Collision strengths Ω from the present ICFT R-matrix calculation and DW data (Zhang et al. 1990) for the dipole 1s²4p   ²P_1/2  →  1s²2s   ²S_1/2 transition (left) and non-dipole 1s²4d   ²D_3/2  →  1s²2s   ²S_1/2 transition (right) of ions (O⁵⁺, Ar¹⁵⁺, Fe²³⁺ and Kr³³⁺) spanning the sequence. Note: The scattered electron energy is in unit of z² Ryd (z is the residual charge). [Colour online]