A study of electron dynamics timed to millionths of a billionth of a second
reveals the damage radiation can do on a molecular level.
The first-of-its kind study used ultrafast X-ray laser pulses to disrupt the
electrons in a molecule of nitrous oxide and measure the resultant changes
with unprecedented accuracy.
The work, published today in Science, was performed at the Linac Coherent
Light Source (LCLS) at the Stanford Linear Accelerator Centre (SLAC),
Stanford, U.S. and was supported by a team of five scientists from Imperial
College London.
Conventional X-rays used in imaging and radiotherapy can cause damage to
cells, but exactly how on a molecular level is not known. Additionally, new
high-intensity and short-pulse-duration X-ray lasers are being proposed to
image smaller molecules with greater precision, leading to questions about
potential damage this could cause to living tissue.
For the first time, researchers have been able to measure the behavior of
electrons in a molecule as it responded to irradiation by ultrafast X-rays
on attosecond timescales—less than millionths of a billionth of a second.
Understanding to new limits
Co-author Professor Jon Marangos, from the Department of Physics at
Imperial, said: "Being able to reach a few hundred attosecond precision when
timing electron dynamics means we can now begin to understand certain
phenomena to new limits.
"The fact is that some electron dynamics in important issues, like radiation
damage to biomolecules, have so far been too fast for us to understand. With
this new understanding we might, for example, conceivably be better able to
mitigate unwanted radiation damage during radiotherapy."
The researchers fired simultaneously an X-ray pulse and a laser pulse at a
molecule of nitrous oxide—the first to knock an electron out of its place,
and the second to time the resultant changes. Previously, using conventional
X-rays, researchers had observed electron emission in a process known as
Auger-Meitner (AM) decay on a fast timescale.
Now, with the faster X-ray lasers, they were able to observe a new
phenomenon imposed on AM decay. Electrons can be difficult to track because
of quantum effects, which means the electrons can be in several quantum
states at once.
The very short X-ray pulse creates a 'quantum coherence'—a superposition of
different electron quantum states of the highly excited molecule. Measuring
with attosecond precision revealed a 'quantum beat'—a repeated pattern
created by the coherence—imposed on the AM decay as observed in the emitted
electron current.
Co-author Dr. Vitali Averbukh, from the Department of Physics at Imperial,
said: "Tackling individual coherent quantum effects stemming from this
irradiation is necessary for building a new physical picture of radiation
damage by intense ultrafast X-rays. The current work is one of the first
steps in this direction—instead of the familiar AM decay, we observe quantum
beats, which are a completely different type of dynamics that translate into
irreversible chemical change.
"How much the quantum coherence affects this chemical change is a matter of
intense investigation, but the initial results obtained by quantum chemists
here at Imperial suggest that such quantum coherence can indeed steer
chemical change."
Attoclock measurements
The timing of the AM decay and the quantum beat was accomplished using an
'attoclock,' which makes use of the circularly polarized optical laser pulse
fired at the same time as the X-ray pulse. The laser field direction rotates
very fast and imparts momentum to the electron, which tags its emission
time.
The signal from an electron was recorded as a position on a momentum
sensitive detector, recording the time in the laser field's rotation that
the electron appeared. This allowed the team to precisely time the electron
dynamics based on where on the detector the signals appeared.
The ultrafast lasers and attoclock setup used by the team will pave the way
for studies that observe fast electron motion in more complex molecules over
sub-nanometer (one-billionth of a meter) spatial scales, allowing
researchers to track the interactions between different parts of the atoms
and molecules under investigation.
Reference:
Siqi Li et al, Attosecond coherent electron motion in Auger-Meitner decay,
Science (2022).
DOI: 10.1126/science.abj2096
Tags:
Physics