It sounds as surreal as meeting yourself on the subway: Inside a molecule, an orbital containing a pair of electrons can collide with itself. The collision produces light, and now physicists have used that light to generate the best image yet of such an orbital.
The electrons within atoms and molecules do not travel on continuous trajectories like planets around a star. Instead, they inhabit cloud-like orbitals--quantum waves that give the probability of finding the electron at various locations. Atoms and molecules have multiple orbitals, each with a distinct energy. Electrons crowd into them, two electrons per orbital, from the lowest energy upward. The shape and size of the highest occupied orbitals determine how a molecule bonds to other molecules, and researchers can calculate those shapes for simple molecules. Those calculations are less certain for more complicated molecules, however, and researchers have few techniques that can image orbitals directly.
Now, David Villeneuve and Paul Corkum of the National Research Council of Canada in Ottawa and colleagues report that the orbital can be used to image itself. The trick is to use a pulse of laser light lasting only a few millionths of a nanosecond to pull part of the orbital away from the molecule and then accelerate it back toward it. The two pieces of the orbital recombine. But the returning bit has gained energy, and that extra energy emerges in a flash of light. Like a shadow, the light conveys information about the orbital as seen from one direction. By plucking the outer orbital of a nitrogen molecule from 19 different angles, the researchers pieced together an image that confirmed its dumbbell shape, they report in the 16 December issue of Nature.
"It's absolutely beautiful work," says Jon Marangos, a physicist at Imperial College, London. The technique for producing a 3-dimensional orbital from the 2-dimensional "shadows" is essentially the one used to in medical CT scans, he notes. Henrik Stapelfeldt, a physical chemist at Aarhus University in Denmark, adds that, in principle, the laser pulses are so quick the technique should be able to track orbitals as they change during chemical reactions. "If you could see molecular bonds being broken and reformed," he says, "that would be fantastic."
Femto Science at the NRC Canada