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First images of electron orbitals in complex molecules

The areas where electrons in a molecule are likely to be found in space are known as molecular orbitals. These define the chemical and physical traits of matter, so researchers have long strived to image them, but were missing all the details they needed to do so for any molecule more complex than nitrogen. Now scientists reveal a strategy to observe long-unobservable molecular orbitals of more complex molecules. These findings are detailed in the Proceedings of the National Academy of Sciences.

Molecular orbitals describe the way electrons behave in molecules. This behavior is wave-like, according to quantum mechanics. Experiments could directly observe the amplitudes or energy levels of these waves. However, details regarding the phase of these waves, or their position relative to each other,gets lost in these experiments. Both the amplitude and phase of molecular orbitals are needed to image them.

Now Peter Puschnig at the University of Graz in Austria and his colleagues suggest they can recover this lost phase data and image molecular orbitals in complex molecules.

The researchers employ ultraviolet photoelectron spectroscopy to scan molecules clinging onto a metallic surface, such as silver, which helps line all the molecules up the same way. The scientists then analyze the momenta of the orbitals.

The problem is that the researchers cannot measure the wave function directly. Instead, they can only measure the square of the number of the wave function. This means they lose information on the sign of the wave function — whether it has positive or negative amplitude.

“But, since we know the size of the molecule, we can in fact use this information to recover the lost information on the sign of the wave function,” Puschnig says. With the right design for experiments, the sign of a wave function can equal its phase.

The scientists developed images of a number of orbitals in the molecules pentacene and perylene-3,4,9,10-tetracarboxylic dianhydride clinging onto silver that closely matched calculations of how those orbitals should look.

“The most important implication of our findings is that we opened up a new window to look at electron orbitals of molecules,” Puschnig says. “I was really not sure whether it would indeed work.”

The researchers next plan to develop fully 3-D images of molecular orbitals.

“Another way to go is to investigate molecules on different surfaces,” Puschnig says. “We know that on silver, the interactions of the molecules with the metal is rather moderate, so the molecular orbitals that we image look pretty much like that of a free molecule. On the other hand, if we use more reactive surfaces, we should be able to monitor how chemical interactions of molecules with surfaces modify electron orbitals. This will be of fundamental interest to learn more about the way molecules chemically interact with various surfaces.”

Categories: Applied Physical Sciences
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