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.
Rydberg Matter (RM) is a generalized metallic form of matter which forms small clusters, primarily planar clusters. These clusters agglomerate to stacks of clusters and to clouds, both small clouds of the order of cm size in the laboratory and to clouds with any size in the upper atmosphere and in space. RM clusters are stable in vacuum for hours and days in the laboratory, and RM is calculated to be stable in space for periods longer than the assumed lifetime of the Universe. In the lowest excitation level for hydrogen (protium) RM called H(1), RM is not an excited type of matter but can exist indefinitely. The energy level for H(1) is below that for separate H atoms in their ground state, thus H(1) is the lowest energy state for hydrogen atoms. In fact, every type of atom forming RM has a lowest excitation level corresponding to its normal valence state but normally at slightly higher energy, and in this excitation level RM is stable if undisturbed by particle impact and similar energetic processes. The stability of RM at atmospheric pressure is thus not so good due to the impinging small molecules, but layers of RM can survive on surfaces in the atmosphere at least for hours. Here, the focus is on the study of RM clusters in a low-density phase of RM. Methods to form RM at larger densities have been discussed in relation to cold Rydberg gases. Many of these methods give large amounts and large densities of RM, suitable for use in plasma studies and studies of the condensed RM phase as a material. However, to understand the inner structure of such a material other methods are needed to create and study the clusters which aggregate to form RM. In principle, the form of matter called ultra-dense deuterium D(-1) is not of the RM type, but due to its very close relation (in real time) to the RM type D(1) it is mentioned here as well. The main method used to form RM in cluster form is based on the very interesting catalytic properties of alkali promoted solid metal oxide catalysts, often called hydrogen abstraction or hydrogen transfer catalysts. The atom-level description of the catalyst action and of the RM formation at such catalyst surfaces is outside the scope of the present review. Much information already exists in this field. RM was predicted around 1980 by Manykin et al. It was originally described as a condensate of excited atoms called CES (for condensed excited states). However, the format of their prediction was much broader than believed at that time, and the observation of RM from Cs in thermal plasmas and on surfaces demonstrated the usefulness of this concept clearly. However, since excited states are not really required but Rydberg orbits are, the generic name RM seemed more appropriate (Rydberg orbit means hydrogen-like, so all levels in a hydrogen atom are Rydberg levels with degenerate levels since no inner electrons exist). Manykin et al. adapted their theory to the case of RM formed by Cs atoms. They also developed the theory further and for example calculated radiative lifetimes for RM. The first experimental studies of RM were aimed at understanding surface layers of RM and the interface between such surface layers and thermal alkali metal plasmas. Macroscopic parameters like the surface work function and the plasma drop were studied, even with direct probing inside the alkali (Cs) plasma 6 L. Holmlid 123. The highly intriguing dark passage of extremely large electron current densities over very large electrode gaps, and the even more intriguing extremely strong thermal electron emission from the cold electrode were well described by the RM theory due to Manykin et al. . RM has the lowest electronic work function for any material, easily demonstrated both experimentally and theoretically, has not been utilized technologically as it could have been. On the fundamental level, a continuum description of RM was however not adequate to explain many of the experimental observations. Thus, spectroscopy and molecular beam methods were applied instead, starting to demonstrate the interaction between well-defined RM clusters as the reason for the existence of the RM material. The most important result which affirmed the special molecular form attributed to the RM clusters was the observation of quantized energy release in the laser-induced time-of-flight experiments. The success of the general chemical molecular picture in the form of well-defined clusters is apparent in the case of RM as reviewed here: solid-state descriptions and bulk experiments have not been able to provide the same amount of useful information as the cluster studies have been able to do. Certainly, solid-state methods may be useful in the future to describe RM but several special properties need first to be implemented. One such property is the inherent graininess (cluster structure) probably caused by classical-limit retardation effects, and another is the highly variable local material density (many orders of magnitude) and the varying excitation spectrum caused by the choice of excitation level in RM. Several other possibilities exist probably for forming Rydberg clusters which are related to the RM clusters reviewed here. Laser-based studies of Rydberg gases at low temperature have detected many-body interactions. By association of Rydberg atoms in high ns states (thus with l = 0) with ground state atoms, new types of Rydberg clusters Rb2 and Rb3 are observed. They have a bond distance of the order of 50 nm and are thought to be stabilized by reflection of the s electron before it reaches the ion core. The plan of this review is to first summarize the theory necessary to understand the experimental studies, and then to describe the experimental methods and some important results found by these methods. It is then necessary to describe the various types of RM clusters studied so far, and finally describe the most important properties of the RM clusters. In connection with the cluster properties, the observational information about RM clusters in space is also summarized.
