Absolute measurement of dielectronic recombination for C3+ in a known external field

An absolute measurement of the rate coefficient for dielectronic recombination (DR) of C3+, via the 2s-2p core excitation, in an external electric field of 11.4±0.9(1σ) V cm-1 is presented. An inclined-beam arrangement is used and the stabilizing photons at ∼155 nm are detected in delayed coincidence with the recombined ions. The full width at half maximum of the electron energy spread in the ion rest frame is 1.74±0.22(1σ) eV. The measured DR rate, at a mean electron energy of 8.26±0.07(1σ) eV, is (2.76±0.75)×10-10 cm3 s-1. The uncertainty quoted for the DR rate is the total uncertainty, systematic and statistical, at the 1σ level. In comparing the present results to theory, a semiempirical formula is used to determine which recombined ion states are ionized by the 4.65 kV cm-1 fields in the final-charge-state analyzer and not detected. For the present results, any DR of the incident electrons into n levels greater than 44 is assumed to be field ionized in the final-charge-state analyzer. A more precise treatment of field ionization, which includes the lifetime of the C2+ ions before they are ionized and the time evolution and rotation of the fields experienced by the recombined ions, is needed before a definitive comparison between experiment and theory can be made. Our DR measurement, within the limits of that approach, agrees reasonably well with an intermediate coupling calculation that uses an isolated resonance, single-configuration approximation, but does not agree with pure LS-coupling calculations.