Lamb shift measurement in the 1t¹S ground state of helium

With a phase-modulated extreme ultraviolet pulsed laser source the frequency of the 1 ¹S-2 ¹P transition of helium at 58 nm has been measured. The phase modulation scheme enabled measurement and reduction of frequency chirp, usually limiting pulsed precision spectroscopy. From the measured transition frequency of 5 130 495 083(45) MHz, a fourfold improved value of the ground state Lamb shift of 41 224(45) MHz is deduced, in good agreement with a theoretical value of 41 233(35) MHz based on QED calculations up to order α⁵Z⁶. From these measurements, the well-known binding energy of the 2 ¹P state and the previously determined ⁴-³He isotope shift, accurate values for the ionization energies of the helium atom follow: 198 310.6672(15)fcm^-1 for ⁴He and 198 301.8808(15)fcm^-1 for ³He.