Snapshot Yields Inside Look At Molecular Movement

“Nothing stands still. By learning how particles move, we can probe the fundamental properties of molecules and how they work,” says Thorsten Weber, a visiting scientist in Berkeley Lab’s Chemical Sciences Division who conducted the research with several other Berkeley Lab scientists, as well as physicists from Kansas State University and institutions in Australia, Germany, and Spain. Their research is published in the Sept. 24 issue of Nature.

Like the opening shot of a pool game, the team fired a single photon at a two-atom deuterium molecule and broke it into its four charged constituents: two electrons and two nuclei (each containing one proton and one neutron). Next, the energy and direction of each particle was measured as it flew from the explosion and hit a position-sensitive detector. This information enabled the scientists to backtrack in time and determine how the particles were oriented inside the molecule precisely when the photon struck. The result is a multi-dimensional picture of the particles’ movements at the moment the molecule disintegrates, an image that inches physicists one step closer to directly observing the inner workings of a molecule.

“We obtained a fingerprint of the moving particles inside the molecule at the time of photo ionization,” says Weber. “And particle dynamics is crucial to understanding the chemical reactions occurring in our bodies and everywhere.”

The combined momenta of a molecule’s electrons and nuclei dictate its geometry and how it binds with other molecules — in other words, what makes the molecule tick. But pinning down all of the particles’ momenta at the same time has proven extremely elusive. Theoreticians, relying on quantum mechanics, can only predict the probability that an electron will possess a given position or momentum.

To get an inside look at all the particles’ dynamics, experimental physicists are developing ways to fragment a molecule in a manner that preserves at least some of its internal motion, which is no easy task. Hit a molecule with an ion, for example, and the ion’s momentum transfers to the electrons and nuclei, which clouds physicists’ ability to determine their true momenta at the time of impact. As Weber explains, it’s like cutting a stretched rubber band with a hammer. The rubber band breaks, but the set-up is ruined.

But bombard a molecule with a photon, which has no mass and no charge, and the photon mainly deposits its energy and kick-starts the fragmentation process. It’s like cutting the rubber band with a scalpel instead of a hammer. With a scalpel, an observer can watch the band explode into two fragments and, more importantly, get a feel for the tension present in the band before it was cut.

With this in mind, the team used a photon from a polarized light beam generated at the Advanced Light Source to excite a deuterium molecule’s electrons. Usually, this releases only one electron, but sometimes both electrons are liberated. If this happens, the two remaining nuclei fly apart because they are both positively charged. In this manner, a single photon peels open an entire molecule.

The four particles are then guided by a combination of electric and magnetic fields onto large detectors. The time it takes the particles to fly from the explosion to the detectors, and the positions where they hit, are used to construct a three-dimensional rendition of the particles’ momenta at the moment of fragmentation.

“We dream of seeing what’s going on in an unperturbed molecule, and for that we use a very sharp knife, like a photon,” says Weber. “It offers the least momentum transfer to blur the results, and some of the information is preserved from the initial state.”

Although physicists can’t yet peer inside a molecule without altering it, this “microscope for motion,” as Weber calls it, gives scientists the best vantage so far of the simultaneous momenta of all of a molecule’s particles. Its snapshots could help them learn how molecules bind at the most basic level, not chemically, but dynamically. They may also enable scientists to someday peer inside more complex and biologically important molecules such as water and carbon dioxide, and observe how their particles govern life-sustaining reactions. And already, with the fragmentation of a deuterium molecule, the snapshots give theoreticians something new to ponder.

“We experimentalists are ahead right now. Calculating such a few-particle break-up is a big challenge for state-of-the-art quantum mechanics,” says Weber. “The forces, charges, and angular momenta of a molecule’s particles are known. We know the ingredients, but when we observe all of the particles moving together in a few-particle system, an image appears that doesn’t match theoretical predictions.”

In addition to Weber, Berkeley Lab’s Eli Rotenberg, George Meigs, and Michael Prior contributed to the research. Reinhard Dörner of the University of Frankfurt led the work, which was funded in part by the Department of Energy. Their Nature article is entitled “Complete photo-fragmentation of the deuterium molecule.”

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California. Visit our website at http://www.lbl.gov.