Effect of resonances on2s−2pand2l−3l′excitation of Li-like ions by electron impact

Collision strengths are calculated in a five-state close-coupling approximation for excitation of ${\mathrm{C}}^{3+}$, ${\mathrm{O}}^{5+}$, ${\mathrm{Ne}}^{7+}$, and ${\mathrm{Ar}}^{15+}$ by electron impact for energies less than the $3d$ threshold. Resonance contributions enhance the allowed $2p-3s$ collision strength by larger factors than the enhancement of the forbidden $2s-3s$ collision strength. The resonance enhancement is approximately independent of nuclear charge $Z$ for $2s-3s$, $2s-3p$, and $2p-3p$, and increases nearly linearly with $Z$ for $2p-3s$. The average effect of resonance contributions to $\Omega \mathrm{}(2s,2p)\mathrm{}$ is only 4%.