Attoclock turns electrons into movie stars
16 September 2011 by Lisa Grossman
The Steve McQueen of the subatomic world (Image: Jila, University of Colorado)
AN ELECTRON takes just billionths of a billionth of a second to escape its host molecule - mere attoseconds. Now we have the first snapshots of what is the initial step in almost every chemical reaction.
"We can watch not only the atoms and the nuclei in a chemical reaction. Now we can even watch the electrons," says physicist Andreas Becker of the University of Colorado in Boulder. His team, mostly based at Goethe University in Frankfurt, Germany, zapped a molecular hydrogen ion - composed of two protons and one electron, the simplest molecule known - with an ultrafast infrared laser pulse. This booted the electron out of the ion and allowed the researchers to trace the path it took.
It's already possible to make movies of molecules and atoms in motion during chemical reactions using laser strobe lights that flash every few femtoseconds (10-15 seconds). However, before two substances can react, an electron must first make the leap from one atom or ion to another, a process that takes only a matter of attoseconds (10-18 seconds).
Laser pulses of 80 attoseconds duration exist and in theory could be used to make a movie of an electron's motion. Becker's team used laser pulses lasting femtoseconds, which are easier to produce. By rotating the pulses once every 1.55 femtoseconds, their pulses tracked motion every 15 to 20 attoseconds, similar to the way the minute hand on a clock tracks minutes, even though it takes an hour to complete one cycle.
The team expected the electron to break free of the hydrogen ion when the laser's electric field was strongest. Surprisingly, the electron made its escape (see the white arrow) 350 attoseconds later. "It changed our understanding of how a molecule is ionised," says Becker. "We thought we understood this process, at least for the simplest molecule." The findings will be published in Physical Review Letters.
More work is needed to determine what causes the electron's delay, a first step towards precisely controlling reactions via electron motion, says Becker. "You would like to take an electron and push it where you wish it to go," says attosecond pioneer Paul Corkum of the University of Ottawa in Canada. "That's the ultimate dream."
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Electron strobe turns atoms into movie stars
11:47 21 November 2008 by Colin Barras
For similar stories, visit the Nanotechnology Topic Guide
The latest movies from Los Angeles are silent, and filmed in grainy black and white - but they are receiving rave reviews from around the world. No, these films aren't recently rediscovered archives of the antics of Harold Lloyd - they are the clearest ever footage of atoms in motion.
For years, transmission electron microscopes (TEMs) have been able to resolve individual atoms, and even objects just a fraction of a nanometre across.
This is achieved by shooting a finely focused beam of electrons through a specimen, and seeing how it is altered by the object.
But although TEMs can provide an exquisitely detailed 3D picture of a specimen, the resolution in the fourth dimension - time - is less impressive.
The devices are limited by having an electron beam produced by heating a positive electrode - a method that unleashes a large burst of electrons emitted over several milliseconds, resulting in blurred movement.
Strobe effect
Now Ahmed Zewail, a chemist at the California Institute of Technology in Pasadena, has developed a way to produce faster, shorter electron pulses.
These can be used like a strobe light to reveal atoms in motion, letting electron microscopes capture more frequent snapshots, and producing high-resolution footage of atoms in motion.
Zewail is a past master at speeding things up. He was awarded the 1999 Nobel Prize in Chemistry for his work on femtochemistry - studying chemical reactions just a few femtoseconds (billionths of a millionth of a second) long using pulses of laser light.
His latest work has now created another new field - ultrafast electron microscopy. Zewail's team use an oscillating laser to illuminate the cathode in their microscope with pulses of UV light about 100 femtoseconds long.
The cathode's surface has a high quantum efficiency, meaning it readily releases electrons when bombarded with photons. It converts the femtosecond light pulses into femtosecond electron pulses, of just a few tens of electrons.
Pioneering movies
Those short pulses act like a camera flash, creating a crisp image of the target specimen at a very precise moment in time. Zewail's research team can generate movies showing the picosecond (millionths of a millionth of a second) motion of atoms (see video, top).
The results are somewhat like the first pioneering movies of trotting horses created by Eadweard Muybridge in the late 19th century.
One of the "firsts" Zewail has already captured on film is the response of atoms in a graphite sheet to being heated with a laser pulse. Until now, the reaction of materials to such forces has only been watched at much larger scales.
The first generation of the new microscope was made in 2005 (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0502607102), but the latest machine is the first to focus down to the subatomic scale - made possible by the production of electron pulses that are 66% more energetic.
The new microscope also contains a second laser, with pulses a thousand times longer than the femtosecond ones. That is equivalent to an extra set of lenses, making it possible to seamlessly move from the atomic to the micrometer scale in space, and from the picosecond to the millisecond scale in time.
"We consider the second generation [device] as a universal microscope - it's much more versatile," says Zewail.
Revolutionary device
Other leading microscopists say the new design is a highly significant development. John Thomas, at the University of Cambridge, UK, recently described it as a "revolutionary" advance that will change physics, biology, and material science (Thomas's commentary on the first generation device is available here).
"This demonstrates, impressively, how real-time observation of the microcosm can reveal features of matter otherwise inaccessible," says Eleftherios Goulielmakis, a physicist at the Max Planck Institute for Quantum Optics in Garching, Germany.
Earlier this year Goulielmakis' research group generated the shortest ever pulse of light - just 80 attoseconds (a billionth of a billionth) in length - which could be used to go a step further, and capture the motion of electrons within the atoms.
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Fastest-ever flashgun captures image of light wave
Fastest-ever flashgun captures image of light wave
19:00 19 June 2008 by Colin Barras
However hard you stare, you would still miss it. Researchers have found a way to generate the shortest-ever flash of light - 80 attoseconds (billionths of a billionth of a second) long.
Such flashes have already been used to capture an image of a laser pulse too short to be "photographed" before (see right).
The light pulses are produced by firing longer, but still very short laser pulses into a cloud of neon gas. The laser gives a kick of energy to the neon atoms, which then release this energy in the form of brief pulses of extreme ultraviolet light.
The trigger pulses fired at the neon cloud are themselves only 2.5 femtoseconds, billionths of a millionth of a second, long, says team member Eleftherios Goulielmakis at the Max Planck Institute for Quantum Optics in Garching, Germany.
Light punch
The trigger pulses contained only one or two oscillations of a light wave so that they packed a compact energy punch when they reached the neon cloud.
To do this, the researchers had to corral the trigger-pulse photons into a tightly packed bunch using a device called a chirped mirror.
These multilayered mirrors make the photons at the front of a pulse travel further than the slower photons at the rear do. That gives the back markers time to catch up, in this case producing a tight pack of photons that hit the neon atoms at roughly the same time.
Photon finish
To find out how short the light flashes from the neon atoms were, Goulielmakis and colleagues directed them onto a second neon gas cloud.
As each flash is intense enough to completely ionise a neon atom and release an electron, the researchers could use those electrons like a flashgun, to illuminate some of the original 2.5 femtosecond trigger pulses of laser light.
"Only sampling them with a "sampler" way shorter than that can render them visible," explains Goulielmakis.
Recording the energy of the electrons that passed through the pulse generates a crisp side-profile of the short laser beam, not unlike a sporting photo-finish image (see right). The image of the laser clearly shows the single oscillation of the trigger pulse.
Computer analysis of the image reveals that the flashes of light used to make the electrons lasted just 80 attoseconds - the shortest ever made.
Electron 'camera'?
Jonathan Marangos at Imperial College London, UK, says the super-short flashes could let researchers image the movement of electrons around large atoms.
"Any better understanding of the microscopic world is going to have an impact across all of science," he says.
The previous record for the shortest light pulse was 130 attoseconds, set in 2007. "To go from 130 to 80 attoseconds is a major step," says Marangos.
In the future, Goulielmakis hopes to produce light pulses of 24 attoseconds, the atomic unit of time, defined as how long it takes an electron to travel from one side of a hydrogen atom to the other.
But Marangos thinks even shorter pulses are possible. "There's nothing magical about the atomic unit of time," he says, saying zeptosecond pulses of trillionths of a billionth of a second might be possible. These would be capable of imaging the movement of nuclear particles like protons, says Marangos.
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