Absolute Cross Sections and Polarization for Electron-Impact Excitation of theKandHResonance Lines of theCa+Ion

Abstract

Crossed beams of electrons and ${\mathrm{Ca}}^{+}$ ions have been used to measure absolute cross sections for electron-impact excitation of the resonance $K$ and $H$ lines of Ca II at 3934 and 3968 Å, respectively. Polarization fractions of the light were also measured. The cross sections presented are absolute in the sense that all measurables including photon flux, have been compared to relevant standards. The cross section for excitation of the $K$ line is observed to have a value of about $(18\mathrm{}\pm 2)\pi a_0^2$ at the 3.15-eV threshold, and to decrease to a magnitude of $(1.5\mathrm{}\pm 13)\pi a_0^2$ at 700-eV electron energy. Experimental uncertainties have been presented at the 98% confidence level, typically three standard deviations of random fluctuations combined in quadrature with the systematic uncertainties. The experimental results for both the $K$ and $H$ emissions are in agreement at 350 eV with the Coulomb distorted-wave calculation of Burgess and Sheorey but lie about 35% below the low-energy three-state close-coupling calculations of Burke and Moores. The ratio of the cross sections for the $K$ and $H$ emission is found to be 2.0 at all energies. Detailed study of the cross section at low energies demonstrates the expected finite value at threshold (within the accuracy allowed by the electron energy spread of 0.3 eV), and does not indicate the presence of a large (greater than 5%) contribution from cascade. Structure in the $K$ cross section about 2 eV above threshold suggests interactions with autoionizing levels of Ca I, presumably belonging to the $5snl$ or $4dnl$ series. The polarization fraction at low energy is about 25% higher than the calculation of Saraph which is based on the Burke and Moores close-coupling calculation.