The 2s3p3P1-2s21S0intercombination line in the beryllium sequence

Abstract

Configuration interaction wavefunctions are used to evaluate the transition probabilties of this line for members of the beryllium sequence from Z = 6 to Z = 10. The values, which are critically dependent on an accurate evaluation of the 2s3p 3P1-'P1 splitting, are in good agreement with those deduced from recent beam-foil measurements of 2s3p 3PJ lifetimes in N IV, 0 v and F VI. These lifetimes are calculated for 0 V, and it is confirmed that this transition is the principal cause of the 2s3p 3P1 lifetime being lower than the other two. There is also a smaller contribution from the 2s3p 3P1-2pz IDz line. The calculation of the probabilities of the 2s2p 3P1-2s2 'So transition for members of the beryllium sequence has been considered recently by a number of authors (Glass 1979, Glass and Hibbert 1978a, Laughlin er a1 1978, Muhlethaler and Nussbaumer 1976). The values of the probabilities for this intercombination line are several orders of magnitude smaller than those for allowed transitions, and values deduced from beam-foil experiments are available only for the more highly ionised members of the sequence. In these cases agreement between theory and experiment is reasonably good-for MO XXXIX, the agreement is within 10% (Nussbaumer and Storey 1979). Very recently, Engstrom er a1 (1979) have published lifetimes of the 2s3p 3PJ levels of N IV, 0 v and F VI. From these they have deduced values for the 2s3p 3P1-2sz 'So intercombination transition probability. Their values lie between lo6 and 10' s-'. Although the lifetimes of the 3P0 and 3P2 levels are essentially the same, that of the 3P1 level is noticeably lower. These results allow a comparison between theory and experiment for intercombination lines with much lower 2 values than is possible for the 2s2p 3~1-2s2 'so transition. The 2s3p 3P1-2s2 'So transition becomes allowed when relativistic (spin-orbit etc) effects are included in the calculation, whereas the 3P2-1S0 and 3P0-'S0 transitions remain forbidden because of the J values. Engstrom eta1 (1979) attribute the different lifetimes of the 3PJ levels to this effect, and report preliminary calculations which substantiate their conclusions. In this note, we shall present accurate calculations of this transition probability for C 111 through Ne VII and of other relevant transition prob- abilities for 0 v in order to investigate the situation more closely. We have performed several types of calculations with differing degrees of complex- ity on 0 v, using the configuration interaction code c1v3 (Hibbert 1975) including the relativistic effects used in earlier work (Glass and Hibbert 1978a, b). We present the results in table 1. The first of the three ab initio calculations (given respectively as 1-111