Since their first detection 15 years ago, radio recombination lines from several elements have been observed in a wide variety of objects including HII regions, planetary nebulae, molecular clouds, the diffuse interstellar medium, and recently, other galaxies. The observations span almost the entire range from 0.1 to 100 GHz, and employ both single djsh and aperture synthesis techniques. The theory of radio recombination lines has also advanced strongly, to the point where it is perhaps one of the best-understood in astro physics. In a parallel development, it has become possible over the last decade to study these same highly-excited atoms in the laboratory; this work provides further confirmation of the theoretical framework. However there has been continuing controversy over the astrophysical interpre tation of radio recombination line observations, especially regarding the role of stimulated emission. A workshop was held in Ottawa on 24-25 August, 1979, bringing together many of the active scientists to review the field and discuss these questions of interpretation. A broad concensus has emerged: the subtleties of the line-formation process are understood, and the conditions under which reliable in formation can easily be extracted from the line measurements are known. It thus appears likely that the emphasis will shift increasingly from the study of the line phenomenon itself to further application in other areas of astrophysics, ranging from physical processes in plasmas (temperatures, densities, ionization structure), to the large-scale properties of our galaxy (abundances, kinematics, structure), and studies of extragalactic systems.

Inhalt
I Physics of Radio Recombination Lines.- Theory of Recombination Lines.- Laboratory Studies of Rydberg Atoms.- II Radio Recombination Lines from HII Regions.- Interpretation of HII Region Radio Recombination Lines.- The Importance of non-LTE effects to the Interpretation of Radio Recombination Lines.- Determination of Electron Temperatures using Radio Recombination Lines.- Aperture Synthesis Observations of Recombination Lines from Compact HII Regions.- Radio Recombination Lines from Planetary Nebulae.- Helium Recombination Lines.- Systematic Variation of the Helium Ionization in Galactic HII Regions.- Pressure Broadening Effects on the Emission of He Recombination Lines.- III Radio Recombination Lines from CII Regions.- The Partially Ionized Medium Adjacent to HII Regions.- Carbon Radio Recombination Line Emission from Dark Clouds.- Carbon and Sulphur Ionized Regions in Dark Clouds.- Carbon (?) Recombination Lines in W49A (G43.2-0.0).- IV Large Scale Properties of the Galaxy.- The Galactic Center: Results of Radio Recombination Line Surveys.- Low Frequency Recombination Lines.- Low Frequency Recombination Line Surveys of the Galactic Plane.- Surveys of Galactic Hydrogen Radio Recombination Line Emission.- The Evidence for a Gradient of HII Region Electron Temperatures with Galactic Radius.- On the Electron Temperature of Extended, Low Density HII Regions.- V Extragalactic Radio Recombination Lines.- Prospects for Observations of Extragalactic Radio Recombination Lines.- Radio Recombination Lines in Extragalactic Sources.- Closing Remarks.- Name Index.