Freezing electrons in flight

(Phys.org)—Using the world's fastest laser pulses, which can freeze the ultrafast motion of electrons and atoms, University of Arizona physicists have caught the action of molecules breaking apart and electrons getting knocked out of atoms. Their research helps us better understand molecular processes and ultimately be able to control them in many possible applications.

In 1878, a now iconic series of photographs instantly solved a long-standing mystery: Does a galloping horse touch the ground at all times? (It doesn't.) The images of Eadweard Muybridge taken alongside a racetrack marked the beginning of high-speed photography.

Approximately 134 years later, researchers in the University of Arizona department of physics have solved a similar mystery, one in which super-excited oxygen molecules have replaced the horse, and ultrafast, high-energy laser flashes have replaced Muybridge's photo emulsion plates.

Using extreme ultraviolet light bursts lasting 0.0000000000000002 seconds – that's 200 quintillionths of a second – Arvinder Sandhu and his team have managed to freeze the unimaginably fast action that ensues when oxygen molecules are zapped with high energies for incredibly short amounts of time.

Observing ultra-short events in atoms and molecules has become increasingly important as scientists are trying to gain a better understanding of quantum processes on the level of electrons, and ultimately even control those processes to design new light sources, assemble new molecules, or engineer new ultrafast electronic devices, among countless other possibilities.

While Sandhu's group does not hold the world record for generating the shortest light pulses, it has pioneered their use as tools to solve many outstanding scientific questions. Its latest accomplishment, published in Physical Review Letters, is a real-time series of snapshots documenting what happens to an oxygen molecule when it pops apart after absorbing too much energy to maintain the stable bond between its two atoms.

While Sandhu's group does not hold the world record for generating the shortest light pulses, it has pioneered their use as tools to solve many outstanding scientific questions. Its latest accomplishment, published in Physical Review Letters, is a real-time series of snapshots documenting what happens to an oxygen molecule when it pops apart after absorbing too much energy to maintain the stable bond between its two atoms.

In its latest work, Sandhu's team has solved a long-standing debate by measuring how long it takes an oxygen molecule to break apart when zapped with high energy photons: 1,100 femtoseconds. Previous measurements of this phenomenon were in disagreement by as much as 100-fold. In another innovation, this work provides the first experimental measurement of the time it takes for electron to be ejected from a super-excited atom. This process had only been simulated in theory. Sandhu's group found that this spontaneous electron emission happens in about 90 femtoseconds.

The shortest laser pulses achieved so far last 67 attoseconds. According to Sandhu, even shorter "zeptosecond" laser pulses are conceivable, but for now attosecond-pulses get the attention. "We are going to attoseconds because we want to study processes that are faster than the movements of molecules," Sandhu said. "The practical aspects that affect life around us, and the technologies around us are generally governed by electrons and electronic motion."

"The problem we are interested for the future is, what will happen when there are many electrons interacting with each other? Now the experiments become challenging and the theoretical modeling becomes impossible. That is why we have the high energies and the short time resolution. We now can actually look at those processes in real time."

More information: arxiv.org/abs/1207.4740