Angular distributions for single- and double-electron capture in slow C4+ -Ne collisions

In an earlier experiment by H. Cederquist et al. [J. Phys. B 18, 3951 (1985)] an unusual Q-value distribution for true double-electron capture was reported for 400-eV ${\mathrm{C}}^{4+}$-Ne collisions: a strong population of reaction channels resulting in Q values between 27 and 35 eV followed by weakly populated channels in the region 18--25 eV and then again a strong channel at Q=16 eV. The last channel is nearly degenerate in Q value with the dominant single-electron capture reaction. Here, we report measured angular distributions for scattered projectiles in single- and double-electron capture at collision energies 0.6--1.0 keV. At 0.6 keV, both single- and double-electron capture exhibit strong forward peaks for Q\ensuremath{\sim}16 eV. The intensity for the latter, but not the former, decreases drastically when the energy is raised to 1 keV. The energy dependence of the angular distribution for double-electron capture with Q>20 eV can be qualitatively understood in simple terms; single-electron captures to 2s and 2p orbitals, however, yield unexpectedly wide and structured angular distributions.