Physicists have invented an 'optical centrifuge', and used it to pull chlorine molecules apart, Mark Haw reports.
There seems to be no limit to what you can do with a laser. Lasers have been used to move microscopic particles, to trap and cool atoms, and even to stretch biological cells. Now a team of physicists has used lasers to set individual molecules spinning -- so fast that the molecules get pulled apart.
A laser is an intense beam of light, and light is essentially an oscillating electric field. If the light is 'polarized' -- oscillating up and down along one axis -- the electric field can 'grab' a molecule and force it to align with the axis of the polarization. By combining the beams of two lasers, the polarization axis can be made to rotate. Rotate the polarization axis and the molecule rotates with it. This is the principle of the 'optical centrifuge'.
Anyone who has ridden the 'Wall of Death' at a funfair, where you are held against the inside wall of a huge rotating drum, apparently in defiance of gravity, knows what happens inside a centrifuge. The 'centrifugal' forces set up by the spinning push outward, and the faster the rotation the harder the push. To accustom themselves to the enormous forces they suffer during take-off, astronauts train in giant centrifuges. Cultured readers will remember the scene in '_Moonraker_' when James Bond was almost pulled to pieces inside one.
David Villeneuve and colleagues at the National Research Council of Canada, Ottawa, use their optical centrifuge to do the same to molecules of chlorine gas1. The dumb-bell-shaped chlorine molecule is made up of two atoms joined by a bond. Place a cloud of chlorine gas in the optical centrifuge and the molecules begin to spin about their centres, pulled around by the rotating polarization axis of the laser beam.
Villeneuve's team looked at what happens at very high rates of rotation, so fast that the chlorine molecules spin 6 thousand billion times a second. At such a high speed the centrifugal forces -- the same forces that nearly put an end to James Bond -- are enough to snap the bonds holding each molecule together. The molecules shatter into a shower of chlorine atoms.
It's a very elaborate way to break gas molecules apart; you could do the same just by heating, for instance. But potential uses of the optical centrifuge include separating gases of different molecules, or even of isotopes of the same molecule, because heavier molecules break apart at slower spin rates. For some molecules the centrifuge could be used to break only selected bonds, leaving bonds away from the centre of rotation intact, opening the door to direct bond-by-bond 'control' of molecular chemistry.
Spinning molecules into exotic 'rotational energy states' could also provide a laboratory for investigating the 'energy structure' of complex molecules perhaps revealing new routes for chemical processes. More speculatively, a dense gas of spinning molecules might provide a new source of intense high-frequency radiation easily 'tunable' to frequencies not available from lasers.
Villenueve,D. M. et al. Forced molecular rotation in an optical centrifuge. Physical Review Letters 85, 2000.
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Haw, M. Molecules in a spin. Nature (2000). https://doi.org/10.1038/news000720-6