Production of Metastable Mercury Atoms by Electron Impact

Abstract

Excitation of Hg atoms by electron impact, particularly into metastable states, was studied in the energy range 4 to 15 eV. The excitation function for the $6{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ metastable state (5.46 eV) was measured from threshold up to 8.5 eV. The absolute cross-sectional scale for this state was calibrated by monitoring slow inelastically scattered electrons near threshold. The cross section had a maximum value of 3.2×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$ ± 25% (rms) at 5.75 eV and dropped to half this value at about 8 eV. An electron beam of 0.1-eV half-width was produced by the Retarding Potential Difference RPD method. Excited atoms were detected by electron ejection from a tungsten surface. The secondary electron yield for the $6{}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ state was typically 2×${10}^{-4\mathrm{}}$. Pronounced structure in the detector current with maxima at 9.00 and 9.60 eV was attributed to the $7{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ state (8.64 eV). It appears that the contribution of the $6p^{\prime }{}_{}{}^3{}_{}{}^{}{D}_3_{}^0{}_{}{}^{}$ metastable state was minor. An upper limit of a few times ${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$ was estimated for the maximum cross section of this state. Another region of structure in the total detector current occurred close to and above ionization threshold. Experimental evidence indicates that this structure was produced by long-lived auto-ionizing states.