Absolute cross sections and polarization for electron-impact excitation of the resonance multiplet of theBe+ion

Abstract

Crossed beams of electrons and ground state ${\mathrm{Be}}^{+}$ ions have been used to measure absolute cross sections for electron-impact excitation of the 313.1-nm resonance radiation corresponding to the transition ${\mathrm{Be}}^{+}(2p)\ensuremath{\rightarrow}{\mathrm{Be}}^{+}(2s)$. Polarization fractions of the emitted light were also measured. Cross sections are absolute in the sense that all measurables including photon flux at 313 nm have been compared to relevant standards. The doublet emission cross section in units of ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ can be represented as a function of electron energy by $\ensuremath{\sigma}=\ensuremath{-}5.10\mathrm{ln}E+23.0$ from 4.4 to 21 eV (extrapolating to 16\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ at the 3.96-eV threshold) and by $\ensuremath{\sigma}=101\frac{\mathrm{ln}E}{E}\ensuremath{-}\frac{151}{E}$ from 21 to 740 eV. Total uncertainties at a high (approximately 98%) confidence level are about \ifmmode\pm\else\textpm\fi{}10%. Coulomb-Born II calculations of Bely overestimate the value at threshold by a factor of 1.7, and a recent Coulomb-Born calculation by Mann is about 10% higher at 740 eV. Two- and five-state close-coupling calculations lie, respectively, 28% and 19% above the measurement at threshold. For energies from 5 to 300 eV the measured polarization fractions can be represented by $P=\ensuremath{-}0.0708\mathrm{ln}E+0.284$ which given $P=0.19$ at threshold.