Lamb shift and the lifetime of the2S122state of hydrogenlike argon (Z=18)

Abstract

We report a measurement of the Lamb shift in hydrogenlike argon based on the electric-field-quenching method, and a measurement of the unperturbed lifetime of the $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ state. We find the lifetime of the unperturbed $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ state to be 3.487(0.036) ns in agreement with the theoretical value of 3.497 ns. This is the first measurement of the $2{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ state lifetime of sufficient accuracy to observe the contribution to the total decay rate of the single-photon magnetic dipole decay. Our measured value of the Lamb shift is $\delta E(2\mathrm{}{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}-2\mathrm{}{}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{})=37.89\mathrm{}\left(0.38\right)\mathrm{}$ THz, lower than, but in agreement with, Mohr's value of 38.25(0.025) THz, and 2.7 standard deviations below Erickson's value of 39.01(0.16) THz. Both the quenched and natural lifetimes were measured by the beam-foil time-of-flight technique. In this paper we emphasize the treatment of the systematic effects in the beam-foil time-of-flight method including cascades from higher excited states, interference from the spectra of heliumlike ions, collisions in the residual gas, and perturbations from a highly excited extra electron, as well as systematic effects more unique to electric field quenching.