Ultrafast electron diffraction (UED) enables study of very rapid nuclear motions

An ultrafast “electron camera” at the Department of Energy’s SLAC National Accelerator Laboratory at Stanford University 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 they used is called ultrafast electron diffraction (UED).

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% — from 0.27 to 0.39 millionths of a millimeter – before returning to its initial state. One vibrational cycle took about 400 femtoseconds.

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Fig. 1: In the experiment, a laser pulse (green) hit a spray of iodine gas (at right). This caused vibrations in the iodine molecules, which consist of two iodine atoms connected via a chemical bond (top left). The molecules were then hit by an electron beam (blue), generating a characteristic diffraction pattern.

“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.”

Using the UED, an electron beam shines through a gas of iodine molecules, with the distance between the two iodine nuclei in each molecule producing an interference pattern similar to that of a classic double-slit experiment. The beam hits a detector and the resulting intensity pattern makes a diffraction pattern. This immediately tells the distance between the nuclei.

The UED method has been under development by a number of groups throughout the world since the 1980s. However, the quality of electron beams has only recently become good enough to enable femtosecond studies. SLAC’s instrument benefits from a high-energy, ultrabright electron source originally developed for the lab’s femtosecond X-ray laser, the Linac Coherent Light Source (LCLS).

The results will be published in Physical Review Letters. See more from SLAC at http://goo.gl/CzI13I and this video: https://youtu.be/XVvhQIlCft8.