Rydberg matter

Rydberg matter is a metastable state of highly excited atoms (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.