The recombination and level populations of ions

The methods of analysis of the recombination and population structure of ions in a thermal plasma are extended so as to permit unification of the unresolved angular momentum quantum number treatment of highly excited states of Paper I (Burgess and Summers (1976)) with a resolved angular momentum treatment of the lower states. The plasma is assumed to contain ions, electrons and protons and be permeated by external radiation fields. Very many resolved and unresolved levels may be included and the techniques relate to any ion, except that processes involving doubly-excited ion states are reserved for a later paper in the series. An LS coupled picture is used with inclusion of collisional transitions between spin systems. Hydrogen and helium are considered and best available rates for processes involving the lowest levels are incorporated. 'Generalized collisional radiative recombination and ionization coefficients' are graphed and the effect of a strong Lα radiation field in hydrogen investigated. The whole population structure of hydrogen and helium for a representative selection of planetary nebular conditions is presented. The dependence of the 1s, 2s and 2p populations of hydrogen on dilution of the external stellar field and on the degree of containment of Lα in the nebula is examined. (author)