Imaging molecules in action

An international team of researchers has developed an experimental technique to take 3D images of molecules in action. Their method has been described in The Journal of Chemical Physics.

Quantum mechanics dictates how particles and forces interact, and thus how atoms and molecules work — for example, what happens when a molecule goes from a higher energy state to a lower energy one. But beyond the simplest molecules, the details become complex.

“Quantum mechanics describes how all this stuff works,” said Paul Hockett of the National Research Council of Canada. “But as soon as you go beyond the two-body problem, you can’t solve the equations.” Physicists must therefore rely on computer simulations and experiments instead.

Now, Hockett and researchers from Canada, the UK and Germany have developed a 3D imaging technique that they hope will help scientists better understand the quantum mechanics underlying bigger and more complex molecules. The method combines two technologies: a camera developed at Oxford University, called the Pixel-Imaging Mass Spectrometry (PImMS) camera, and a femtosecond vacuum ultraviolet light source built at the NRC femtolabs in Ottawa.

In most types of mass spectrometry, a molecule is fragmented into atoms and smaller molecules that are then separated by molecular weight. In time-of-flight mass spectrometry, for example, an electric field accelerates the fragmented molecule. The speed of those fragments depends on their mass and charge; to weigh them, you measure how long it takes for them to hit the detector.

Most conventional imaging detectors, however, can’t discern exactly when one particular particle hits. To measure timing, researchers must use methods that effectively act as shutters, which let particles through over a short time period. But this method can only measure particles of the same mass, corresponding to the short time the shutter is open.

The PImMS camera, on the other hand, can measure particles of multiple masses all at once. Each pixel of the camera’s detector can time when a particle strikes it. That timing information produces a 3D map of the particles’ velocities, providing a detailed 3D image of the fragmentation pattern of the molecule.

To probe molecules, the researchers used this camera with a femtosecond vacuum ultraviolet laser. A laser pulse excites the molecule into a higher energy state and, just as the molecule starts its quantum mechanical evolution, another pulse is fired. The molecule absorbs a single photon, a process that causes it to fall apart. The PImMS camera then snaps a 3D picture of the molecular debris.

By firing a laser pulse at later and later times at excited molecules, the researchers can use the PImMS camera to take snapshots of molecules at various stages while they fall into lower energy states. The result is a series of blow-by-blow images of a molecule changing states.

The researchers tested their approach on a molecule called C₂F₃I, which fragmented into five different products in their experiments. The experiments demonstrate the power of this technique, said Hockett, noting, “It’s effectively an enabling technology to actually do these types of experiments at all.”

The technique means it now takes only a few hours to collect the kind of data that would take a few days using conventional methods. This allows for experiments with larger molecules that were previously impossible, enabling researchers to better answer questions like: How does quantum mechanics work in larger, more complex systems? How do excited molecules behave? How do they evolve?

“People have been trying to understand these things since the 1920s,” Hockett said. “It’s still a very open field of investigation, research and debate, because molecules are really complicated. We have to keep trying to understand them.”

The data and analysis software is available online as part of an open science initiative.

Image caption: 3D images of molecules in action. Image credit: Paul Hockett.