Thermal ionization of barium

The theory of thermal ionization of gases was first given by M. N. Saha in a series of papers (1920a , 1920b , 1921) and widely applied by him and others to explain the spectrum of the sun and furnish a satisfactory physical theory of stellar spectra and their classification. Saha considered the ionization of a unicomponent system, but it is evident that in actual practice electrons may generally be obtained from the ionization of other atoms or from other surfaces and must therefore be considered as an independent constituent. This was done by E. A. Milne (1921) and H. N. Russell (1922). The theory was further developed by Darwin and Fowler (1923) to include the different excited states of the atom and of the ionized atom, all of which are simultaneously in thermodynamical equilibrium. There may also be multiple ionization. At the time when the theory was first formulated the ionization potentials and the spectral characteristics of only a few elements were known and hence it could be applied only to those few elements, but thanks to the recent developments in atomic physics we are now in possession of the values of ionization potentials as well as the spectra of almost all the elements in their different stages of ionization. So a detailed comparison of the theory with experimental results can now be satisfactorily done. A full account of the theory and its applications to astrophysical problems has been given by Pannekoek (1930).