Crossed beams of electrons and ${\mathrm{Hg}}^{+}$ ions have been employed to measure absolute cross sections for 165.0-nm emission, corresponding to the transition ${\mathrm{Hg}}^{+}\left(6p{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^0{}_{}{}^{}\right)\to {\mathrm{Hg}}^{+}\left(6s{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}\right)$. The emission cross section, which is nearly identical to the excitation cross section for ${\mathrm{Hg}}^{+}\left(6p{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^0{}_{}{}^{}\right)$ near threshold, has a value of about 1.2 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ at the 7.51-eV threshold, rises to a maximum of 2.26 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ at 13 eV, and falls to 0.64 × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ at 274 eV. The cross section exhibits considerable structure in the energy region from threshold to the onset of the first cascade ($7s{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$) at 11.9 eV; this structure is likely due to interference from autoionizing states of neutral Hg. The total uncertainty at a "good" confidence level is about 18%, taken as the quadrature sum of random uncertainty (5% at 90% confidence level) with systematic uncertainties (17% at a level equivalent to 90% confidence level). At threshold, the Gaunt-factor formula of Seaton predicts a value of about 3 times that measured, but the two converge to within about 15% for electron energies ranging from 13 to 274 eV. The results are also in reasonable agreement with a semiclassical Gryzinski-type calculation for energies above several times threshold. The data give an emission cross section of (2.8 ± 0.5) × ${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ at 6.8 eV for 194.2 nm ($6p{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^0{}_{}{}^{}\to 6s{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$). Analysis of data taken with some metastable $6s^2{}_{}{}^2{}_{}{}^{}D$ ions in the target beam indicates that the mean cross section for excitation of the $6s{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^0{}_{}{}^{}$ level from the $6s^2{}_{}{}^2{}_{}{}^{}D$ metastable states is larger than that for excitation from the $6s{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ ground state. Additional measurements of 398.4-nm emission ($6p{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^0{}_{}{}^{}\to 6s^2{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}$) yield a cross section of about 6 × ${10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$ near threshold, and a branching ratio $\frac{({}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\to {}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{})}{({}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}\to {}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{})}$ of $\frac{350}{1}$.

• Received 25 November 1974

DOI:https://doi.org/10.1103/PhysRevA.11.1223