The isotopic shift (also called isotope shift) is the shift in atomic spectra which occurs when one nuclear isotope is replaced by another. If the spectra have also hyperfine structure the shift refers to the center of gravity of the spectra. There are two effects which contribute to this shift:
- The mass difference. This difference induces a change in the reduced electronic mass. This purely kinematical effect, studied theoretically by Hughes and Eckart is important for light elements.
- D. J. Hughes, C. Eckart (1930). "The Effect of the Motion of the Nucleus on the Spectra of Li I and Li II". Phys. Rev. 36: 694. doi:10.1103/PhysRev.36.694.
- W. Pauli, R. E. Peierls, Phys. Z. 32 (1931) 670
- P. Brix, H. Kopfermann, Festschrift Göttinger Akad. Springer, 17 (1951)
- H. Kopfermann, Nuclear Moments, Academic Press 1958