Single N₂ Bonding Orbital Imaged

It's tough to get electron orbitals to smile for a camera. But that hasn't stopped scientists from photographing them. Researchers in Canada have developed a technique for recording three-dimensional images of molecular orbitals. The procedure, which is based on ultrafast laser methods, may lead to new probes of chemical reaction dynamics and techniques for studying the motions of individual electrons.

Analytical methods such as X-ray diffraction and scanning tunneling microscopy are invaluable to scientists for their ability to probe electron density. The atomic structures of an enormous number of chemical systems have been determined by those techniques. Yet researchers would benefit from a procedure that's capable of probing individual electron orbitals with a resolution on the timescale of chemical reactions. Now they have one.

Scientists at the Canadian National Research Council (CNRC) in Ottawa have demonstrated a procedure in which femtosecond (10^–15 second) laser pulses are used to construct a 3-D image of a single molecular orbital. The technique, which bears similarities to medical tomography, was used to image the highest occupied molecular orbital (HOMO) of a simple test system: dinitrogen [Nature, 432, 867 (2004)].

The study was conducted by postdoctoral associate Jiro Itatani, staff member David M. Villeneuve, and group leader Paul B. Corkum, all of whom are at CNRC's Steacie Institute of Molecular Sciences, and their coworkers at other institutions in Canada and Japan.

Constructing an image of N₂'s HOMO is a multistep process. First, the Ottawa group aligns the nitrogen molecules in a particular orientation by exposing them to a brief pulse of linearly polarized laser light. An instant later, the researchers deliver an intense femtosecond laser pulse to the gas molecules. Within the 10^–15-second period of the intense pulse, an electron in N₂'s HOMO is forced away from the molecule and then driven to recollide with the molecule energetically. The collision angle between the electron and the molecule is fixed by controlling the angle between the two laser pulses.

The team members explain that the laser-molecule interactions produce a series of high-order harmonics--radiation with overtone frequencies that are multiples of the initial laser pulse. They note that the spectrum of overtones carries 2-D information about the electron orbital structure. So by varying the collision angle (via the alignment laser) and recording numerous overtone spectra, the group is able to use tomographic methods to construct a 3-D image of the HOMO.

Now that the group has shown that, in a simple molecule, the HOMO--the orbital directly involved in bonding--can be imaged with femtosecond resolution, more complex chemical systems may soon be within reach.

Henrik Stapelfeldt, an associate professor of chemistry at Aarhus University in Denmark, agrees. Commenting on the study in the same issue of Nature, Stapelfeldt notes that "in the near future, it should be possible to watch directly how electron clouds change during chemical reactions. This would be progress, indeed, and provide insight into one of the most fundamental steps in chemistry."