Radio recombination lines

Over the past forty years, investigations of radio recombination lines have become an effective way of studying the interstellar medium. The electron temperatures, densities, emission measures, kinematics parameters, elemental abundances and other physical characteristics of emission processes in planetary nebulae, distributed ionized gas, stellar winds, the partially-ionized interstellar medium and external galaxies are determined with the lines of hydrogen, helium, carbon, etc. This astrophysical phenomenon is unique because of the large (>1000) number of possible quantum transitions and because of the wide observable frequency range (from ~150 GHz to ~10 MHz). In this short review the main results from recombination radio line investigations are discussed. Special attention is given to the areas which have been given least attention in the literature such as the radio recombination lines of very excited carbon atoms at the extremely low-frequency (v < 30 MHz) end of the decameter wavelength range. This is important to stress in the 70-th jubilee of radio astronomy because cosmic radio emission was discovered by Karl Jansky at decameter wavelengths. Investigations of the extremely low-frequency radio recombination lines are also very interesting for the prospective LOFAR system