Page last updated at 16:28 GMT, Saturday, 7 February 2009

Photosynthesis viewed in a flash

By Jason Palmer
Science and technology reporter, BBC News

Imperial laser lab
The approach hinges on a method of putting more colours into a laser pulse

A new method of examining the inner workings of plants has shed light on how they harvest the Sun's energy.

Researchers have taken laser snapshots lasting just one ten-thousandth of a billionth of a second to examine the role of electrons in energy transfer.

The approach will be key in discovering how energy trickles through other systems, such as electronic devices, and could lead to better solar cells.

The work is published in the current issue of Physical Review Letters.

Ian Mercer of University College Dublin, Ireland, collaborating with researchers at Imperial College London, UK, examined the protein LH2, a well-known photosynthetic system.

The protein harvests light, helping along the molecular machinery that pulls electrons out of water, in turn driving the reaction that makes sugars from carbon dioxide.

"More generally, we're trying to understand how nature can transport energy across large molecules, and photosynthesis is a good example of where nature does it remarkably efficiently," Dr Mercer told BBC News.

Significant research has been performed to assess the role of electrons in that process with a view to increasing the performance of solar cells, most of which currently operate at an efficiency around just 10%.

Colour full

What has remained unclear, though, is the way in which electrons interact with each other or with the molecules of the machinery.

It's a whole new world of what you can look at
Ian Mercer
University College Dublin

A number of laser-based methods have been developed to examine that electron coupling, but they require that the delicate proteins are subject to thousands or millions of laser pulses, which can change their structure or destroy them altogether.

Dr Mercer's method can look at those electron couplings directly with just one "ultrafast" laser pulse lasting 100 femtoseconds - or ten thousand million times shorter than an average camera flash.

Such short pulses are made up of a broad spectrum of colours, with each colour corresponding to the particular energy of the photons that make it.

The new method works by splitting powerful laser pulses into three beams and crossing them in the protein samples in a specific geometry.

The light that comes out gives for the first time an unambiguous picture of how the different colours - and thus energies - interact inside the protein.

"The fact that it's instantaneous is not just a detail, not just a nicety," Dr Mercer said. "It means we're able to take a picture of any system before that molecule has had a chance for its atoms to move significantly.

"We're looking at the shape of something before the laser was even there - it's a whole new world of what you can look at."

Wide application

Arc spectrum
The data sheds light on how electrons move in chloroplasts

The new method is applicable across a wide array of samples; the intricate details of electron and energy transport are the subject of study in disciplines ranging from electronics to drug design.

The results could be used to help mimic the photosynthesis process in the design of more efficient solar cells - a pursuit in which Dr Mercer says "the impact of a small increase in efficiency is very large for the world".

The method was only made possible by developments at Imperial College London in lasers that can provide the huge range of colours in the laser pulses, a pursuit headed up by John Tisch and Jon Marangos.

The technology was subsequently used at the Rutherford Appleton Laboratory's Astra laser.

"The laser source has opened up a new frontier of optics," Dr Tisch said.

"The beauty of it is that you really can extract the information in a single shot - the data was coming out of this much faster than it could be viewed."

Dr Mercer said that the team is now in discussion with a number of researchers who are keen to apply the method in their own work.

"There isn't a conversation I've had that hasn't resulted in someone saying, 'well, let's get a sample in there'."

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