| Abstract: | Radio recombination lines (RRL) discovered about 25 years ago became a very effective tool for the investigation of interstellar matter (ISM). These lines are unique by the number of transitions and the spectral range observed: from 2 mm to 20 m wavelength. This allows to study the ISM under variety of physical conditions which differ by 7 to 8 orders of magnitude in density and 3 orders of magnitude in temperature. RRL allow to determine the main physical conditions in HII and HI-CII regions, to map the galactic distribution of ionized hydrogen, and to obtain the helium abundance of the ISM. Furthermore, the information contained in the RRL allows to estimate the intensity of cosmic rays and to draw conclusions on the galactic evolution. The physical properties of the RRL and the results obtained by observations of RRLs are reviewed. |
