Precision measurements in helium at 58 nm: Ground state lamb shift and the 1 S-2 P transition isotope shift.

Abstract

A source of narrow bandwidth ( $l800\mathrm{MHz}$) tunable laser radiation at 58.4 nm has been developed and is applied to record the $^{1}1S\ensuremath{-}^{1}2P$ transition in ${}^{3}$He and ${}^{4}$He. From the ${}^{4}$He transition frequency of $171134.8936(58){\mathrm{cm}}^{\ensuremath{-}1}$ a fivefold improved ground state Lamb shift of $1.3763(58){\mathrm{cm}}^{\ensuremath{-}1}$ is deduced, in good agreement with the theoretical value of $1.3755(10){\mathrm{cm}}^{\ensuremath{-}1}$. The measured $^{1}1S\ensuremath{-}^{1}2P$ transition isotope shift of 263 410(7) MHz presents a more than 2 order of magnitude improvement over a previous value and agrees with a theoretical value of 263 411.26(11) MHz.