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 the ground and excited states of atomic helium up to principal quantum number n = 25 and angular momentum quantum number l = 5. These cross-sections are used to calculate total recombination and cooling coefficients for atomic helium for electron temperatures given by log(T) = 1(0.2)4.4. A comparison of the threshold photoionization cross-sections obtained here with extrapolations of the highly accurate bound-bound oscillator strength calculations by Drake shows that the new calculations are in error by no more than 1 per cent. The accuracy of the photoionization cross-sections used by previous workers to derive recombination coefficients is also discussed.