Breaking the Bonds You Choose
Chemists have long dreamed of targeting one atom in a dense molecular thicket and slicing its bonds with surgical precision, perhaps to nudge a chemical reaction toward desired products or trigger some atomic rearrangement. Many researchers have been developing the use of lasers to directly and selectively break bonds. Now, in the 10 January issue of PRL, a team of physicists reports success with a different kind of selective bond breaking in molecules stuck to metal surfaces. They used x rays from a synchrotron to excite core electrons and reshuffle the electron distribution, rather than targeting the bonds directly. The work represents the most precise control of selective bond breaking ever achieved.
Dumping laser energy directly into a chemical bond might seem a sensible approach to getting a clean cut, but experimenters have found that the energy often dissipates too quickly to blow the bond apart. The route taken by a group led by Dietrich Menzel and Peter Feulner at the Technical University of Munich is different, involving excitation of inner core electrons deep in the atomic shell structure. Exciting these electrons leaves core “hole” states, which are immediately filled as outer electrons fall into them. If these outer electrons are sucked out of the bond by the core hole, the bond vanishes. The trick is to target core electron transitions that somehow lead to the breaking of a selected atom’s bonds.
Such selective bond breaking has been observed previously in gases and other free systems, but the Munich group worked with nitrogen molecules bound to a ruthenium surface. Although nitrogen is a symmetric molecule, only one of the two nitrogen atoms sticks to the surface, and the asymmetric environment slightly alters the electronic energy levels in both atoms. But the change is small: A core electron transition of about 400 eV differs by just 0.7 eV between the two nitrogen atoms. To excite the 1s core electrons, the team used a precisely tunable x-ray beam from the HASYLAB synchrotron in Hamburg. As they varied the photon energy, they detected electrons and nitrogen atoms and molecules that left the surface.
The data showed that the team could selectively break either the N-N bond or the bond with the ruthenium surface, depending on the photon energy, but the routes to these bond breaking events were surprising. Intuitively, one might expect that excitation of the outer atom would split the molecule; likewise, excitation of the inner one should eject the molecule from the surface. But exactly the opposite happens. “We now understand why,” says Menzel. “The states that break the bonds are really the ones you get after the core hole has [been filled],” and the unexpected behavior comes from the way electrons rearrange in response.
Although the result is unlikely to foster new chemical synthesis methods any time soon, it does show a completely different path to bond breaking, according to Menzel. William Egelhoff of the National Institute of Standards and Technology in Gaithersburg, MD, agrees. “It’s neat that you can cleave nitrogen bonds this way, and they did a great job in mapping out the decay paths,” he says. “But it is an extraordinarily expensive way to break bonds; the cost of the photons is enormous.” Still, says Egelhoff, “it’s a nice piece of physics and shows the power of being able to manipulate atoms in a new way.”
–David Voss
David Voss is an editor for Physics.