Adapted from L. Gross et al., PRL, 107 (2011)

Shadow of the orbitals. The pictures on the left show the highest occupied molecular orbital (top) and the lowest unoccupied molecular orbital (bottom) of pentacene, as mapped by the STM. The pictures on the right show the same orbital structures, calculated mathematically.

Molecules Imaged Most Intimately

If you took high school chemistry, then you undoubtedly recall the bizarre drawings of the "orbitals" that describe where in an atom or a molecule an electron is likely to be found. Resembling strange clouds with multiple lobes, the shapes and orientation of the orbitals control where electrons can go and how molecules can share or exchange them in chemical bonding and interactions. Now, a team of researchers has taken a key step toward directly measuring the orbitals of molecules lying on a surface, an advance that should let theorists test the results of their high-precision quantum mechanics calculations and could pave the way to designer molecular devices.

To make the breakthrough, a team of physicists at IBM Research Zurich in Switzerland and the University of Liverpool in the United Kingdom used a device called a scanning tunneling microscope (STM). It consists of a tiny metal finger with a tip only a few atoms wide that moves back and forth just above the surface of a sample. When scientists apply a voltage to the finger, electrons can hop between it and the surface through a process called quantum tunneling. In the simplest setup, the size of the current reveals the density of electrons in the surface, allowing it to be mapped out.

But that's not good enough to map the orbitals of an individual molecule. First off, the density of electrons doesn't directly reveal the mathematical structure of the orbital. That's because the density at a given point depends only on the mathematical square of the orbital, whereas the orbital can also have a positive or negative sign. In fact, it can generally be a complex number with both a "real" part that's an ordinary number and an "imaginary" part that's multiplied by the square root of one. That complex number defines the "phase" of that spot in the orbital. More practically, the run-of-the-mill STM doesn't have the spatial resolution to detect the fine details of the orbital. And the surface beneath the sample molecule is usually metal, too, and its smooth, featureless mash of electron orbitals can camouflage the molecule lying on top of it.

But Leo Gross and colleagues from IBM Research Zurich and the Surface Science Research Centre at the University of Liverpool in the United Kingdom found ways around these problems. Building on this prior research, they first isolated the molecules they wanted to study—organic molecules called pentacene and naphthalocyanine—by coating the surface below with an ultrathin layer of insulating salt. Then, to improve the resolution of the STM and make it sensitive to the phase of the molecule's orbital, the researchers stuck a single carbon monoxide molecule on the metal STM tip. Carbon monoxide has a simple but distinctive outermost orbital structure with two side-by-side lobes sticking out from the end of the tip, one with a positive phase and the other a negative phase.

That phase difference from one side of the tip to the other makes the current running through the tip sensitive to changes in the phase of the orbital of the molecule below, too. In fact, the current maxes out when the plus-and-minus lobes in the tip line up over plus-and-minus lobes in the orbital. The current drops to zero when the tip passes over a single lobe dense with charge because the charge and phase of two lobes of the carbon monoxide molecule interact with the molecule's orbital and cancel out, preventing electrons from tunneling through. That means the tip is particularly good at mapping out the "nodes" or places where the molecule's orbital changes sign. Those are also the places where the orbital goes to zero and the electron is sure not to be found. Thus, the researchers mapped the "nodal structure" of the underlying molecule's orbital, essentially sketching an outline of the spaces that contained electrons.

"They built up nice little pictures," says David Villeneuve, program leader of attosecond science at the National Research Council of Canada in Ottawa. The STM technique is a valuable new way to image a molecule, Villeneuve says. "Sometimes quantum calculations are not correct, and sometimes you see things you don't expect," he says. The researchers themselves hope the technique will lead to engineering designer orbitals for molecule-sized machinery.

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