The two-photon decay rates and photon energy distributions of the metastable 2S1 and 2S3 states of the helium isoelectronic sequence are calculated by variational procedures for the ions He i through Ne ix. © 1969 The American Physical Society.
Drake, Gordon W. F.; Victor, G. A.; and Dalgarno, A.. (1969). Two-photon decay of the singlet and triplet metastable states of helium-like ions. Physical Review, 180 (1), 25-32.