RECOMBINATION OF HELIUM-LIKE IONS - I - PHOTOIONIZATION CROSS-SECTIONS AND TOTAL RECOMBINATION AND COOLING COEFFICIENTS FOR ATOMIC HELIUM

A new calculation of photoionization cross-sections is described for t he ground and excited states of atomic helium up to principal quantum number n = 25 and angular momentum quantum number l = 5. These cross-s ections are used to calculate total recombination and cooling coeffici ents for atomic helium for electron temperatures given by log(T) = 1(0 .2)4.4. A comparison of the threshold photoionization cross-sections o btained here with extrapolations of the highly accurate bound-bound os cillator strength calculations by Drake shows that the new calculation s are in error by no more than 1 per cent. The accuracy of the photoio nization cross-sections used by previous workers to derive recombinati on coefficients is also discussed.