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Paper XXII

Radiative rates for the intercombination transitions in the carbon isoelectronic sequence
C. Mendoza, C.J. Zeippen and P.J. Storey
Astronomy & Astrophysics Supplement Series, 1999, Vol.135, pp.159-169

Radiative decay rates are computed with the atomic structure code SUPERSTRUCTURE for the 2s2p$^3$ $^5$S$^\circ_2$ - 2s$^2$2p$^2$ $^3$P$_1$,$^3$P$_2$ and $^1$D$_2$ intercombination transitions of the carbon isoelectronic sequence ($6\le Z \le 28$). Contributions from configuration interaction and relativistic corrections are carefully studied, in particular the spin-spin interaction that is shown to be of considerable importance for low $Z$. By extensive comparisons with other theoretical datasets and measurements of the $^5$S$^\circ_2$ radiative lifetime and the B = $A(^5S^\circ_2 - ^3P_2) /A(^5S^\circ_2 - ^3P_1)$ branching ratio, we are able to assign accuracy ratings to the present transition probabilities. The transitions to the ground term are believed to be accurate to better than 10% whereas the smaller $A$-values belonging to transitions to the $^1$D$_2$ are not expected to be accurate to more than 20%.