I liked the introduction of this article by Röthlisberger because it nicely explains the processes in excited state proton or hydrogen atom transfer. Most of it is well explained in this Figure.
In the ground state the n and π orbitals (shown on the left-top and right-bottom) are each doubly occupied. Excitations into the two virtual orbitals shown (left-bottom and right-top) lead to three states of interest: ππ*, nπ*, πσ*.
In the ππ* and nπ* states it can be seen that electron density is shifted from the O to the N. This increases both the acidity of the O and the basicity of the N. In the cluster shown this induces proton transfer through the ammonia molecules. Actually there is a very nice movie showing this transfer in their supporting information.
The situation is completely different in the πσ* state. If the anti-bonding σ* orbital is populated, the bond is no longer stable. The molecule stabilizes by dissociation of a hydrogen radical (i.e. hydrogen atom). If the hydrogen atom takes part in a hydrogen bond, you can have excited state hydrogen atom transfer. The orbital corresponding to this is shown in the left bottom. It is very diffuse and has probably also some Rydberg character.
When I decided to write the post, I had only read the introduction which is very nice and helped me finally understand the difference between excited state proton and hydrogen atom transfer. The part that seems kind of strange is that they only computed one trajectory. And for this one trajectory they had 1024 processors on a Blue Gene/L. With atom centered basis sets in Turbomole you could almost do it in real time if you had 1024 CPUs (or pretty fast anyway). So it may be a problem with the plane waves. Or I got something wrong. Or I am just jealous because I never had 1024 CPUs at my service.
 Call it advertisement ...
 Not quite real time as one CPU cycle is already about 3 orders of magnitude longer than the process observed.
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