Dynamic force microscopy (DFM) is by now well established, at least since Gross et al. at IBM Zurich attached a carbon monoxide molecule to the sharp tip of their AFM apparatus. By passivating and stabilizing the tip in this manner, the tip could approach closely enough to the substrate so that the electrons interacted without causing any chemical or physical modifications, either to the substrate or the tip. In this mode of operation, the electron orbitals of the CO molecule interact with those of the substrate by way of the Pauli exclusion principle.
We should compare with the earlier technique of scanning tunnelling microscopy (STM), which probes the tip/substrate interaction within a particular energy range. As the authors remind us, this means that in STM one usually obtains an image of the frontier molecular orbitals, which do not map easily onto atomic positions. Indeed, STM is often interpreted by way of quantum mechanical computations of the hypothesized substrate surface. That is, one needs to have a fairly good idea of what the surface looks like in terms of molecular orbitals in order to make sense of an STM image.
By contrast, DFM responds to the total electron density, and this normally corresponds directly to the “ball and stick” model with which we visualize molecules and surfaces. Now, that this mental model is based in classical physics sometimes leads us astray. That the force sensed in DFM is due to the Pauli exclusion principle means that it is fundamentally a quantum mechanical phenomenon. But there is little doubt that in most situations, we can make sense of the image without the heavy machinery of quantum mechanical calculations.
The paper presents a quite readable summary of Pauli’s exclusion principle and how it is used in imaging.
Reference: Jarvis et al., “Pauli’s principle in probe microscopy”, arXiv:1408.1026.