This is a special issue on Rydberg-Matter and Excited-State Clusters. It contains four invited reviews and six contributed papers. The theme of the invited reviews (and one contributed paper) is Rydberg Matter Clusters while the remaining contributed papers deal with excited-state and/or unusual clusters with interesting properties.
Rydberg matter (RM) is an exotic phase (or state) of matter formed by energetic, so-called Rydberg atoms (i.e. atoms in excited states). RM can be likened to dusty plasmas with small excited-state clusters. Rydberg matter was predicted in the early 1980s by EA Manykin, MI Ozhovan and PP Poluéktov and is now an intensive field of cluster research. This special issue provides an overview of the current development of the field (with special emphasis on certain aspects of RM). Rydberg matter normally consists of planar clusters of excited-state atoms arranged in two-dimensional hexagonal patterns with magic numbers of 7, 19, 37, 61, 91, etc.; though close-packed clusters or stacks or chains of clusters have also been observed. The largest RM cluster reported to date consists of around 100 atoms. The most studied RMs often comprise of one-electron atoms such as H and D and alkali metals, or two-electron atoms such as He. Bonding in Rydberg matter is caused by delocalization of the high-energy, excited-state electrons to form a generalized metallic state.[1, 2]. However, unlike ordinary metals, RMs also exhibit strong electron exchange–correlation properties like covalent bonding, making them a highly unusual class of cluster materials. The properties of Rydberg matter clusters vary greatly with the degree of excitation. A key variable in determining their properties is the principal quantum number.
