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I have a Lennard-Jones diagram here where I can't figure out, what is on the x-axis.

enter image description here G. Ertl, Angew. Chem. Int. Ed. 2008, 47

We are looking at a diagram for a molecule $\mathrm A_2$ composed of two $\mathrm A$ atoms that comes into close contact with a surface.

  • If the molecule $\mathrm A_2$ forms a bond with the surface, a shallow minimum ($\mathrm A_{2,ad}$) is reached.

  • $E_{diss}$ is the dissociation energy needed to seperate/dissociate the two $\mathrm A$ atoms from each other (breaking the molecule). In this form (as two individual atoms), they could both seperately bind to the surface, resulting in much stronger bond formation $2 \mathrm A_{ad}$. This would be called chemisorption, if I'm not wrong.

  • Now combinding these two processes, we would have dissociative chemisorption. The crossing point between the two curves markes the activation energy of this.

This was just an explanation of the graph. My question is, how I should understand the horizontal dimension here - the x-axis. On a google search many diagrams pop up, but never with the dissociation included. Many of those as well as the Wikipedia article about Lennard-Jones potential show the x-axis as a distance between atoms - the interatomic distance $r$.

I cannot make sense of that on the diagram I am showing above. The distance between atoms should be smaller for the molecule than for the seperated atoms. What is on the x-axis in this case?

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Ahh, silly me. The x-coordinate is of course just the distance between the $\mathrm A$ atoms / $\mathrm A_2$ molecule and the surface, to which they bind. This diagram doesn't show the interaction between $\mathrm A$ atoms but between them and the surface. I will close this question as solved.

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