Adiabatic description of nonadiabatic isotopic asymmetry effects. The dipole moment of HD

A new method is proposed for describing the effects of the asymmetry in those electronic properties of a molecule which are induced by the isotopic asymmetry of the nuclear core when this core consists of different isotopes of the same chemical element. Whereas in the traditional approach the computations of the corrections to the adiabatic wave function of the molecule are laborious, in the new approach the isotopic asymmetry of the electronic properties fits into the framework of the adiabatic approximation. For this purpose use is made of the fact that the accuracy of the adiabatic approximation depends on the specific choice of the dynamic variables describing the ''fast'' and ''slow'' subsystems in the molecule. It is shown that in the case of diatomic isotopically asymmetric molecules the coordinates can be chosen such that all the asymmetry effects will be included in the potential energy. As a result, the adiabatic approximation can be used, and this simplifies greatly the theory of the isotopic asymmetry of the electronic properties. The new approach is illustrated with the molecules HD and HD+. In the case of HD the mean value of the component of the dipole moment along the line joining the nuclei and the dipole moments determining the intensities of the rotational and vibrational-rotational transitions are computed. In the case of the one-electron molecule HD+ a connection is established between the isotopic asymmetry effects and the physical characteristics (the electric polarizability, the quadrupole moment, the diamagnetic susceptibility, and the change that occurs in the nuclear magnetic screening in an electric field) of the homonuclear molecular ion H2+. The displacement of the center of the electron cloud from the center of the nuclear core in HD+ is computed