The ORNL ion-atom merged-beams apparatus has measured benchmark electron capture cross sections for the multicharged ion C⁴⁺ on H(and D) from 6 eV/u to 1000 eV/u (F. W. Bliek, R. Hoekstra, M. E. Bannister, and C. C. Havener, Physical Review A, vol. 56, page 426 (1997). Low energy electron capture by highly charged ions from atomic hydrogen continues to generate considerable interest, not only because of practical applications in studies of astrophysical plasmas and particle transport in the divertor region of thermonuclear fusion devies, but also because of the fundamental atomic physics processes that occur during these low energy collisions.

In this experiment the fraction of C⁴⁺ metastables was measured (using the e-ion crossed beam apparatus) to be 5% which resulted in a correction to the cross section of only a few percent. The measurements are in good agreement with previous measurements for total electron capture; however, the reduced uncertainties of these measurements allow a more detailed comparison with theory. Our observations show that fully quantal molecular orbital theory which includes rotational coupling, overestimates the cross section by approximately 25%. Better agreement is found with a recent semiclassical, impact-parameter, coupled-channel calculation. While all theory predicts that the cross section varies smoothly with energy around the maximum, a sharp structure is observed around 400 eV/u.