Isotope shift in the 2S01-2P11 line of helium

The isotope shift in the 2S01-2P11 (2.06) line of helium has been measured for the first time and found to be 4200 140 MHz. This measurement is a sensitive test of the mass-polarization contribution to the isotope shift and is found to be in excellent agreement with the calculations of Pekeris. © 1969 The American Physical Society.