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Molecules at the Movies

Atoms and molecules are not the most photogenic of subjects. Once you’ve zoomed in on their miniscule size, you still have to find a way of snapping electrons hurtling around at breakneck speeds of up to 10 million km/h.

Physicists at Imperial College London have however made hydrogen and methane molecules into unsuspecting movie stars thanks to a revolutionary new measurement technique.

Using a process known as high harmonic generation (HHG), this technique can capture ‘films’ of chemical processes at timescales that almost defy the human imagination.

The principle bears similarities to strobe photography, where rapid motion is freeze-framed by leaving the camera’s shutter open in a darkened environment and then producing a bright flash of light. The shorter this flash, the faster the motion that can be recorded.

Freezing the movement of a speeding bullet, for example, requires a flash of light no longer than 1 microsecond (see image above).

In HHG, pulses of laser are fired into a jet of gas and the high intensity wave of light sweeps an electron off each atom. But the electron quickly smashes back into the resulting ion. ‘In this collision, the electron’s kinetic energy is released in an extremely short but very bright burst of x-ray radiation,’ says Dr John Tisch, who worked with Imperial College colleagues Dr Sarah Baker and Professor Jon Marangos to develop this new technique.

Blink and you’ll miss it

This x-ray ‘flash’ can be squeezed down to a staggering 100 attoseconds long - pretty impressive when you consider that in the time it takes for you to blink, a whopping 300 million billion attoseconds have already slipped by.

By analysing certain characteristics of the x-ray beam, researchers build up a ‘movie’ of the molecule, for example as it changes its structure after being rapidly ionised. The time interval between the individual ‘frames’ in these ‘movies’ is also 100 attoseconds - resulting in an effective framing-rate of 10 million billion frames per second, compared to around 2000 frames per second for the super-slow motion used in TV sports coverage.

This new measurement system and related techniques being developed by a number of groups around the world could revolutionise our fundamental understanding across science.

‘The kind of dynamics that we are now beginning to be able to capture control the function of many molecules', comments Dr Tisch. And putting more complex molecules under the spotlight in years to come could yield ‘footage’ of processes that have never been seen before.

So it might not be long before scientists attend the world premiere screening of photosynthesis or entangled electron states. Such understanding could help to create quantum computers, faster electronics, designer materials or artificial photosynthesis.

Imperial College’s physicists may be the current world record holders for the fastest ever observation of molecular dynamics, but, as Dr Tisch puts it, ‘the relentless drive for shorter and shorter measurement techniques’ is still on. With molecules swiping all the leading roles, it’s clear that slow motion takes are truly no longer the preserve of film stars and football players.

Dr Sarah Baker checks the alignment in the HHG chamber

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