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Gold surface ablated by femtosecond laser beam under chloroform.

SLAC National Accelerator Laboratory has a love of the femtosecond.

In fact, they are dedicating an entire week to the femtosecond—one quadrillionth of a second—from April 17 to April 21. Each day SLAC scientists explain how science can be used to examine things on the femtosecond scale, which helps better explain virtually every reaction in nature.

On Monday, April 17, Ryan Coffee, a physicist at SLAC, explained in an online video posted to the SLAC website the importance of being able to break things down at the femtosecond level.

“The first steps in the chemical reactions that happen in your blood with oxygen moving around, or photoabsorption in light harvesting in the plants in your garden or even the formation of plaques in your blood that give you diabetes or degenerative diseases happen on the femtosecond time scale,” Coffee said.

As part of Femtosecond Week, Phil Bucksbaum, director of the Pulse Institute at SLAC and Stanford University explained  that until recently the tiny, rapid motion of every molecule, atom and electron needed to obey the laws of quantum physics was hidden from view because it is too small and too fast to record in real time.

However, new tools including the bright X-ray pulses at the Linac Coherent Light Source and the advanced ultrashort pulse lasers and electron beams at SLAC, can capture snapshot images that last only femtoseconds.

Coffee also said recent advancements led to a better understanding of molecular nature.

“The field of ultra-fast science has been used in chemistry for probably 10, 20 years now and it has delivered much of the fundamental understanding of how light interacts with molecules,” Coffee said. “The idea is now we can make these molecular movies because we have now X-rays that can probe the molecules at exactly that natural timeframe of the reactions.

“Now that we have x-rays to see at the scale of molecules and we have the timeframe with femtoseconds that are on the scale of how fast these molecules move, how they change their shape, we can start to look at why nature made the molecules the shapes that she did,” he added. 

According to Bucksbaum, this is a flash of light short enough to freeze the motion of atoms in molecules. Faster probes under development by scientists at SLAC will soon be able to track the motion of electrons as they cross single chemical bonds in less than a single femtosecond.

Scientists can piece these snapshots together to make slow-motion molecular movies that show how nature works and these femtosecond movies can also help scientists develop novel materials and new chemicals and help better explain how all processes in nature depend on femtosecond motion on the atomic scale.

Also included in Femtosecond Week is a Q&A with Agostino Marinelli, who is involved in research and developed related to the femtosecond at LCLS.

“It’s the fastest we can reach now with X-rays, and as an accelerator physicist, I get excited about technical things like that,” Marinelli said in the Q&A posted on the SLAC website on April 17.

Marinelli explained how the femtosecond is broken down.

“Normally LCLS shoots 120 X-ray pulses a second. But you can also make it send two pulses of different energies, separated by a few to 100 femtoseconds,” he said. “You excite your sample with the first one and probe it with the second. You have to observe it within femtoseconds after you excite it because reactions happen that fast.

According to Marinelli, there are several things being studied on the femtosecond scale including chemical reactions where you can see the positions of the atoms rearranging as it happens.

However, Marinelli said the next progression might be to go beyond the femtosecond scale.

“I’m really excited about what I’m about to do, which is this sub-femtosecond project called XLEAP,” he said. “We will shape the LCLS electron beam with a high-power infrared laser and use it to generate pulses that are shorter than a femtosecond! What we will be looking at is energy and electrons moving around a molecule, which happens even faster than the atoms rearranging.

“Right now we’re really blind to all of this. To me, the way I understand it is, going to that timescale, you’re peeking into the very fundamental, quantum nature of the electrons in the molecule.”

In another Q&A posted by SLAC on Tuesday, April 18, Gabriella Carini, Ph.D., of LCLS, explained some of the challenges in working on the femtosecond scale.

“In more traditional X-ray sources the photons arrive distributed over time, one after the other, but when you work with ultrafast laser pulses like the ones from LCLS, all your information about a sample arrives in a few femtoseconds,” she said. “Your detector has to digest this entire signal at once, process the information and send it out before another pulse comes.

“This requires deep understanding of the detector physics and needs careful engineering. You need to optimize the whole signal chain from the sensor to the readout electronics to the data transmission.”

Each day during the week another interview with a SLAC scientist on the miniscule time measurement will be posted online. Some of the topics will include how the femtosecond relates to biology, chemistry, high energy density science and materials. Other activities include a virtual tour of the undulators and near experimental hall at LCLS, which will be posted April 19 and a Twitter chat with the hashtag #femtoweek with Mike Dunne, director of LCLS held on April 18, along with other factsheets and interviews that will be posted throughout the week.

For more information about Femtosecond Week: https://home.slac.stanford.edu/femtosecond-week/

 

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