High-Speed Film of Vibrating Molecules

Using SLAC’s instrument for ultrafast electron diffraction (UED), researchers were able to directly see the motions of atomic nuclei in vibrating molecules for the first time. In the experiment, a laser pulse (green) hit a spray of iodine gas (at right). This stimulated vibrations in the iodine molecules, which consist of two iodine atoms connected via a chemical bond (top left). (Source: SLAC)

Using SLAC’s instrument for ultrafast electron diffraction (UED), researchers were able to directly see the motions of atomic nuclei in vibrating molecules for the first time. In the experiment, a laser pulse (green) hit a spray of iodine gas (at right). This stimulated vibrations in the iodine molecules, which consist of two iodine atoms connected via a chemical bond (top left). (Source: SLAC)

An ultra­fast “electron camera” at the Depart­ment of Energy’s SLAC National Acce­lerator Labo­ratory has made the first direct snapshots of atomic nuclei in molecules that are vibrating within millionths of a billionth of a second after being hit by a laser pulse. The method, called ultrafast electron dif­fraction UED, could help scientists better under­stand the role of nuclear motions in light-driven processes that naturally occur on extremely fast time­scales.

Researchers used the UED instrument’s electron beam to look at iodine molecules at different points in time after the laser pulse. By stitching the images together, they obtained a molecular movie that shows the molecule vibrating and the bond between the two iodine nuclei stretching almost 50 percent from 0.27 to 0.39 nanometer before returning to its initial state. One vibra­tional cycle took about 400 femto­seconds. “We’ve pushed the speed limit of the technique so that we can now see nuclear motions in gases in real time,” said co-principal inves­tigator Xijie Wang, SLAC’s lead scientist for UED. “This break­through creates new oppor­tunities for precise studies of dynamic processes in biology, chemistry and materials science.”

Physicists have long known that chemical bonds between atoms are flexible. This flexibility allows molecules to change shape in ways that are crucial for bio­logical and chemical functions, such as vision and photo­synthesis. However, methods to study these motions on a femtosecond timescale have so far been indirect. Spectro­scopy infers these changes from the way laser light interacts with electron clouds around atomic nuclei, and requires theo­retical calcu­lations to turn these data into a picture of the nuclear geometry. This can be done very precisely for small molecules but quickly becomes very challenging for larger molecules.

Researchers also use X-rays to study ultrafast molecular motions. Although X-rays deeply penetrate the electron clouds, interacting with the electrons closest to the nuclei, they don’t yet do so with high enough reso­lution to precisely determine the nuclear positions in current femto­second X-ray studies. In contrast, UED uses a beam of very energetic electrons that interacts with both electrons and atomic nuclei in molecules. Therefore, it can directly probe the nuclear geometry with high reso­lution. “We previously used the method to look at the rotation of molecules – a motion that doesn’t change the nuclear structure,” said Jie Yang from SLAC. “Now we have demon­strated that we can also see bond changes due to vi­brations.”

The concept behind the iodine UED experiment is similar to the classical double-slit experiment. In that experiment, a laser beam passes through a pair of vertical slits, producing an inter­ference pattern of bright and dark areas on a screen. The pattern depends on the distance between the two slits. In the case of UED, an electron beam shines through a gas of iodine mole­cules, with the distance between the two iodine nuclei in each molecule defining the double slit, and hits a detector instead of a screen. The resulting intensity pattern on the detector is called a dif­fraction pattern.

“The charac­teristic pattern tells us imme­diately the distance between the nuclei,” said co-principal investigator Markus Guehr from Potsdam University in Germany and the Stanford PULSE Institute. “But we can learn even more. As the iodine molecules vibrate, the dif­fraction pattern changes, and we can follow the changes in nuclear separation in real time.”

Co-principal inves­tigator Martin Centurion from the Uni­versity of Nebraska, Lincoln, said, “What’s also great about our method is that it works for every molecule. Unlike other techniques that depend on the ability to calculate the nuclear structure from the original data, which works best for small molecules, we only need to know the pro­perties of our electron beam and expe­rimental setup.” Following their first steps using the relatively simple iodine molecule, the team is now planning to expand their studies to molecules with more than two atoms.

“The development of UED into a technique that can probe changes in inter­nuclear distances of a dilute gas sample in real time truly is a great achieve­ment,” said Jianming Cao, a UED expert from Florida State Univer­sity and a former member of the Zewail lab at the Cali­fornia Institute of Techno­logy, who was not involved in the study. “This opens the door to studies of atomic-level motions in many systems – structural dynamics that are at the heart of the corre­lation between structure and function in matter.” (Source: SLAC)

Reference: J. Yang et al.: Diffractive Imaging of Coherent Nuclear Motion in Isolated Molecules, Phys. Rev. Lett. accepted; preprint: arXiv: 1608.07725

Link: SLAC National Accelerator Laboratory, Menlo Park, USA

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