Monday, August 15, 2011

STM Images Molecular Orbital

Scanning tunneling microscopy has been extremely useful in condensed matter/material science. Now, it might finally be an important tool in chemistry. A new report shows a very neat experiment using a STM with a tip that has been "functionalized" with a CO molecule. So in this case the tunneling goes through the p-wave orbital of the molecule and allows for an imaging of the nodal direction.

With their new results, Gross et al. show that combining CO-functionalized tips and alkali-halide interlayers allows them to image the nodal pattern of the orbitals of flat organic molecules (Fig. 1). Here, again, the two CO 2π* orbitals play an important role. These derive from the p orbitals of the carbon and oxygen atoms that stand perpendicular to the molecular axis. Figure 1 shows one of the 2π* orbitals: it has four lobes (drawn in red and light blue, representing their phase or sign) separated by two nodal planes. The other 2π* orbital, perpendicular to the one in Fig. 1, is not shown. By bringing the tip close to a sample molecule (pentacene in Fig. 1) separated by a salt layer from a metallic substrate, and tuning the voltage between tip and substrate to address a particular orbital of the pentacene molecule, the lobes of the CO 2π* orbital and those of the pentacene orbital (shown in purple and rose) come into and out of registry, as the tip is scanned across the molecule.

Amazing work. You can get the paper for free at the link above.


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