Lamb Shift in the(Li6)++Ion

The energy difference between the $2S_{\frac{1}{2}}$ and $2P_{\frac{1}{2}}$ levels of the hydrogen-like atom ${\left({\mathrm{Li}}^6\right)}^{++}$ was determined by measuring the lifetime of the metastable $2S_{\frac{1}{2}}$ state in an electrostatic field (Stark quenching). The metastable ions were produced by charge equilibrating a lithium beam of energy ∼3 MeV in nitrogen gas and were then directed through the electrostatic field (about 10 kV/cm) between a pair of parallel plates. The field mixed the $2S_{\frac{1}{2}}$, $2P_{\frac{1}{2}}$, and $2P_{\frac{3}{2}}$ states and the photons from the subsequent decay from $2P$ to the ground state were detected with two thin-window GM counters, one of them fixed and the other movable along the quenching chamber in the beam direction. From the normalized counting rates of the movable counter and the separation between the two counters, the lifetime of the perturbed $2S$ state could be determined. It was found to be (2.629±0.021) ×${10}^{-9\mathrm{}}$ sec and (1.764±0.035) ×${10}^{-9\mathrm{}}$ sec for the field strengths 7425±2 and 9173±2 V/cm, respectively. From these values, the Bethe-Lamb theory of Stark quenching yields an averaged Lamb shift in the ${\left({\mathrm{Li}}^6\right)}^{++}$ ion of 63 031±327 Mc/sec. This result agrees with the theoretical value 62 740±47 Mc/sec recently calculated by Erickson.