Behavior of the He II $3s$, $p$, $d-4s$, $p$, $d$, $f$ Line Complex in a Hollow Cathode Plasma

The fine structure of the He ii $3s$, $p$, $d-4s$, $p$, $d$, $f$ line complex ($\sigma =21\mathrm{}335$ K, $\lambda =4686\mathrm{}$ Å) as observed in a hollow cathode cooled in liquid helium has been studied with a photoelectric double Fabry-Perot spectrometer. The traces show excellent resolution. Among the component lines: (1) The relative intensities tend, with increasing pressure in the discharge region, toward the calculated statistical intensities, and with decreasing pressure, not toward Bethe's dynamical intensities but in each case toward a dynamical intensity determined by the lifetime and rate of excitation of the initial state. (2) The measured relative positions are in good agreement with those calculated from quantum electrodynamics; small apparent discrepancies are believed to be associated with the excitation conditions. (3) The widths of the lines are appreciably greater than the widths of the He i lines at $\sigma =21\mathrm{}218$ K, the derived widths being 74 and 44 mK, respectively. (4) The positions are significantly shifted (∼4 mK) due to a drift of the ${\mathrm{He}}^{+}$ ions toward the cathode, and there is evidence that the lines of the neutral atoms are shifted (∼1 mK) due to a motion of the emitters toward the anode.