A theorem is derived concerning the energy shift of an atomic level due to nuclear motion. Since the expression obtained (Eq. (12) below) does not involve derivatives, it may be useful for calculations of isotope shifts due to nuclear motion in those cases where the radial wave functions are known not as analytical expressions but only as tables of numerical values.

• Received 7 September 1940

DOI:https://doi.org/10.1103/PhysRev.58.882