ELECTRON ORBITAL of a nitrogen molecule as imaged (top) agrees quite well with the orbital as calculated from theoretical models (bottom). Each image is 0.6 nanometer by 0.6 nanometer in size. Colors represent the amplitude of the quantum wave function--the electron is most likely to be found at the red and dark blue locations. Image: DAVID M. VILLENEUVE
In the quantum world, objects are described by wave functions. Electrons around a molecule, for example, exist in wavelike orbitals, smeared-out shapes that determine properties such as the electrons' energy and the propensity for the molecule to undergo various chemical reactions. But orbitals are slippery critters that, because of Heisenberg's uncertainty principle, defy routine efforts to image them completely and accurately. Now, however, researchers at Canada's National Research Council in Ottawa have produced a three-dimensional scan of the outermost electron orbital around a nitrogen molecule. The "shutter speed" of the imaging method is fast enough that scans might one day be taken of molecules caught in the middle of a chemical reaction.
The group, led by Paul B. Corkum and David M. Villeneuve, uses a laser pulse lasting just 30 femtoseconds (3 x 10¿14 second). During the course of the laser pulse, the electric field of the light wave oscillates about a dozen times. Each oscillation drives the outermost electron of the nitrogen molecule away from the molecule and back again.
Although it might seem that the team relies on a laser to "light up" the electron, it is actually the electron on its way back toward the molecule that acts as the imaging beam. More precisely, the laser's field drives a small proportion of the electron's wave function away and back. Think of it as the electron being in two places at once; mostly it is still in place in its original orbital around the nitrogen, but partly it is being ripped away.
The sharp acceleration turns the traveling electron wave into a plane wave, like a nice regular pulse of an electron beam with an extremely short wavelength--exactly the kind of beam useful for imaging. When the plane wave returns and crosses the molecule, it produces an interference pattern with the stationary part of the electron wave function, like two trains of water waves crossing and forming a checkerboard disturbance.
To complete the imaging, that interference pattern must be detected. As the plane wave travels along, the pattern oscillates rapidly, causing it to emit ultraviolet radiation that the researchers observe. Information about the shadow of the electron orbital as seen by the traveling electron wave is imprinted on the ultraviolet emission. Producing a three-dimensional image requires repeating the process at different angles, like a hospital CT scanner. The angles are set by aligning all the nitrogen molecules in the sample with a somewhat weaker laser pulse a few picoseconds (10¿12 second) before the imaging pulse arrives.
The result of the imaging agrees quite well with the shape of the electron orbital computed theoretically. "I was very excited when I saw the experimentally obtained images of molecular orbitals for the first time," says Ferenc Krausz of the Max Planck Institute of Quantum Optics near Munich. "The technique has great potential." In late 2003 Krausz's group demonstrated another kind of imaging using 250-attosecond (2.5 x 10¿16 second) pulses of extreme ultraviolet light, the shortest light pulses ever produced. The two methods are complementary--Krausz's involving the dynamics of inner electrons, Corkum and Villeneuve's working on the outermost electrons.
Of great interest will be the application of the technique to more complicated molecules and to molecules caught in the process of undergoing a chemical reaction. Villeneuve says he is considering trifluoromethyl iodide, which can be broken up by pulses from the group's laser. "Then we could follow the dissociation," he says, "and measure how the atoms move."