Helium isotopes yield touchstone spectroscopy

Atomic energy levels in different isotopes of an element have slightly different energies. This difference is termed the isotope shift and is a result of the different masses (the mass shift) and charge distributions (the volume shift) in the nuclei. For light elements the mass shift is dominant and is the sum of two components. One, the normal mass shift, arises from the reduced mass correction and can be calculated exactly. The other, the specific mass shift, is sensitive to the correlation between the motions of the atomic electrons and is less well understood. Now Ping Zhao and colleagues at Harvard University, USA, have measured the isotope shift in helium 100 times more accurately than ever before (Phys. Rev. Lett.. (1991) 66 592). This is the most precise isotope-shift measurement ever made for a multi-electron system and provides a rigorous check for calculations of atomic structure in two-electron atoms.