Analysis of the He ii 4686-Å (n = 4 to n = 3) Line Complex Excited in an Atomic-Beam Light Source*

The fine structure of the 4686-A (n = 4 to n = 3) line complex of ionized helium has been studied using a gaseous atomic-beam light source and a pressure-scanned Fabry–Perot spectrometer. The separations, intensities, and widths of the fine-structure components were found by computer analysis of the recorded spectra. With this technique we located some components with uncertainties as small as ± 0.2 mK, only about 1/500 of their full width at half intensity. In all, 11 of the 13 predicted fine-structure components were found at their quantum-electrodynamic positions, including two very weak, optically unresolved components whose existence has not been conclusively shown in past studies. The Lamb shifts of the 4S and 3S levels of He ii, calculated indirectly from the observed optical transitions, are, respectively, 60 ± 4 mK and 138.5 ± 1.6 mK. The observed relative intensities, characteristic of a gas at near zero pressure, appear to depend on at least two basic excitation mechanisms: simultaneous ionization–excitation from the ground state of the helium atom and excitation from the ground state of the helium ion. The first quantitative study of the widths of the components has shown that the excessive widths that have plagued the study of this line in the past are most likely due to momentum transfer during excitation of the helium atom by energetic electrons. For most of the components, source widths of 85 mK were observed, about ten times that expected in the atomic-beam source, while transitions arising from the 4S level were even broader, about 100 mK.