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I am aware that outer shell electrons in rubidium atoms in an optical lattice can be excited to Rydberg levels, in which the electrons orbit well beyond the atoms to which they are bound. Is this something that can happen within the bulk of a metal as well?

If this is not something that has been experimentally examined, assume there are one or more straightforward mechanisms that could provide the requisite excitation—naturally occurring alpha or beta emitters, for example.

If Rydberg levels within the bulk of a metal are not possible, why is this the case? If they are possible, how would the electron charge density be affected? In the non-Rydberg case, I understand the d-electron density will be quite low in the interstitial regions. Would this change in the Rydberg case? How long would the excited state last? How would the blockade effect factor in? Can you recommend a suitable approach for modeling the charge density?

(It seems a 1996 paper did some calculations that are relevant here, which I will take a look at. I am still interested in any information that people can provide.)

Eric Walker
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Some quick thoughts, but hopefully useful:

The direct analogue of Rydberg states would just be exciting an electron to a very high energy band. I don't believe this does anything terribly interesting, other than decay quickly. There are just too many decay channels. Note, the electrons are delocalized in a metal, so there is no sense in which one gets a "big atom" from such a high energy electrons. I think the opposite, such an electron would look essentially free, and very "small" because of its short wavelength.

A better analogue of a Rydberg state, (and perhaps this is what you have in mind),might be a donor site in the gap of a semiconductor. This would be a positively charged impurity in the lattice. If the material is right this hosts hydrogen-like states (if I recall correctly you need a small effective mass). These states lie in the gap, so there is no delocalization like in the electronic bands. In the naivest approximation (which is all that I know) you get precisely the Rydberg series, but with a different mass and dielectric constant, so that your orbits are very big and your "Rydberg" is very small.

Largely eqivalent is a Wannier exciton, which is a bound state of electron and hole in the same fashion.

Haven't really seen anything about highly excited states of these things, but that could be ignorance on my part. Again hard to imagine that the lifetimes would be long, but excitons themselves can have surprisingly long lifetimes, up to a millisecond.

BebopButUnsteady
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I assume the question traces back somehow to Leif Holmlid's claims to have discovered "Rydberg matter" and "ultradense deuterium" in the lab.

Can Rydberg states exist within the bulk of a metal?

The answer is no, for straightforward reasons. For example, Rydberg states in monoatomic hydrogen can only exist at low densities. This is simply because the radius of state $n$ goes like $n^2$. There isn't enough room for arbitrarily high $n$ states to exist, and they would be disrupted by collisions, whose cross-section goes like $n^4$. In the sun's absorption spectrum, for example, there is a cutoff in the $n$ values that are observed, because the density of the gas is fairly high. This is why there's no way we're going to see them in condensed matter.

Holmlid is a kook who is very persistent about pushing his claims. For example, he tried to promote himself in a Wikipedia article, "Rydberg matter," writing the article himself and citing his own papers extensively. Although he has managed to get his articles published in journals, a literature search showed that out of 2154 references to his papers (presumably not all on Rydberg matter), 1863 were self-citations. What little recognition his work has received from others seems to have been mainly from cold-fusion kooks, such as Hora and Miley, with whom he has co-authored papers.