Crossed beams of electrons and ${\mathrm{Ga}}^{+}$ ions have been used to measure the absolute emission cross section for the process $e^{-}+{\mathrm{Ga}}^{+} \left(4\mathrm{}{}_{}{}^1{}_{}{}^{}S\right)\to e^{-}+{\mathrm{Ga}}^{+} \left(4\mathrm{}{}_{}{}^1{}_{}{}^{}P\right)$ from below thresh-old (8.77 eV) to 400 eV. Total uncertainties at 68% confidence level are typically 16%. The cross section exhibits the sharp onset at a threshold characteristic of positive-ion excitation processes. Evidence of structure is observed from just above threshold to 15 eV. The semiempirical Gaunt-factor formula is in reasonable agreement with the measurements over the energy range measured. A lower limit for the lifetime of the $4{}_{}{}^3{}_{}{}^{}P_{0,2}^{}{}_{}{}^{} {\mathrm{Ga}}^{+}$ metastable state has been established at $\tau \ge 0.4$ sec.

• Received 21 December 1981

DOI:https://doi.org/10.1103/PhysRevA.25.2996