High-resolution doubly differential cross-section measurements and calculations for quasifree electrons elastically scattered through 180°from ground-state He-and H-like boron ions are presented. The measurements, covering the entire (for B 3+ ) and (for B 4+ ) Rydberg series populated by resonant excitation, were performed by zero-degree Auger projectile electron spectroscopy of 3.91 MeV B 4+ and 3.08-7.48 MeV B 3+ ions in collisions with H 2 targets. The projectile energy dependence study is used to further investigate other background contributions such as direct electron capture, direct excitation, and nonresonant transfer excitation. R-matrix calculations, particularly sensitive at this large scattering angle to electron correlation, exchange, and interference effects, are found to be in excellent overall agreement within the electron scattering model.

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In an earlier publication ͓1͔, we included an appendix that derived the ratio of direct electron capture of a 2p electron onto a 1s2s͑ 3 S͒ ion ending up in the three possible states 1s͓2s2p͑ 3 P͔͒͑ 4 P͒ ,1s͓2s2p͑ 3 P͔͒͑ 2 P͒, and 1s͓2s2p͑ 1 P͔͒͑ 2 P͒ as being 8:1:3. While this result still holds, the appendix contained five typographical errors and also erroneously used incorrect reasoning at one step of the derivation ͑"The averaged, uncoupled, captured state 1s2s͑ 3 S͒2p…".͒. Here we correct all 5 typographical errors and use correct reasoning to deduce the probabilities of each of the initially captured states in the amended Appendix. APPENDIXIn this appendix, we show how the capture of a 2p electron to a 1s2s͑ 3 S͒ ion results in a 8:1:3 ratio of probabilities to the states, respectively. We first have a 1s electron, with spin and magnetic quantum numbers s 1 =1/2 and m 1 , coupled to a 2s electron, with spin and magnetic quantum numbers s 2 =1/2 and m 2 , to yield a coupled 1s2s͑ 3 S͒ state with quantum numbers s 12 = 1 and m 12 , denoted as the ket vector ͉s 12 ,m 12 ͘ = ͚ m 1 +m 2 =m 12 C m 1 m 2 m 12 s 1 s 2 s 12 ͉s 1 ,m 1 ͉͘s 2 ,m 2 ͘. ͑A1͒The uncoupled captured states 1s2s͑ 3 S͒2p can be denoted as ͉͓1s2s͑ 3 S͔͒2p͘ = ͉s 12 ,m 12 ͉͘s 3 ,m 3 ͘ = ͚ s C m 12 m 3 m s 12 s 3 s ͉s,m͘ ͑ A2͒ with s 3 =1/2, s =1/2,3/2, and m = m 12 + m 3 . The probability of ending up in a state with final spin s is therefore given as the average of the probabilities that each of the possible ͑2s 12 +1͒͑2s 3 +1͒ uncoupled captured states ͉s 12 , m 12 ͉͘s 3 , m 3 ͘ has spin s and any magnetic quantum number −s ഛ m ഛ s: P͑s͒ = 1 ͑2s 12 + 1͒͑2s 3 + 1͒ ͚ m 12 ,m 3 ͚ m=−s m=s ͑C m 12 m 3 m s 12 s 3 s ͒ 2 = 1 ͑2s 12 + 1͒͑2s 3 + 1͒ ͚ m=−s m=s 1 = 2s + 1 6 = ͭ 4/6 for the ͓1s2s͑ 3 S͔͒2p͑ 4 P͒ state 2/6 for the ͓1s2s͑ 3 S͔͒2p͑ 2 P͒ state, ͮ ͑A3͒i.e., it breaks down according to spin statistics ͓2͔. The orthogonal property of the Clebsch-Gordon coefficients is used in the first step. The captured doublet state ͓1s2s͑ 3 S͔͒2p͑ 2 P͒ is not pure, but can be recoupled from a ͕͓͑s 1 , s 2 ͒s 12 ͔ , s 3 ͖s coupling scheme to a ͕s 1 , ͓͑s 2 , s 3 ͒s 23 ͔͖s one, where s 23 = 1 for the 1s͓2s2p͑ 3 P͔͒͑ 2 P͒ state and s 23 = 0 for the 1s͓2s2p͑ 1 P͔͒͑ 2 P͒ state. These latter states are essentially pure due to the dominance of the correlation between the n = 2 electrons. The initially captured ͓1s2s͑ 3 S͔͒2p͑ 2 P͒ state can thus be written as ͉͕͓͑s 1 ,s 2 ͒s 12 ͔,s 3 ͖s͘ = ͚ s 23 ͑− 1͒ s 1 +s 2 +s 3 +s ͱ ͑2s 12 + 1͒͑2s 23 + 1͒ ͭ s 1 s 2 s 12 s 3 s s 23 ͮ ͉͕s 1 ,͓͑s 2 ,s 3 ͒s 23 ͔͖s͘, ͑A4͒ and the probability of populating the state with spin s 23 is thusP͑s 23 ͒ = ͑2s 12 + 1͒͑2s 23 + 1͒ ͭ s 1 s 2 s 12 s 3 s s 23 ͮ 2 = 3͑2s 23 + 1͒ ͭ 1/2 1/2 1 1/2 1/2 s 23 ͮ 2 = ͭ 1/4 for the 1s͓2s2p͑ 3 P͔͒͑ 2 P͒ state 3/4 for the 1s͓2s2p͑ 1 P͔͒͑ 2 P͒ state, ͮ ͑A5͒ *Present address: Institute of Electronic Structure and Laser,

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We have investigated the reason for significant discrepancies between the results of two recent, similar computational methods ͓Zatsarinny et al., Astron. Astrophys. 426, 699 ͑2004͒; Gu, Astrophys. J. 590, 1131 ͑2003͔͒ for dielectronic recombination ͑DR͒ of Mg 2+. It is found that the choice of orbital description can lead to discrepancies by as much as a factor of 2 between total peak DR rate coefficients resulting from otherwiseidentical computations. These unexpected differences are attributed to the large sensitivity to bound-orbital relaxation and continuum-orbital description effects on the computed radiative and autoionizing transitions arising from accidental cancellation. In order to obviate these effects, an approach, using a separate, nonorthogonal orbital basis for each configuration, is employed to yield a DR rate coefficient that we assess to be more reliable than all earlier published results.