In 2018, researchers at the SLAC National Accelerator Laboratory developed a way to generate X-ray laser bursts lasting hundreds of attoseconds (or billionths of a second). Let's get to know a little about X-ray laser-enhanced attosecond pulse generation (LEAP). This process is a technology that allows scientists to study how electrons racing around molecules trigger fundamental events in biology, chemistry, materials science and other fields.
"Electron movement is an important process by which nature can move energy," says SLAC scientist James Cryan. “A charge is created in one part of a molecule and transferred to another part of the molecule, potentially starting a chemical reaction. It's an important piece of the puzzle when you start thinking about photovoltaic devices for artificial photosynthesis or charge transfer within a molecule."
Researchers at Slac's Linac Coherent Light Source(LCLS) used attosecond pulses to jolt electrons in a molecule, create an excited quantum state, and measure in unprecedented detail how electrons behave in this state. The findings were recently published in the journal Science.
“XLEAP allows us to look deep into molecules and monitor electron transport on the natural time scale,” explains Agostino Marinelli, XLEAP project leader from SLAC. "This can shed light on a variety of important quantum mechanical phenomena in which electrons are often involved."
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X-ray free electron lasers such as LCLS emit attosecond pulses, which are the shortest possible pulses. The LEAP project broke new ground by generating attosecond pulses of the right wavelength to peer inside the most important tiny atoms, such as carbon, nitrogen, and oxygen. XLEAP pulses can capture electrons and other motion on a previously undecipherable very fast time scale, like cameras with ultra-fast shutter speeds.
When X-ray pulses come into contact with matter, they can excite some of the sample's most securely bound core electrons, known as core excited states. Nuclear excited states are extremely unstable due to their high energies, and they decay rapidly, releasing energy in the form of a fast electron, usually known as an Auger-Meitner electron. This process was previously Auger Decay It used to be known as , but scientists recently decided to include the name of Lise Meitner, who was the first person to identify it for her contributions to contemporary atomic physics.
The researchers changed the wavelength of the LCLS X-rays to create a quantum state of matter called a coherent superposition, a manifestation of the wavelet nature of matter. Excited electrons were indeterminate nuclear excited states at the same time, similar to Schrödinger's cat, which was both dead and alive at the same time. This means that they simultaneously orbit the molecule in different orbits.
To monitor how this coherent superposition of core excited states evolves over time, the researchers created an ultrafast clock called the 'attoclock', which uses a rapidly rotating electric field from a circularly polarized laser pulse as the clock hand. The circularly polarized laser pulse rotated Auger-Meitner electrons emitted during the death of core excited states before settling in the detector. The moment an electron was ejected from the molecule was determined by the position it landed on the detector. By measuring the ejection periods of several Burgu-Meitner electrons, the researchers found consistent superposition state they were able to construct a picture of how it changed with a time resolution of just a few hundred attoseconds.
“This is the first time we have been able to monitor this process and directly measure the electron emission rate,” explains lead author and SLAC scientist Siqi Li. “With our technology, we can observe the detailed electron activity that occurs in the molecule in a few millionths of a billionth of a second.” rather than just observing the process.” It gives us a great way to see what's going on inside the molecule in a very short timeline.”
The researchers are now working on further measurements of more complex quantum behavior as a follow-up to this experiment.
SLAC "In this experiment we're looking at the electronic behavior of a really simple model that you can almost solve with a pen and paper," says lead scientist and co-author Taran Driver. "Now that we've shown we can make these ultrafast observations, the next step is to investigate more complex phenomena that theories have yet to adequately describe."
Cryan argues that the capacity to measure on faster timelines is fascinating because it may contain the key to knowing what the first events in a chemical reaction will be.
“This is the first time solved application of these ultrashort X-ray pulses,” he explains. “It has the potential to become popular in the scientific community that will keep many people intrigued for years to come.”