Excitation of Ionized Helium States in a Cooled Hollow-Cathode Discharge

A high-resolution study of the He II $\lambda 4686$ Å ($n=3-4\mathrm{}$) line complex emitted from a cooled hollow cathode has been made using a double-etalon spectrometer with photoelectric detection. Computer analysis of the digitally recorded spectra of this line yields fine-structure and isotope-shift measurements for the isotopes ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ which agree with quantum electrodynamic theory to within ±0.0005 ${\mathrm{cm}}^{-1\mathrm{}}$ for the best measured components. The excitation and deexcitation processes for this transition, which are active in the hollow-cathode, have been analyzed. The results show that the anomalously large widths observed in ionized helium lines which have been electronically excited arise from large momentum transfers in the excitation process. Calculation of the momentum-transfer line profile in the process $\mathrm{He}+e\to {{\mathrm{He}}^{+}}^{*}+e+e$ gives reasonable agreement with the observed profiles when the instrumental profile and neutral He Doppler profile are included. Measurements further confirm that the origin of the small anode-drift shift is a residual electric field within the discharge.