RADIATIVE STABILIZATION OF THE DOUBLY-EXCITED 4L4L' AND 4L5L' SINGLETTERMS IN NE8+

Calculations concerning the competition between radiative and non-radi ative decay for doubly-excited 4lnl' (n = 4, 5) singlet states in Ne8 are reported. Recent coincidence experiments have been interpreted as showing that for the 4l4l' states in Ne8+, as well as in the isoelect ronic N5+ and O6+ systems, radiation contributes more than 50% to the total decay. The 4l4l' states in Ne8+ are, as in N5+ and O6+, degenera te with the 3lnl' Rydberg states and a considerable mixing exists betw een the two, which leads to an increase in the radiative decay probabi lity. In the free Ne8+ ion we find that radiative decay accounts for 2 9% of the total decay for the singlet terms assuming a statistical pop ulation distribution. The results vary considerably between terms and the total radiative yield is strongly dependent on the initial populat ion distribution. The branching ratio is similar to the ratio obtained under similar conditions for O6+. The similarity is ascribed to two c ompeting effects: the lowering of the 4l4l' states relative to the 3ln l' series with increasing Z, which favours decay through autoionizatio n, and the Z(4) behaviour of the radiative decay rates, which has the opposite effect. The free ion results are compared to results includin g the auto-transfer to Rydberg states (ATR) mechanism. In N5+ we have shown that this effect leads to an appreciable reduction in the radiat ive yield. For Ne8+, the ATR mechanism is less important but we conclu de that also hem the effect on average is a (small) reduction in radia tive yield. This is because the ATR mechanism is usually so effective that the 4l4l' population is transferred to low Rydberg states. These autoionize in light ions while they have a larger probability for radi ative decay with increasing Z. This seems to account for a large part of the difference between the ATR results in N5+ and Ne8+. We report t he results of an alternative approach in which part of the collision d ynamics is included and we find that the ATR model is rather sensitive to a number of parameters, which at the moment are not known very wel l. For the 4l5l' states, which are located above the N = 3 threshold, radiative decay accounts for 6% of the total decay, much less than for the 4l4l' states, due to the opening of autoionization channels to th e N = 3 thresholds for the 4l5l' states. Assuming a statistical popula tion distribution, the theoretical results are in clear disagreement w ith recent observations for both 4l4l' and 4l5l' manifolds. However, w e conclude that it is necessary to obtain a more definite idea about t he real population distribution before it is possible to make meaningf ul comparisons with the observed radiative yields.