Isotopic field shift of the rotational energy of the Pb-chalcogenides and Tl-Halides

Abstract

The formulas for the isotopic field shift which is due to the finite nuclear charge distribution, are deduced for the electronic, vibrational, and rotational energy of diatomic molecules following similar lines as in atomic physics. The result is applied to a reanalysis of the precise mm-wave measurements on the Pb-chalcogenides and Tl-halides by Tiemann et al. It is demonstrated that the isotope shift of lead and thallium in the rotational constant is mainly caused by the field shift and not by the breakdown of the Born-Oppenheimer approximation as it was interpreted earlier. This conclusion removes the unusual behavior of the adiabatic correction of the Born-Oppenheimer approximation in the group of diatomic molecules with ten valence electrons reported by Tiemann et al.