Absolute emission cross section for electron-impact excitation ofLi+to the (2P3) level

Abstract

Crossed beams of electrons and ${\mathrm{Li}}^{+}$ ions have been used to measure the absolute emission cross section for the process, $e+{\mathrm{Li}}^{+}\left(1\mathrm{}{}_{}{}^1{}_{}{}^{}S\right)\to e+{\mathrm{Li}}^{+}\left(n{}_{}{}^3{}_{}{}^{}l\right)\to h\nu$ (548.5 nm), from below the threshold at 61.26 to 162 eV. The cross section exhibits the sharp onset at threshold characteristic of positive-ion excitation. The cross section at threshold is deduced to be 2.0 × ${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$, decreasing to 0.2 × ${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$ at 162 eV. Total uncertainties at a 68% confidence level are typically about ±15%. Pronounced structure is observed in the near-threshold region. From about 1.5 times the threshold energy to the highest energies measured, the cross section follows the generally predicted $E^{-3\mathrm{}}$ behavior for ${}_{}{}^1{}_{}{}^{}S$ to ${}_{}{}^3{}_{}{}^{}P$ transitions. Theoretical predictions of the cross sections differ from one another by up to a factor of 2, and none appears to fit the data really well.