Measurement of Collisional Rate Coefficients for Heliumlike Carbon Ions in a Plasma

Abstract

Measurements of the emitted line intensities from a $\theta$ pinch discharge are interpreted in terms of a modified corona model for heliumlike ions. The relative intensities of the resonance and intercombination lines have been measured for C V and O VII as a function of time, as has the absolute intensity of the $C \mathrm{v} 2{}_{}{}^3{}_{}{}^{}S-2{}_{}{}^3{}_{}{}^{}P$ multiplet. Electron temperatures and densities are obtained as functions of radius and time from the spectrum of Thomson-scattered laser light, and the densities also from Abel-inverted absolute continuum intensities. The derived model allows for population of the $n=2\mathrm{}$ levels by direct impact excitation and by cascading, for collision-induced (exchange) transitions from $n=2\mathrm{}$ triplets to singlets, and for collisional ionization from both the ground and the $n=2\mathrm{}$ triplet levels. Collisional de-excitation of the triplets is shown to be negligible compared with exchange collisions and radiative transitions. Singlet excitation rates (near threshold) are derived with an accuracy of 40% which agree with Seaton and Van Regemorter's semiempirical formulas, triplet excitation rates which are smaller than the singlet rates by a factor 1.8±0.2, and ground-state ionization rates to an accuracy of 25% agreeing with those estimated from the Born-exchange approximation for hydrogenic ions, suitably modified for two-electron systems. The measured value for the triplet-singlet exchange rate for C V is equal to ⅕ of the value calculated for He I using the close-coupling approximation, and it has an estimated accuracy of 25%.