On Isotopic Shifts in the Spectra of Diatomic Molecules

An inspection of the theory of diatomic molecules shows that the elementary theory of the dependence of the energy of diatomic molecules on the mass of the nuclei must be supplemented in two ways. In the first place the reaction of the nuclei to the precession of the electronic angular momentum about the internuclear axis gives a term [Bₑ-α(V+1/2)][L(L+1)-Λ²]. Furthermore there is a second positive term of uncertain magnitude. Both these terms are of importance only for very light molecules especially for the hydrogen molecule. The large electronic shifts observed for some band systems of H₂, HD and D₂ are well accounted for by this theory. A very accurate comparison of the constants of very light isotopic molecules necessitates also taking into account small corrections which appear in the higher order approximations of the energy values for a rotating anharmonic oscillator.