Camera captures atoms moving in a molecule

Ashley N. Paddock, ashley.paddock@photonics.com

The first real-time image of two atoms vibrating in a molecule was recorded with a new ultrafast camera.

A team from Kansas and The Ohio state universities used ultrafast laser pulses to knock one electron out of its natural orbit in a molecule, and the electron then fell back toward the molecule scattered off it. The use of the energy of a molecule’s own electron acted as a “flashbulb” to illuminate the molecular motion.

The feat marks the first step toward observing chemical reactions and controlling them on an atomic scale, said principal investigator Louis DiMauro, a physics professor at Ohio State.

“Through these experiments, we realized that we can control the quantum trajectory of the electron when it comes back to the molecule by adjusting the laser that launches it,” he said. “The next step will be to see if we can steer the electron in just the right way to actually control a chemical reaction.”

Standard imaging methods for a still object involve shooting it with an electron beam, which bombards the object with millions of electrons per second. The new single-electron quantum approach, however, enabled the researchers to take images of rapid molecular motion based on theoretical developments by scientists at Kansas State.

Laser-induced electron diffraction (LIED) provides a new method for imaging gas-phase molecular imaging with picometer spatial resolution and femtosecond timing. In panel 1, the LIED approach is enabled by strong-field molecular tunnel ionization that occurs in a low-frequency laser field (green line). In panel 2, the ionized electron is driven back by the laser field and diffracts from the molecular structure. In panel 3, measurement of the electron’s momentum distribution conveys the structural information at the diffraction time. Courtesy of Cosmin Blaga, The Ohio State University.

The scientists used laser-induced electron diffraction (LIED) – a technique commonly used in surface science to study solid materials – to study the movement of atoms in a single molecule of nitrogen and oxygen, two common gases.

In each case, they hit the molecule with 50-fs laser light pulses, knocking a single electron out of the shell of the molecule and detecting the scattered signal of the electron as it recollided with the molecule.

DiMauro and Ohio State postdoctoral researcher Cosmin Blaga likened the scattered electron signal to the diffraction pattern that light forms when it passes through slits. Given only the diffraction pattern, scientists can reconstruct the size and shape of the slits. In this case, given the diffraction pattern of the electron, the physicists reconstructed the size and shape of the molecule – i.e., the locations of the constituent atoms’ nuclei.

The key element of the experiment was that, during the brief time between when the electron was knocked out of the molecule and when it recollided, the atoms in the molecules had moved, Blaga said. The LIED method can capture this movement – “similar to making a movie of the quantum world,” he added.

The method also provides a new tool to study a matter’s structure and dynamics. Ultimately, the scientists want to understand how chemical reactions occur.

“You could use this to study individual atoms,” DiMauro noted, “but the greater impact to science will come when we can study reactions between more complex molecules. Looking at two atoms – that’s a long way from studying a more interesting molecule like a protein.”

The experiment was published in the March issue of Nature (doi:10.1038/nature10820).