LUMO stands for lowest unoccupied molecular orbital. If it's unoccupied, how is it possible to image it?

See for example in this paper ("Pentacene imaged with STM and NC-AFM").

Pentacene imaged with STM and NC-AFM


Why not? It's not like we poke them with our finger, anyway. All we can really see are the electron transfers. Now, the transfers from the molecule are as good as the transfers to it. If we adjust our setup so as to look at the latter, then we may see the shape of the orbital which is now empty but ready to accommodate our electron.

In one setup, the electrons tunnel from the HOMO to the tip, thus visualizing the HOMO. In another setup, they tunnel in reverse direction (hence the change in the sign of voltage), from the tip to the LUMO, thus visualizing the LUMO.

  • $\begingroup$ So the some of the tunnelling electrons that the STM measures pass from the HOMO to the LUMO, and only then tunnels to the STM tip? $\endgroup$ – Sparkler Oct 7 '15 at 19:12
  • $\begingroup$ Got it! Your phrasing made the difference... $\endgroup$ – Sparkler Oct 7 '15 at 19:27
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    $\begingroup$ There are quite a few people that would strongly object against MOs as objects that could be visualized. They are not in any sense QM observables (and equivalent sets of MOs could be used to describe the same states. But the pictures with canonical orbitals are quite compelling, right? Indeed the difference between HOMO and LUMO ('orbital density'?) would be the direction of the charge transfer. $\endgroup$ – Fedor Oct 8 '15 at 10:20
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    $\begingroup$ OK, then what are the pictures a and b? X-ray images of a fossilized Cambrian shrimp, maybe? $\endgroup$ – Ivan Neretin Oct 8 '15 at 10:25
  • $\begingroup$ A wonder often skipped by chemists too is that chemical structures - often snobbery treated, are 1:1 correspondence of what the molecule look like by means of these techniques. But they were just deduced by conducting reactions, taking melting points, and thinking wisely. $\endgroup$ – Alchimista Feb 20 at 10:23

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