I'd like to give an update on the question of electron screening and Rydberg states in metals, recently discussed here [1]. As a background to this note, I encourage anyone who has the time and inclination to read a 1989 paper by X.W. Wang, S. Louie and M.L. Cohen, published in Physical Review B [2]. The authors are/were a group at UC Berkeley, which is just down the street from where I live. I recently got in touch with one of them, and he was kind enough to get back to me.
That paper sought to model the electron charge density surrounding the lattice atoms in a palladium system. One thought was that if some of the electron orbitals extended out into the interstitial sites between the palladium atoms, where the hydrogen is, you might get electron screening, which would reduce Coulomb repulsion between fusion precursors (d+p, d+d), allowing them to approach closer and possibly increase the tunnelling rate (dramatically, by some accounts). This is one of the questions that the 1989 Physical Review B paper looked. The authors modeled the d-electron charge density in the palladium lattice and found that the density was very low in the interstitial sites, so they ruled out an increased tunnelling rate due to screening. What the 1989 paper did not look at was the possibility that you might be able to excite lattice atoms into Rydberg states, where some of the electrons have additional energy, but not enough to ionize and be ejected from the atom. If so, some of those electron clouds might extend further out, possibly into the interstitial sites, and lead to the screening the authors of the 1989 paper were looking for. We know that the expanded reach of the Rydberg orbitals can be dramatic, as can be seen in the image in this phys.org article [3], where the orbitals go out many atoms beyond the ones to which they are bound. The idea of the Rydberg excitation of the metal substrate is inspired by details provided by Defkalion. Defkalion have mentioned Rydberg hydrogen as an ingredient in the reaction they are seeing. The discussion in [1] inverts this, and asks whether Rydberg excitation of the metal substrate itself could be behind the reaction (without saying anything about whether Rydberg hydrogen is also involved). I recently wrote Steven Louie, one of the authors of the 1989 paper mentioned above and a professor at UC Berkeley, to get his thoughts on the general possibility. (This feels a little like travelling in a time machine back to 1989 and interviewing someone twenty four years ago; think *Back to the Future*.) The mode of excitation I suggested was through inelastic collisions between deuterons in the palladium lattice (they were looking at PdD) and outer shell palladium electrons -- perhaps at higher temperatures the deuterons would scatter with the electrons, bumping them into higher orbitals. Dr. Louie was kind enough to reply, and he thought that the deuterons would have to have temperatures on the order of eV (1 eV = 11604 K) in order to alter the DFT all that much. (A "DFT" is a density functional theory -- a model of the electron density.) What I take away from this is that (1) the general idea of altering the electron charge density through Rydberg excitation of the outer shell metal electrons is not in itself crazy, and (2) the hydrogen or deuterium nuclei might not have enough energy to accomplish this in general, since the electronic structure requires a lot more energy to perturb in this way than the hydrogen/deuterium nuclei are likely to have dissolved within a metal. But there might be other ways to accomplish this kind of perturbation that come to mind -- through the decay of naturally occurring alpha and beta emitters, for example, and through sparks or other voltage perturbations. (Which is where Defkalion's spark plugs become very interesting.) I assume that at higher temperatures you start to see occasional excursions to very high energies for individual hydrogen nuclei in the Boltzmann tail, but I don't know much about this, so temperature might be a nonstarter. One last point that is interesting and worth mentioning in this connection is that in an fcc lattice, you have octahedral and tetrahedral sites, and in palladium these sites keep the hydrogen nuclei further apart from one another than if they were bound in molecules. So even if you had increased charge density and screening, I suppose the hydrogen might not be permitted to approach closely enough, due to Coulomb repulsion with the lattice sites. But if you had vacancies, this might change. It is interesting to note in this context that Defkalion have sought to create a kind of altered crystal structure, where instead of the normal nickel fcc you have something else, with a lot more vacancies. Eric [1] http://www.mail-archive.com/vortex-l@eskimo.com/msg84908.html [2] http://adsabs.harvard.edu/abs/1989PhRvB..40.5822W [3] http://phys.org/news/2012-11-super-atoms-rydberg-quantum-gas.html. The rubidium atoms in this instance are not in a metal but in an optical trap.
