Progress in Molecular Film Production Shows How Molecules React to Two Photons of Light

Diffraction pattern obtained by scattering of x-rays on an iodine molecule in a detector at the National Accelerator Laboratory SLAC. Hundreds of these samples from a free-electron laboratory X-ray laser were joined together to create a “molecular film” showing how the molecules reacted unexpectedly when two photons of light hit at once. Scientists say this new approach should work with larger and more complex molecules. Credit: Bucksbaum group / PULSE Institute

Over the past few years, scientists have developed amazing tools – “cameras,” which use x-rays or electrons instead of ordinary light to take quick pictures of molecules in motion and turn them into molecular films.

Now scientists from the SLAC National Accelerator Laboratory of the Department of Energy and Stanford University have added another feature: by tuning their lasers to get iodine molecules with two photons light coloured immediately, instead of the usual single photon, they caused completely unexpected phenomena that were captured in slow motion films in just trillions of a second.

The first film they made with this approach is described on March 17 in Physical Review X, shows how two atoms in the iodine molecule are mixed back and forth, as if connected by a spring, and sometimes fly apart when intense laser radiation hits. The action was detected by a Linac Coherent Light Source (LCLS) laboratory hard X-ray free-electron laser. Researchers noted that some of the reactions of the molecules were amazing, while others were noticed earlier with other methods, but never in such detail and not directly, without relying on prior knowledge of how they should look.

The researchers added that a preliminary analysis of larger molecules that contain different atoms also allows them to be removed in such a way that they can take a fresh look at molecular behavior and fill the gap in which previous methods fail.

“The picture we got this way was very rich,” said Philip Bucksbaum, a professor at SLAC and Stanford University and a researcher at the Stanford PULSE Institute, who led the study with PULSE doctoral student Matthew Ware. “The molecules gave us enough information so you can see how the atoms move at distances less than an angstrom, which is about the width of two hydrogen atoms, in less than a trillionth of a second. We need a very fast shutter speed and high resolution to see this level of detail, and now it is only possible with a hard free electron X-ray laser such as LCLS. "

Double-barreled photons

Iodine molecules are a favorite subject for this kind of research, because they are simple – only two atoms are connected by an elastic chemical bond. Previous studies, such as with the SLAC “electronic camera,” investigated their response to light. But so far, these experiments have been created to initiate motion in molecules using single photons or particles of light.

In this study, scientists tuned the intensity and color of an ultrafast infrared laser so that about one tenth of the iodine molecules interacted with two photons of light – enough to make them vibrate, but not enough to separate their electrons.

Progress in Molecular Film Production Shows How Molecules React to Two Photons of Light

This image contains hundreds of images or frames of a “molecular film” shot with a free electron x-ray laser at the SLAC National Accelerator Laboratory. He shows how simple iodine molecules sometimes react in unexpected ways when two photons of light hit at once, and a new approach, which, according to scientists, should work for larger and more complex molecules. Each image is a single diffraction pattern created by x-rays scattering atoms in one molecule, and looks like a thin horizontal line, just one pixel in depth. When you look at one color bar from the bottom up, subtle line changes show how the positions of the atoms of the molecules move back and forth many times in a picosecond or trillionth of a second. Courtesy: (Bucksbaum group / PULSE Institute

Each hit was immediately followed by an X-ray laser pulse from LCLS, which scattered the atomic nuclei of iodine into the detector to record the reaction of the molecule. Changing the time intervals between light and x-ray pulses, the scientists created a series of images that were combined into a film about the reaction of a molecule to a stop, with frames of only 50 femtoseconds or millionths of a billionth of a second.

Researchers knew that colliding with iodine molecules with more than one photon at a time would produce what is known as a non-linear response that can deviate in unexpected directions. “We wanted to look at something more complex, what we could see, which might not correspond to what we planned,” as Baksbaum put it. And this is actually what they found.

Unexpected vibes

The results showed that the light energy caused vibrations, as expected, with two iodine molecules rapidly approaching and moving away from each other. “This is a really big effect, and of course we saw it,” said Bucksbaum.

But another, much weaker type of vibration was also found in the data, “the process is weak enough for us not to expect to see it,” he said. "This confirms the discovery potential of this technique."

They were also able to see how far the atoms are from each other and how they move at the very beginning of each vibration – compressing or expanding the connection between them – and also how long each type of vibration lasts.

Only in a few percent of the molecules did the light pulses make the iodine atoms fly apart, rather than vibrate, being thrown in opposite directions at a high or low speed. As with vibrations, fast flights were expected, but slow flights were not.

Bucksbaum said he expects chemists and materials scientists to be able to use these methods effectively. Meanwhile, his team and other laboratory staff will continue to develop tools to see how more and more is happening in the molecules and understand how they move. "This is the goal here," he said. “We are filmmakers, not writers, producers or actors. The value of what we do is that all these things happen in collaboration with other scientists. ”

Molecules "cat Schrödinger" give rise to exquisitely detailed films

Additional Information:
Philip H. Bucksbaum et al. Characterization of multiphoton excitation using time-resolved X-ray scattering, Physical Review X (2020). DOI: 10.1103 / PhysRevX.10.011065

Progress in molecular film production shows how molecules react to two photons of light (2020, March 18)
restored March 18, 2020

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