Isotopic shift in helium

Abstract

Using Doppler-free two-photon spectroscopy, the authors have measured the 3He-4He isotopic shift on the 23S-n3D (n=3-6) and 23S-n3S (n=4-6) transitions. The accuracy of their data is more than one order of magnitude better than the previous ones and permits a valuable comparison with theory. Thus they have been able to show evidence for relativistic effects and to check Stone's theory (1963) for the first time.