Grotrian diagrams of the Mg  levels and transitions representing the model atom adopted for this study. Energy is relative to the ground state in each case. The nl-configuration is shown for some levels. The dashed line in the diagrams is the ionised state. The vertical line in the ³P term connects the 3s 3p ³P^o to the 3p²³P level. Parity is not distinguished, and so these two terms are shown in the same column.