Photoionization Energies and Oscillator Strengths of Helium and Helium-Like Ions with Z = 6-8

In the photoionization process of helium and helium-like ions with Z 6-8(C4+, N5+, O6+), resonance energies of doubly excited autoionizing 1,3P0 states converging on the n = 4 and n = 5 hydrogenic thresholds were calculated by use of the diagonalization method. We also evaluated the oscillator strengths for transitions from 1s2 (1Se) to continuum 1 skp (1P0;) state. The calculations were done with the dipole length and velocity formulas using the six-parameter Hylleraas wavefunction for the ground state. The continuum state was represented by an antisymmetrized product of hydrogenics wavefunctions with completely screened Coulomb functions, and the autoionizing states by wavefunctions constructed in the diagonalization approximation within the framework of the theory of configuration interaction.