Analysis of the photoionisation of an atom in a¹Sestate leaving an ion in a²P⁰state. Application to helium

It is shown that the photoionisation of an atom in the 1 S e ground state leaving the ion in an excited 2 P 0 state which decays to a 2 S e state can be completely described in terms of three independent parameters sigma , beta and xi . Expressions for the polarisation and angular distribution of the emitted radiation are obtained in terms of sigma , beta and xi when the photoelectron and photon decay are observed in coincidence and when the photoelectron is not observed. As an example the photoionisation of He leaving the He + ion in its 2p state is considered and values for sigma , beta and xi are obtained using a six-state R-matrix calculation. These results are compared with previous theoretical calculations and with experimental measurements.