Cross sections and rate coefficients for excitation of the 2s²2p⁵ ²P_3/2-2s²2p⁵ ²P_1/2 transition in fluorine-like Fe XVIII and Ni XX by proton (p), deuteron (d), triton (t) and alpha -particle ( alpha ) impact have been calculated using the close-coupled impact parameter method. At temperatures close to or below those of maximum Fe XVIII and Ni XX fractional abundance in ionisation equilibrium, the p, d and t rates are found to be comparable and are much greater than the rates due to alpha collisions. However, at high temperatures the situation is reversed, with the alpha rates being about a factor of two larger than those due to the other particles. The effects of adopting the present atomic data in calculations of the electron density or ion temperature sensitive emission line ratios R₁=I(2s²2p⁵ ²P_3/2-2s²2p⁵ ²P_1/2)/I(2s²2p⁵ ²P_3/2-2s2p⁶ ²S_1/2) and R₂=I(2s²2p⁵ ²P_3/2-2s²2p⁵ ²P_1/2)/I(2s²2p⁵ ²P_1/2-2s2p⁶ ²S_1/2) are briefly discussed.