A blink of the eye, a fraction of a second and a flash are all way too slow to describe the speed at which electrons move. Electrons change their position on timescales measured in attoseconds, or millionths of a billionth of a second – a period of time that is so brief that there are as many attoseconds in one second as there have been seconds since the birth of the universe. It’s a blip of time that for most of us is impossible to comprehend, but not to Taran Driver, a staff scientist at the Department of Energy’s SLAC National Accelerator Laboratory.
Driver has spent the past few years working with SLAC’s Linac Coherent Light Source (LCLS) using attosecond X-ray pulses to initiate and capture electron changes in molecules excited by light. In recognition of his excellent work and contributions to the field, the LCLS User Executive Committee has honored Driver with its 2023 LCLS Young Investigator Award. The award recognizes outstanding contributions by early career scientists who perform research using LCLS.
“I’m delighted to receive the award,” Driver says, “but it was an extremely collaborative effort to get here. So many people are putting in so much time and effort and ingenuity to make this amazing machine work, and give us the opportunity to do this research in the first place.”
Capturing tiny moments
Learning how electrons move during a reaction is the first step toward being able to modify a chemical reaction, Driver says. “Can we change how chemistry is happening by changing what the electrons do on this very, very, very fast attosecond timescale?”
To take a snapshot of the super speedy electrons in action, Driver and his colleagues use a method called pump-probe spectroscopy. The scientists program LCLS to shoot out attosecond X-ray pulses that first stimulate, or pump, a molecule with a photon to prompt a rearrangement of the molecule's electrons. This is analogous to kickstarting a chemical reaction.
Then the facility shoots out another X-ray pulse to probe what electrons are doing in response to the initial pump. “The probe allows us to see where the electrons are in the molecule at a given moment in time. For example, we can ask whether the electrons have moved from one end of the molecule to the other as a result of the pump”, Driver explains. By changing the time delay between pump and probe, a time history of the reaction can be scanned with attosecond precision.
“Studying electrons at this speed is a very new capability,” Driver says, “The goal for now is to understand what the electrons are doing. As we learn more about this, we hope one day to be able to alter a chemical reaction with such small-scale tweaks of electrons.”
Ruaridh Forbes, Driver’s colleague and staff scientist at SLAC, nominated Driver for his work in pushing forward the experimental capabilities and scientific output of the attosecond program at LCLS.
“Taran was critical in developing new instruments, made several important discoveries, has driven several projects to completion and has shown leadership in interacting with our scientific user community,” Forbes said. “The achievement is particularly noteworthy since Taran joined the field of ultrafast X-ray science recently from previous graduate work in biostatistics, which highlights his ability as a talented multidisciplinary scientist.”
A beam of excitement
Driver, who holds undergraduate degrees in physics and philosophy, completed his doctorate at Imperial College London, where he spent most of his time analyzing the structure of proteins using statistics. Toward the end of his PhD program, Driver joined his advisor for an experiment at LCLS with a few other students from his group.
It was during that visit when Driver met his current supervisor, James Cryan, senior scientist at SLAC, who soon offered Driver a position with his research team. “I gratefully accepted,” Driver says, “because I was very excited by the science that was happening here, and I still am.”
Driver is thankful to Cryan for his continued support and mentorship, as well as the mentorship of SLAC assistant professor Agostino Marinelli. “The people who I work with on a day-to-day basis are incredibly motivated and smart and have phenomenal ideas,” he says.
He also noted what an amazing time this is to be in the field. The ability to produce attosecond pulses was awarded the Nobel Prize in Physics this year, and at SLAC, the culmination of over a decade’s work has just delivered a major upgrade to LCLS which will provide the brightest source of attosecond X-ray pulses in the world – moving from 120 pulses per second to 1 million per second.
Driver and his colleagues are planning to get the first beam time from this new facility in the next couple of days and he says he can’t wait. “It's all very exciting. We’re sitting downstairs in the basement, playing around with this amazing machine with the aim of answering some really fundamental questions in science.”
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