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

A source of narrow bandwidth (<800 MHz) tunable laser radiation at 58.4 nm has been developed and is applied to record the 1'S-2'P transition in He-3 and He-4. From the He-4 transition frequency of 171134.8936(58) cm(-1) a fivefold improved ground state Lamb shift of 1.3763(58) cm(-1) is deduced, in good agreement with the theoretical value of 1.3755(10) cm(-1). The measured 1'S-2'P transition isotope shift of 263410(7) MHz presents a more than 2 order of magnitude improvement over a previous value and agrees with a theoretical value of 263411.26(11) MHz.