4 Conclusions

We have performed a detailed study of the radiative decay properties of the $\sp5 {\rm S}_2\sp{\rm o}$ metastable state in the carbon sequence. Ample evidence has been provided to support the conclusion by Fleming & Brage (1997) regarding the importance of the Breit interaction in obtaining accurate results for the low Z members and to illustrate its isoelectronic trend as theoretically predicted by Jones (1970). An extensive comparison with previous datasets for this sequence has allowed us to assign accuracy ratings to the present A-values. The transitions to the ground term are given a 10% ranking for $Z\leq 8$ and 5% for Z>8 whereas the transitions to the $\sp1{\rm D}_2$ are not accurate to more than 20%. We therefore assert that the present dataset, that will be included in the public databases of the IP, is the most reliable to date. Also, we would welcome further theoretical and experimental benchmarks that would clarify the inconclusive situation regarding the lifetimes and branching ratios for Z<8.

Acknowledgements

We would like to thank Dr. Elmar Träbert from the Ruhr-Universität Bochum, Germany, for very useful discussions on different experimental aspects of this work, and for allowing us to quote a measurement of the N II($\sp5 {\rm S}_2\sp0$) lifetime prior to publication. Part of the present work was carried out during visits by CM to the Observatoire de Paris, Meudon, France, and University College London, England. The hospitality received is gratefully acknowledged. The visits were funded by the CNRS, IVIC, CONICIT and the Observatoire de Paris. This research has been supported by CONICIT under contract No. S1-95000521.