The project was supported by the EU’s Marie Skłodowska-Curie fellowship programme.