Experimental Studies and Observations of Clusters of Rydberg Matter and Its Extreme Forms

Abstract

A review is given of experimental studies of clusters of Rydberg matter (RM) with applications. This is a metallic type of matter with angular momentum l ≥ 1 for the conduction band electrons. The atoms in RM can be one-electron atoms like K, H and D which are most used in the laboratory, or two-electron atoms like He. Mixed clusters of one-electron atoms are easily studied. This material is stable in a vacuum both in the laboratory and in interstellar space. The large stability is valid for the lowest excitation level for each atom type, at l = 1 for H(1) which is the lowest energy state for protium. Typical clusters of RM are planar with magic numbers 7, 19, 37, 61 and 91. Small planar cluster can form stacks of clusters. Close-packed clusters exist for H(1) with magic numbers 4, 6, and 12, and chain clusters mainly formed by D–D “beads” are common for D(1). Several methods to study RM clusters are reviewed. Stimulated spectroscopy methods like stimulated emission and stimulated Raman are emphasized. Cluster properties like large polarizability and giant magnetic dipoles are described. Results on rotational levels, vibrational levels and electronic levels are reviewed. The observational evidence for RM clusters in space is discussed in relation to the fundamental experimental studies of the specific cluster properties.