Electron Density at the Nucleus and the Lamb Shift ofP3Levels of Two-Electron Ions

Abstract

The Lamb shifts of the $2{}_{}{}^3{}_{}{}^{}S$ and $2{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ levels in He I and Li II have been calculated by a method of effective-oscillator-strength sums. The computed shifts are in a good agreement with experiment. For Li II, the theoretical Lamb shift of the $2{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ level is +0.291 ±0.041 ${\mathrm{cm}}^{-1\mathrm{}}$ to be compared with the experimental value of +0.230±0.060 ${\mathrm{cm}}^{-1\mathrm{}}$ derived from the $2{}_{}{}^3{}_{}{}^{}S-2\mathrm{}{}_{}{}^3{}_{}{}^{}P_1^{}{}_{}{}^{}$ interval measured by Bacis and Berry. A significant Lamb shift of ${}_{}{}^3{}_{}{}^{}P$ levels is attributed to an anomalously low electron density at the nucleus in these states.