Hyperfine Structure of the Metastable State of Singly Ionized Helium-3

The hfs splitting Δν of the metastable state of singly ionized helium 3 is determined by an ion-beam method. The conventional inhomogeneous molecular-beams A and B fields are replaced by radio-frequency fields at 13 350 Mc/sec. At this frequency transitions are preferentially induced between the $F=1$ component of the 2²S1/2 state and the short-lived 2²P1/2 state. All those ions which have made the transition decay immediately to the ground state. Thus, as the beam passes through such a field, state-selection is accomplished by selective quenching of the $F=1$ metastable ions. Two types of detection have been used: photoelectric detection of the Lyman-alpha photons emitted in the second state selector, and metastable ion detection by the surface Auger effect.Eighty-six determinations of the hfs splitting have been made under a range of experimental conditions. The final result is Δν=1083.35499±0.00020 Mc/sec. This value is compared with the theoretical hfs splitting for a point nucleus, which includes the known electrodynamic, relativistic, and reduced-mass corrections, and which may be computed precisely, since the atomic wave function of He³⁺ is hydrogenic and known exactly. The observed hfs splitting is smaller by 186 ppm than the theoretical value. This anomaly is attributed to the effects of finite nuclear and nucleon size, higher order radiative effects, and possibly also to nuclear interaction currents. Estimates are made of some of these effects.The hfs splitting Δν(1s) of the ground state of He³⁺ is estimated on the basis of the present result to be Δν(1s)=8665.628±0.013 Mc/sec.The experimental result is also compared with the experimentally determined hfs splitting of the $1s2s$ (³S₁) state of the He³ atom. The ratio of these two quantities yields a precise estimate of the electronic charge density at the nucleus of the 2³S₁ state of the He³ atom.Finally, the monoenergetic nature of the ion beam has allowed observation of a new molecular beams resonance power sharpening effect.