Oscillator strengths and photoionisation cross sections for positive ions in the sodium isoelectronic sequence

Photoionisation cross sections of the ground state and oscillator strengths for the 3s-np transitions of several numbers of the sodium isoelectronic sequence are presented. The need for careful consideration of the effects of polarisation in these ions is demonstrated by the use of two different approximations. In particular, semiempirical corrections to the dipole matrix element are shown to be unreliable.