Ultrafast Laser Reveals Molecular Mysteries

Eadweard Muybridge used high-speed photography to determine whether a galloping horse touches the ground at all times (it doesn’t). Now, 134 years later, University of Arizona physicists used ultrafast, high-energy laser pulses to solve a similar mystery: What happens to atoms and electrons when they break apart?

Observing ultrashort events in atoms and molecules has become increasingly important as scientists try to gain a better understanding of quantum processes on the electron level and, ultimately, even control those processes to design new light sources, engineer new ultrafast electronic devices or assemble new molecules.

Eadweard Muybridge’s famous “Horse in Motion” marked the beginning of high-speed photography. “In essence, we are following the same principle, only with modern techniques,” Arvinder Sandhu said.

Using extreme ultraviolet light bursts lasting 200 quintillionths of a second, Arvinder Sandhu and colleagues in the Department of Physics froze the action of oxygen molecules zapped with high energies for incredibly short amounts of time. The real-time series of snapshots document what happens to the molecule when it pops apart after absorbing too much energy to maintain the stable bond between its two atoms. The findings were published in Physical Review Letters.

Being able to resolve molecular processes on such short timescales helps scientists to better understand the microscopic dynamics behind the formation and destruction of ozone in the Earth’s atmosphere, for example.

Think baseball
Sandhu compared the principle with trying to take a picture of a fast-pitched baseball hurtling toward a batter. Conventional cameras would either blur the image or not capture the baseball at all. “But we want to study the ball in all its details, its surface, its seam, exactly where it is at any given time and so on,” he said.

“There are two ways to achieve this. You can build a camera with a very fast shutter that opens and closes before the ball has a chance to move, or use a technique called stroboscopy, in which you shine a light on the baseball for a very brief time during which you’ll capture its image,” he added.

Substituting atoms or electrons for the baseball ends the analogy. Microscopic objects move too fast to be captured with either mechanical or electronic devices.

Cue the light pulses
The only way to freeze the action at the level of atoms is to use short packets of femtosecond or attosecond light pulses.

The power delivered by a femtosecond laser pulse used in Sandhu’s lab amounts to 1 TW, the equivalent of the entire US power grid, except that it lasts extremely briefly.

UA physicists led by Arvinder Sandhu (right) take advantage of the world’s fastest laser pulses to take snapshots of ultrafast processes such as chemical reactions. Courtesy of Beatriz Verdugo/UANews.

Although femtosecond pulses are snappy enough to resolve molecular motions such as the rhodopsin in our eyes — which has been clocked at changing its conformation in response to an incoming photon in 200 fs — they don’t cut it when it comes to capturing images of much lighter, faster electrons.

“We were able to study what happens to the atomic structure of helium in the presence of a strong electric field, in real time, as the laser pulse goes through,” said Niranjan Shivaram, a graduate student in Sandhu’s lab. The findings were reported in an earlier Physical Review Letters article.

Electron ejected
In the team’s latest work, the researchers solved a long-standing debate by measuring how long it takes oxygen molecules to break apart when zapped with high-energy photons: 1100 fs. Previous measurements of this phenomenon were in disagreement by as much as a hundredfold.

The work provides the first experimental measurement of the time it takes for an electron to be ejected from a superexcited atom. The process previously had been simulated only in theory. Sandhu’s group discovered that this spontaneous electron emission happens in about 90 fs.

“It is often assumed that if you pump enough energy into a molecule, you’d simply force the electron out,” Sandhu said. “But, instead, we observe the molecule shifting the excess energy around by talking to other electrons and nearby atoms, trying to share energy with them and keep its electron, until it suddenly breaks up.”

An attosecond laser also was applied to study the dynamics in oxygen molecules.

“Not much is known about the physics on the level of excited molecules because they’re very difficult to model mathematically,” said graduate student Henry Timmers. “When you pump the oxygen molecule to these high-energy states, there are multiple pathways to release its excess energy. We were able to analyze each of these pathways individually and what goes on when we drive the electrons out of the atoms.”

Electron controlled
To understand physical and chemical processes — both natural and artificial — it is important to track the movements of molecules, atoms and electrons, Sandhu said.

“Being able to measure the dynamics of electrons and atoms inside of molecules on the shortest possible timescale would allow us to understand the fundamental interactions that govern these molecules,” he said. “But, more importantly, it will give us a knob to control or alter the properties of the dynamics of these atoms or molecules, because now we have a light pulse that can influence the motion in real time.

High-energy lasers are needed to generate the supershort pulses needed to “freeze the action” of molecular processes. Courtesy of Beatriz Verdugo/UANews.

“We are no longer just measuring interactions after they have occurred. We’re actually trying to get into that interaction and control it; for example, to steer chemical reactions in certain directions.”

The shortest laser pulses achieved so far last 67 attoseconds (See: Record Laser Pulse the Key to Hidden Quantum World). Even shorter “zeptosecond” laser pulses are conceivable, Sandhu said, 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,” he said. “The practical aspects that affect life around us and the technologies around us are generally governed by electrons and electronic motion.”

The team will have another mystery to solve: What happens when there are many electrons interacting with each other?

“Now the experiments become challenging and the theoretical modeling becomes impossible,” Sandhu said. “That is why we have the high energies and the short time resolution. We now can actually look at those processes in real time.”

For more information, visit: www.arizona.edu