Rydberg matter

Rydberg matter is a metastable state of highly excited atoms (see Rydberg atom) which are condensed in a solid- or liquid-like very low density matter. For example Cs atoms with main quantum-mechanical level of excitation ~ 10-15 form a Rydberg matter with the density 10^17-18 cm^-3 which is used in thermionic converters (see thermionic emission).   Rydberg matter is formed because energetically the condensed state is more favourable compared with the initial system of non-interacting excited atoms. Bonding of excited atoms in the Rydberg matter is caused by delocalisation of valence electrons and its nature in many respects is similar to that of metallic bonding in metals.

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The exception is that in the Rydberg matter the collective valence electrons are at many orders of magnitude lower average densities compared with ordinary metals. E.g. the Rydberg matter made of Cs atoms with main quantum-mechanical level of excitation 12 has the average equilibrium density of electrons 1.1 10¹⁸ cm^-3. Additionally the valence electrons are distributed extremely non-uniformly in the Rydberg matter causing a significant delay in the decay of excitations compared non-interacting excited atoms. Consequently both life-time and stability of Rydberg matter against impurity recombination do increase with the quantum-mechanical level of excitation. E.g. the half-life of Rydberg matter made of Cs atoms with main quantum-mechanical level of excitation 12 is as high as 17 s.

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References:

1. L. Holmlid. Direct observation of circular Rydberg electrons in a Rydberg Matter surface layer by electronic circular dichroism J. Phys.: Condens. Matter., 19 (2007) 276206.
2. S. Badiei, L. Holmlid. Experimental studies of fast fragments of H Rydberg Matter. J. Phys. B: At. Mol. Opt. Phys. 39 (2006) 4191-4212.
3. E.A. Manykin, M.I. Ojovan, P.P. Poluektov. Rydberg matter: properties and decay. Proc. SPIE, 6181 (2006) 618105.