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> the probe is much much bigger than 1 atom I guess I didn't clearly state an additional assumption of my explanation. I assumed the oxygen atom has an electr

by csense 13y ago

> the probe is much much bigger than 1 atom I guess I didn't clearly state an additional assumption of my explanation. I assumed the oxygen atom has an electric charge, the rest of the probe structure is electrically neutral, and the electrostatic attraction/repulsion of charges is the mechanism by which the oxygen atom on the probe reacts to the sample [1]. So while the rest of the probe structure does exist, it's effectively [2] electrically neutral. [1] I think we can agree that the gravitational attraction of the probe and sample to each other is way too small to matter. [2] I think the large probe structure is close enough to neutral and/or far enough away from the sample to have an effect which is smaller than the effect of the oxygen atom. The structure may contribute some nontrivial amount of noise due to being neither exactly neutral nor infinitely far away, but not enough to totally drown out the signal from the oxygen atom.

gus_massa 13y ago
I've found a previous article with more information and the full article available: "The chemical structure of a molecule resolved by atomic force microscopy" http://www.sciencemag.org/content/325/5944/1110 http://www.sciencemag.org/content/325/5944/1110 In this article, they explain that to get a good image, the distance between the tip and the molecule should be only ~1A. It's similar to the distance between atoms in the molecule, so the force is (relatively) big. Another important point is that using simulations to fit the data, they found that the main contribution to the force is not electrostatic, it's the Pauli repulsion (that is a quantum effect). Nevertheless if the tip "see" the molecule, then the molecule "see" the tip. An important detail is that the Oxygen atom is small and it's not possible to measure how it is moved by the forces. The molecule applies a small force to the Oxygen atom that is attached to the Carbon atom that is attached to the big tip. The tip resonates like a tuning fork, and the small force on the Oxygen atom changes slightly the resonating frequency of the whole system. They measure this frequency shift and use it to calculate the force on the Oxygen atom.

by csense 13y ago

gus_massa 13y ago

I've found a previous article with more information and the full article available: "The chemical structure of a molecule resolved by atomic force microscopy" http://www.sciencemag.org/content/325/5944/1110 http://www.sciencemag.org/content/325/5944/1110 In this article, they explain that to get a good image, the distance between the tip and the molecule should be only ~1A. It's similar to the distance between atoms in the molecule, so the force is (relatively) big. Another important point is that using simulations to fit the data, they found that the main contribution to the force is not electrostatic, it's the Pauli repulsion (that is a quantum effect). Nevertheless if the tip "see" the molecule, then the molecule "see" the tip. An important detail is that the Oxygen atom is small and it's not possible to measure how it is moved by the forces. The molecule applies a small force to the Oxygen atom that is attached to the Carbon atom that is attached to the big tip. The tip resonates like a tuning fork, and the small force on the Oxygen atom changes slightly the resonating frequency of the whole system. They measure this frequency shift and use it to calculate the force on the Oxygen atom.