Just as the hypothetical Schroedinger’s Cat is alive and dead at the same time, molecules hit with a burst of laser light exist in two states at once – excited (top) and unexcited. This weird quantum property allowed scientists at SLAC to make a molecular movie of excited iodine atoms in unprecedented detail. Credit: SLAC National Accelerator Laboratory
Scientists have known for a long time that an atom or molecule can also be in2 different states at once. Now researchers at the Stanford PULSE Institute and the Department of Energy’s SLAC National Accelerator Laboratory have exploited this Schroedinger’s Cat behavior to create Xray movies of atomic motion with much more detail than ever before. The first test of this idea, at SLAC’s Linac Coherent Light Source (LCLS) X-ray laser, created the world’s most detailed X-ray movie of the inner machinery of a molecule – ie a 2-atom molecule of iodine.
The team was able to see details of the molecule’s behavior as small as .3 angstrom – < width of an atom – and as brief as 30 millionths of a billionth of a second, a timescale that captures the vibrations of atoms and molecules. What’s more, their method can be retroactively applied to data from past experiments, not just to future studies. “Our method is fundamental to quantum mechanics, so we are eager to try it on other small molecular systems, including systems involved in vision, photosynthesis, protecting DNA from UV damage and other important functions in living things,” said Prof. Phil Bucksbaum.
The new technique is based on the fact that when a molecule absorbs a short burst of energy, it splits into 2 versions of itself – one excited, the other not. A follow-up burst of X-ray laser light scatters off both versions of the molecule and recombines to form an X-ray hologram that, after some clever processing, reveals the excited state of the molecule in stunning detail. By stringing together a series of these X-ray snapshots, scientists can make a stop-action movie.
“Our movie, which is based on images from billions of iodine gas molecules, shows all the possible ways the iodine molecule behaves when it’s excited with this amount of energy,” Bucksbaum said. “We see it start to vibrate, with the two atoms veering toward and away from each other like they were joined by a spring. At the same time, we see the bond between the atoms break, and the atoms fly off into the void. Simultaneously we see them still connected, but hanging out for a while at some distance from each other before moving back in. As time goes on, we see the vibrations die down until the molecule is at rest again. All these possible outcomes happen within a few trillionths of a second.”
Although the initial laser pulse hits only 4 or 5% of the molecules in the iodine gas cloud, it would be incorrect to say that only this small fraction was excited and the rest were not. In quantum mechanical terms, every single molecule was excited a little bit, like a Schroedinger’s Cat that’s both dead and alive.
This dual state was key to making the molecular movie. It allowed the X-rays to bounce off both states of a molecule at once and recombine to form a hologram pattern of concentric rings that are brighter where the two signals reinforce each other and darker where they cancel each other out. The fact that this pattern formed in the LCLS detector proves that the excited and unexcited states were simultaneously present in each and every molecule, Bucksbaum said; if they had been separated by even a tiny distance, the pattern could not have formed.
The team used mathematical techniques borrowed from atomic physics to amplify the signal from the excited state, which would form the basis of the movie. But the signal from the unexcited state also served as a reference point that helped them reconstruct the behavior of the excited molecule in 3D in a process known as “phasing.”
Any group of molecules hit with a laser pulse will respond the same way, splitting into the equivalent of live and dead cats, Bucksbaum said. But the process can only be clearly and directly observed with intense, ultrashort pulses of coherent light like those from an X-ray laser, and until now no one had thought to take advantage of the Schroedinger’s Cat connection to sharpen images taken with X-rays. The team is already applying their method to data from previous experiments at LCLS to see if they can create more molecular movies.
https://www6.slac.stanford.edu/news/2016-09-21-schroedingers-cat-molecules-give-rise-exquisitely-detailed-movies.aspx
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